Engine Controls - Tests W/codes - 4.3l Chevrolet Astro I

MODEL IDENTIFICATION

Vehicle model is identified by fifth character of Vehicle Identification Number (VIN). VIN is stamped on metal pad on top of left end of instrument panel, near windshield. See MODEL IDENTIFICATION .

MODEL IDENTIFICATION

INTRODUCTION

Most engine control problems result from mechanical failures, poor electrical connections or damaged vacuum hoses. Before condemning the computer system, perform checks and inspections covered in BASIC TESTING - 4.3L article. Failure to do so may result in lost diagnostic time.

If no faults were found while performing BASIC TESTING - 4.3L, proceed with DIAGNOSTIC PROCEDURE. If no fault codes or only a non-running Code 12 is present and driveability problems exist, proceed to TESTS W/O CODES - GASOLINE article for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.). If only intermittent codes are present, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

TERMINOLOGY

Due to Federal government requirements, manufacturers may use names and acronyms for systems and components different than those used in previous years. The following table will help eliminate confusion when dealing with these components and systems. Only relevant components and systems whose names have changed from current General Motors Corp. terminology have been listed.

SAE TERMINOLOGY

SELF-DIAGNOSTIC SYSTEM

All vehicle are equipped with either an Electronic Control Module (ECM), Powertrain Control Module (PCM) or Vehicle Control Module (VCM). Unless specifically stated, references to ECM also apply to PCM or VCM equipped vehicles.

The control module is equipped with a self-diagnostic system, which detects system failures or abnormalities. When a malfunction occurs, control module will illuminate the SERVICE ENGINE SOON light located on instrument panel. This light is also referred to as the Malfunction Indicator Light (MIL). When malfunction is detected and MIL is turned on, a corresponding trouble code will be stored in control module memory. To retrieve stored codes, see READING TROUBLE CODES or RETRIEVING CODES (NON-SCAN) . Malfunctions are recorded as HARD FAILURES or as INTERMITTENT FAILURES.

Note. Not all trouble codes will turn on the SERVICE ENGINE SOON light. All emission-related codes will turn on the SERVICE ENGINE SOON light.

HARD FAILURES

Most hard failures cause MIL to illuminate and remain on until the malfunction is repaired. If light comes on and remains on (light may flash) during vehicle operation, cause must be found using diagnostic (code) charts. If a sensor fails, control unit will use a substitute value in its calculations to continue engine operation. In this condition, vehicle is functional, but driveability can be poor.

INTERMITTENT FAILURES

Intermittent failures may cause MIL to flicker or illuminate and go out about 10 seconds after the intermittent fault goes away. The corresponding trouble code, however, will be retained in control module memory. If related fault does not reoccur within 50 engine restarts, it will be erased from ECM memory. Intermittent failures may be caused by faulty sensor, connector or wiring. See INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

DIAGNOSTIC PROCEDURE

Diagnosis of the computerized engine control system should be performed in the following order

1. Ensure all engine systems not related to the computer are operating properly. DO NOT proceed with testing unless all other problems have been repaired. Perform diagnostic circuit check before using trouble code charts. See «BASIC TESTING - 4.3L»(/chevrolet/astro/i-1985-2005/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-43l) article.
2. If trouble codes were displayed (other than Code 12), determine whether codes are hard or intermittent. Hard codes may cause Malfunction Indicator Light (MIL) to illuminate continuously with engine running. For diagnosing hard codes, proceed to appropriate trouble code chart. For diagnosing intermittent codes, use DIAGNOSTIC AIDS in appropriate code chart or proceed to INTERMITTENTS in «TESTS W/O CODES - GASOLINE»(/chevrolet/astro/i-1985-2005/remont/testing-diagnostics/#engine-controls-tests-wo-codes-gasoline) article.
3. If trouble codes were not displayed and a driveability problem exists, refer to SYMPTOMS in «TESTS W/O CODES - GASOLINE»(/chevrolet/astro/i-1985-2005/remont/testing-diagnostics/#engine-controls-tests-wo-codes-gasoline) article in this section. From there you will be sent to «SYSTEM/COMPONENT TESTS - 4.3L»(/chevrolet/astro/i-1985-2005/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-43l) article.
4. After repairs are made, clear trouble codes and perform FIELD SERVICE MODE CHECK in «BASIC TESTING - 4.3L»(/chevrolet/astro/i-1985-2005/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-43l) article.

RETRIEVING CODES (NON-SCAN)

Note. The Assembly Line Data Link (ALDL) connector may also be referred to as the Data Link Connector (DLC) in flow charts. This is the same connector.

Note. Vehicles equipped with VCM's incorporating the 16-pin DLC, will not have the capability to flash trouble codes.

1. Turn ignition on with engine off. Malfunction Indicator Light (MIL) should glow. Locate Data Link Connector (DLC), attached to control module wiring harness. Most DLCs are located under dash on driver's side of vehicle. For exact location of DLC, see appropriate COMPONENT LOCATIONS illustration in «SYSTEM/COMPONENT TESTS - 4.3L»(/chevrolet/astro/i-1985-2005/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-43l) article. Insert jumper wire from terminal "B" (diagnostic test terminal) to terminal "A" (ground) of DLC (terminals No. 5 and 6 of 16-terminal connector). (Scheme 105)and (Scheme 106). Turn ignition on. NOTE: Inserting jumper wire into test and ground terminals of DLC with engine running will cause fuel-injected vehicles to enter field service mode and codes will not flash. See FIELD SERVICE MODE in «BASIC TESTING - 4.3L»(/chevrolet/astro/i-1985-2005/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-43l) article.
2. Malfunction Indicator Light (MIL) should flash codes. Each code is flashed 3 times. If codes DO NOT flash, perform DIAGNOSTIC CIRCUIT CHECK in «BASIC TESTING - 4.3L»(/chevrolet/astro/i-1985-2005/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-43l) article. To exit diagnostic mode, turn ignition off and remove jumper wire from DLC.

Scheme 105

Scheme 106

READING TROUBLE CODES

The control module stores component failure information under a related trouble code which can be recalled for diagnosis and repair. Read trouble codes by counting Malfunction Indicator Light (MIL) flashes or with diagnostic scan tester connected to the Data Link Connector (DLC). The tester is faster, and capable of reading information which would require testing individual control module and sensor/solenoid connector terminals with a digital voltmeter. Some models REQUIRE a scan tester to pull trouble codes. See SCAN TESTER DATA and SCAN TESTER USAGE .

Note. When using a scan tester, there is a time delay between serial data updates. For instantaneous response, a digital voltmeter must be used.

If scan tester is not available, MIL flashes can be read by grounding DLC terminal with ignition on and engine off. For example, FLASH, FLASH, pause, FLASH, longer pause, indicates Code 21. The first series of flashes are the first digit of trouble code. The second series of flashes are the second digit of trouble code. Trouble codes are displayed starting with the lowest code. Each code is displayed 3 times and will continue as long as DLC is grounded.

Note. Trouble codes will be recorded at various operating times. Some codes require sensor or switch operation for 5 seconds and others may require longer under certain conditions. Some codes may not set in a service bay operational mode.

TROUBLE CODE IDENTIFICATION

Note. On models not using "P" series codes, Code 12 should always exist when DLC test terminal is grounded with key on and engine off, but it may not be indicated by all makes of scan tester.

CLEARING TROUBLE CODES

Turn ignition switch to ON position and ground diagnostic test terminal "B" at DLC. (Scheme 105)and (Scheme 106). Turn ignition switch to OFF posit ion and remove ECM fuse from fuse block for 10 seconds. Replace fuse. Remove diagnostic terminal ground lead. Codes may also be cleared using the General Motors Tech 1 scan tester and is REQUIRED to clear codes on some vehicles.

ECM LOCATION

The term Engine Control Module (ECM) also applies to Vehicle Control Module (VCM) and Powertrain Control Module (PCM). For control module locations, see appropriate COMPONENT LOCATIONS illustration in SYSTEM/COMPONENT TESTS - 4.3L article.

Diagnostic Aids

Diagnostic aids (located in many trouble code charts) are provided as additional tips to help with diagnosis when inspected circuit is okay and an intermittent condition exists.

Field Service Mode Check

Malfunction Indicator Light (MIL) indicates operational mode of engine if DLC is grounded while engine is running. Light response confirms proper fuel system operation and verifies closed loop operation. Clear codes and perform this test after any repair is completed. Field service mode check can be found by proceeding to FIELD SERVICE MODE CHECK in BASIC TESTING - 4.3L article.

SPECIAL TOOLS (DIAGNOSTIC)

Note. A special scan tester, plugged into the DLC, can read trouble codes, check system voltages on the serial data line and save a great deal of time. For additional information, see tester owner's manual. Also, see SCAN TESTER USAGE and SCAN TESTER DATA .

The computerized engine control system is most easily diagnosed using a scan tester. However, other tools may aid in diagnosing problems if a scan tester is unavailable. These tools are a tachometer, test light, ohmmeter, digital voltmeter with 10-megohm input impedance (minimum), vacuum pump, vacuum gauge, fuel injector test lights and 6 jumper wires 6" long (one wire with female connectors at both ends, one wire with male connector at both ends and 4 wires with male and female connectors at opposite ends). A test light, rather than a voltmeter, must be used when indicated by a diagnostic chart.

SCAN TESTER USAGE

Note. Before connecting scan tester, check diagnostic system and ensure accurate information is received by scan tester. Perform DIAGNOSTIC CIRCUIT (GASOLINE) CHECK in BASIC TESTING - 4.3L article. If vehicle does not pass diagnostic circuit check, information received by scan tester may be invalid.

The scan tester is a specialized tester which can diagnose on-board computer control systems by providing access to circuit voltage information without crawling under dash or hood to backprobe sensors and connectors. scan testers reduce diagnostic time by furnishing input data (voltage signals) which can be compared to specification parameters. See SCAN TESTER DATA .

Scan testers also furnish information on output device (solenoids and motors) status. However, status parameters are only an indication output signals have been sent to devices by the control module. They do not indicate whether devices respond properly to that signal. This must be verified at output device using a voltmeter or test light.

Note. On models not using "P" series codes, Code 12 should always exist when DLC test terminal is grounded with key on and engine off, but it may not be indicated by all makes of scan tester.

If trouble codes are not present, a problem may still exist. Driveability-related problems with codes displayed occur about 20 percent of the time, while driveability problems without codes occur about 80 percent of the time. Out-of-calibration sensors WILL NOT set a trouble code, but WILL cause driveability problems. A scan tester is the easiest method of checking sensor specifications and other data parameters. Tester is also useful in finding intermittent wiring problems by wiggling wiring harnesses and connections (key on, engine off) while observing data parameters. See SCAN TESTER DATA .

Note. Information obtained by scan tester is only as accurate as the tester itself. If erroneous voltage signals are suspected, verify tester information using a digital voltmeter and wiring schematic. If non-existent codes are displayed, turn ignition off and remove tester. Turn ignition on and ground DLC test terminal. If same codes are not flashed by Malfunction Indicator Light (MIL) as were indicated by scan tester, tester cannot be used on vehicle and information obtained by it will not be guaranteed accurate. If tested vehicle does not provide for grounding diagnostic test terminal, verify tester accuracy by connecting it to a known good vehicle.

SCAN TESTER DATA

Note. Information contained in the following table is typical of readings taken on vehicle with engine idling, upper radiator hose hot, throttle closed, transmission in Park or Neutral, closed loop status achieved and all accessories off (except as noted in tables). Not all devices and systems are used on all models. For additional information, see tester owner's manual.

SCAN TESTER DATA

TROUBLE CODE CHARTS

Note. The following mini-schematics are courtesy of General Motors Corp.

CODE 13, OPEN OXYGEN SENSOR CIRCUIT - ALL MODELS

When exhaust temperature is less than 600°F (316°C), O2 sensor is open and produces no voltage. An open sensor circuit or cold sensor will not allow system to enter closed loop. On some models, oxygen sensor is equipped with an internal heating unit. This allows sensor to reach operating temperature quicker and maintain closed loop operation even during extended idle. Heating element resistance should be 3.5-14 ohms at 662°F (350°C).

Note. Test numbers refer to test numbers on diagnostic chart.

1. Code 13 will set at normal operating temperature if at least 2 minutes have passed since engine start, Code 21 or 22 is not present, O2 signal voltage is steady at .35-.55 volt and throttle position sensor signal is greater than idle. All conditions must be met for at least one minute.
2. This determines if fault is in O2 sensor, control module or wiring.
3. Use only a high-impedance Digital Volt-Ohmmeter (DVOM) while checking for continuity in signal and ground circuits. If ground circuit is open, voltage on signal circuit will be greater than .6 volt.

Verify a clean, tight connection for sensor ground. An open circuit at sensor signal terminal or ground terminal will result in a Code 13.

CODE 13 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 13 Schematic (4.3L) Open Oxygen Sensor Circuit. Scheme 107

Code 13 Flow Chart (4.3L) Open Oxygen Sensor Circuit. Scheme 108

CODE 14, COOLANT SENSOR SIGNAL VOLTAGE LOW - ALL MODELS

Coolant temperature sensor input is used to determining control of fuel delivery, engine timing, idle speed and converter clutch (TCC) application. As engine warms, sensor resistance reduces. At normal operating temperature, voltage signal will be about 1.5-2.0 volts at coolant sensor signal terminal.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This tests if code was set because of a hard failure or intermittent condition.
2. This simulates conditions for a Code 15. If scan tester displays a low temperature, control module and wiring are not at fault.

After engine is started, temperature should rise steadily to about 194°F (90°C), then stabilize when thermostat opens. If engine is allowed to cool overnight, coolant temperature sensor and intake air temperature sensor (if equipped) should read close to each other, when measured with a scan tester.

CODE 14 TERMINAL & CIRCUIT WIRING IDENTIFICATION

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

Code 14 Schematic (4.3L) Coolant Sensor Signal Voltage Low. Scheme 109

Code 14 Flow Chart (4.3L) Coolant Sensor Signal Voltage Low. Scheme 110

CODE 15, COOLANT SENSOR SIGNAL VOLTAGE HIGH - ALL MODELS

As engine warms, sensor resistance reduces and voltage drops. At normal operating temperature, voltage signal will be about 1.5-2.0 volts at control module coolant sensor signal terminal. If sensor signal circuit opens, control module will see -56°F (-49°C) and deliver fuel for this temperature.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This checks if code was set as a result of a hard failure or intermittent condition.
2. This simulates conditions for a Code 14. If control module recognizes grounded circuit and displays a high temperature, control module and wiring are okay.
3. This determines if problem is control module or wiring. There should be 5 volts present at sensor when measured with a DVOM.

After engine starts, temperature should rise steadily to about 194°F (90°C) and stabilize when thermostat opens. If engine is allowed to cool overnight, coolant temperature sensor and intake air temperature sensor (if equipped) should read close to each other when measured with a scan tester. Code 15 will also set if sensor signal or ground circuit is open.

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

CODE 15 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 15 Flow Chart (4.3L) Coolant Sensor Signal Voltage High. Scheme 111

CODE 16, VSS BUFFER FAULT - "S" & "T" SERIES, (VIN W) UTILITY A/T & (VIN Z) PICKUP A/T

Note. Test number refers to number on diagnostic chart.

1. Checks for battery voltage at VSS buffer.
2. Tests for proper ground path for VSS buffer
3. Tests for vss buffer signal to control module
4. Tests for faulty connections and a faulty VSS buffer

Check for poor connections or damaged harness.

Code 16 Schematic ("S" & "T" Series, 4.3L (VIN W) Utility A/T & 4.3L (VIN Z) Pickup A/T) VSS Buffer Fault. Scheme 112

Code 16 Flow Chart ("S" & "T" Series, 4.3L (VIN W) Utility A/T & 4.3L (VIN Z) Pickup A/T) VSS Buffer Fault. Scheme 113

CODE 21, TPS SIGNAL VOLTAGE HIGH - ALL MODELS

Throttle Position Sensor (TPS) provides a varying voltage signal depending on throttle valve angle. Signal voltage varies from about .50 volt at idle to 4 volts at wide open throttle. Each time TPS voltage drops to less than 1.25 volts and stops, control module assumes this is zero degrees throttle angle and measures throttle percentage angle from this point.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test confirms Code 21 and checks if fault is a hard failure or an intermittent condition.
2. This test simulates conditions for Code 22. If control module recognizes low voltage signal and sets Code 22, control module and power and signal circuits are not at fault.
3. This step isolates a faulty sensor, control module or an open ground circuit.

A scan tester displays throttle position in volts. Closed throttle voltage should be less than 1.25 volts. TPS voltage should increase at a steady rate to about 4.5 volts as throttle angle increases. Code 21 will also result if ground circuit is open or TPS signal circuit is shorted to voltage.

CODE 21 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 21 Schematic (4.3L) TPS Signal Voltage High. Scheme 114

Code 21 Flow Chart (4.3L) TPS Signal Voltage High. Scheme 115

CODE 22, TPS SIGNAL VOLTAGE LOW - ALL MODELS

Throttle Position Sensor (TPS) provides a varying voltage signal depending on throttle valve angle. Signal voltage varies from less than about .50 volt at idle to 4 volts at wide open throttle.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test confirms Code 22 and tests if fault is a hard failure or an intermittent condition.
2. This simulates Code 21. If control module recognizes a high voltage signal and sets Code 21, control module and wiring are not at fault. Replace TPS.
3. This simulates a high voltage signal to check for on open TPS signal circuit.

A scan tester displays throttle position in volts. Closed throttle voltage should be less than 1.0 volt. TPS voltage should increase at a steady rate to about 4.5 volts as throttle angle increases. Code 22 will also set if TPS signal or ground circuits are open or grounded.

CODE 22 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 22 Flow Chart (4.3L) TPS Signal Voltage Low. Scheme 116

CODE 23, INTAKE AIR TEMP. LOW - "L", "M", "S" & "T" SERIES

Control module supplies and monitors a voltage signal (4-6 volts) to sensor. When temperatures are low, sensor resistance is high and control module will see a high-monitored voltage signal. As temperature increases, sensor resistance decreases and voltage sensed by control module drops.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This checks if Code 23 is a hard failure or an intermittent condition.
2. This simulates conditions for a Code 25. If scan tester displays a high temperature, control module and wiring are not at fault.
3. This checks for continuity of sensor signal and ground circuits.

If engine is allowed to cool overnight, coolant and MAT sensors should read close to each other, when measured with a scan tester. A Code 23 will also result if signal and ground circuits become open.

CODE 23 TERMINAL & CIRCUIT WIRING IDENTIFICATION

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

Code 23 Schematic ("L", "M", "S" & "T" Series - 4.3L) Intake Air Temperature Low. Scheme 117

Code 23 Flow Chart ("L", "M", "S" & "T" Series - 4.3L) Intake Air Temperature Low. Scheme 118

CODE 24, VSS TRANSMISSION OUTPUT SIGNAL - A/T - ALL MODELS

Vehicle Speed Sensor (VSS) output sensor is a magnetic induction type. Gear teeth pressed on outside diameter of output carrier assembly induce an alternating current in sensor when drive wheels are turning. Since vehicle speed is taken from transfer case on 4WD vehicles, output speed sensor signal on these units goes directly to control module. Code 24 will set if gear selector is not in Park or Neutral, engine speed is at least 3000 RPM and output speed is less than 250 RPM for at least 1.5 seconds.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Test verifies voltage at buffer module.
2. Test checks VSS buffer ground circuit.
3. Test checks VSS circuit at buffer module.
4. Test verifies VSS signal at from module.

Code 24 will set when no vehicle speed is detected at vehicle start off. Code 72 will set when VSS signal is present and is lost. Check all connections, especially those at transmission pass-through connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article. While Code 24 is set, scan tester will display an RPM derived from input speed. If input speed sensor is not operational at start-up, this can cause VSS to read zero.

Code 24 Schematic ("C" & "K" Series) VSS Transmission Output Signal. Scheme 119

Code 24 Schematic ("G" & "P" Series) VSS Transmission Output Signal. Scheme 120

Code 24 Schematic ("L" & "M" Series) VSS Transmission Output Signal. Scheme 121

Code 24 Schematic ("S" & "T" Series) VSS Transmission Output Signal. Scheme 122

Code 24 Flow Chart (All Models) VSS Transmission Output Signal. Scheme 123

CODE 25, INTAKE AIR TEMPERATURE HIGH - "L", "M", "S" & "T" SERIES

Control module applies and monitors a voltage signal (4-6 volts) to intake air temperature sensor. When manifold air is cold, sensor resistance is high and control module sees a high signal voltage. As air warms, resistance decreases and voltage sensed by control module drops. Sensor resistance can be measured at sensor terminals with harness disconnected.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This checks if code is a hard failure or an intermittent condition.
2. This simulates conditions for a Code 23. If scan tester displays a low temperature, control module and wiring are not at fault.

If engine is allowed to cool overnight, coolant temperature sensor and intake air temperature sensor should read close to each other, when measured with a scan tester. A Code 25 will also result if sensor signal circuit is shorted to ground.

CODE 25 TERMINAL & CIRCUIT WIRING IDENTIFICATION

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

Code 25 Flow Chart ("L", "M", "S" & "T" Series - 4.3L) Intake Air Temperature High. Scheme 124

CODE 28, TRANSMISSION RANGE PRESSURE SWITCH - ALL MODELS

Transmission range pressure switch assembly consists of 5 pressure switches (2 normally closed, 3 normally open), and a fluid temperature sensor combined into one unit and mounted on valve body. Control module supplies voltage to each range signal. By grounding one or more of these circuits through various combinations of pressure switches, control module detects which manual valve position has been selected. With ignition on and engine off, Park/Neutral will be indicated. Code 28 will set when range signals "A" and "C" are both zero volts (on) for 2 seconds.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks indicated range signal to valve position signal actually selected.
2. Checks for correct voltage from control module to transmission external connector.
3. Checks for short to ground in any one of 3 valve position circuits.

Code will set if control module detects one of 2 illegal combinations. Check all connections for good contact. An intermittent may be caused by a poor connection, chaffed wire insulation or a broken wire. Monitor voltage of each terminal while moving related harness connectors. If failure is induced, voltage reading will change.

VALID PSM COMBINATION (4L60-E)

VALID PSM COMBINATION (4L80-E)

Code 28 Schematic ("C", "K" & "P" Series) Transmission Range Pressure Switch. Scheme 125

Code 28 Schematic ("G", "L", "M", "S" & "T" Series) Transmission Range Pressure Switch. Scheme 126

Code 28 Flow Chart (All Models) Transmission Range Pressure Switch. Scheme 127

CODE 32, EGR CIRCUIT ERROR BACKPRESSURE EGR WITH SOLENOID - "C", "K" & "P" SERIES

Control module controls a solenoid regulating vacuum to EGR valve. Normally closed solenoid prevents vacuum from passing until it is energized by control module. A properly operating EGR will directly affect fuel integrator counts. With EGR valve open, integrator counts will be less than without EGR operation. If monitored integrator counts do not change with EGR commanded, Code 32 will set.

Note. Test numbers refer to test numbers on diagnostic chart.

1. EGR valve should open when vacuum is applied to solenoid. Vacuum should hold.
2. When Tech 1 energizes EGR solenoid, vacuum to EGR valve should bleed through a vent in solenoid and EGR valve should close. Vacuum gauge may or may not bleed off vacuum. However, this does not indicate a problem.
3. Determines if fault lies in electrical control part of system, connector or solenoid.
4. This system uses a negative backpressure EGR valve. Valve should hold vacuum with engine off.
5. When engine is started, backpressure should cause vacuum to bleed off and valve should fully close.

Prior to replacing control module, check resistance of all control module-controlled solenoids and relays. Replace any with a resistance value less than 20 ohms.

Code 32 Schematic ("C" & "K" Series - 4.3L) EGR Circuit Error Backpressure EGR With Solenoid. Scheme 128

Code 32 Schematic ("P" Series - 4.3L) EGR Circuit Error Backpressure EGR With Solenoid. Scheme 129

Code 32 Flow Chart (All Models) EGR Circuit Error Backpressure EGR With Solenoid. Scheme 130

CODE 32, EGR CIRCUIT ERROR WITH EVRV SOLENOID - "C", "G", "K" & "P" SERIES

Control module controls a solenoid that regulates vacuum to EGR valve. The normally closed solenoid prevents vacuum from passing until it is energized by control module. A properly operating EGR will directly affect fuel integrator counts. With EGR valve open, integrator counts will be less than without EGR operation. If monitored integrator counts do not change with EGR commanded, Code 32 will set.

Control module checks EGR operation when engine speed is greater than 1600 RPM, MAP sensor signal indicates cruise condition and throttle position are constant.

Note. Test numbers refer to test numbers on diagnostic chart.

1. With ignition on and engine off, solenoid should not be energized or allow vacuum to pass to EGR valve. When Tech 1 energizes solenoid, vacuum should pass through solenoid to EGR valve. Vacuum should hold.
2. Checks for plugged EGR passages. If passages are plugged, engine may have severe detonation on acceleration.
3. Vehicle must be driven during this test to produce sufficient load to operate EGR. Lightly accelerating (about 1/4 throttle) will produce a large and stable enough reading to determine if control module is commanding system on.

Prior to replacing control module, check resistance of all control module-controlled solenoids and relays. Replace any with a resistance value less than 20 ohms. EVRV solenoid cannot be checked with a DVOM due to solid state circuitry.

Code 32 Schematic ("C", "G", "K" & "P" Series A/T) EGR Circuit Error With EVRV Solenoid. Scheme 131

Code 32 Schematic ("P" Series M/T) EGR Circuit Error With EVRV Solenoid. Scheme 132

Code 32 Flow Chart (All Models) EGR Circuit Error With EVRV Solenoid. Scheme 133

CODE 32, EGR CIRCUIT ERROR - "L", "M", "S" & "T" SERIES W/ LINEAR EGR

Control module controls linear EGR valve using a pulse width modulated signal. Proper operation of EGR valve is determined using 2 separate diagnostic tests. A properly operating EGR system will directly affect fuel integrator counts. With EGR activated, integrator counts will be less than without EGR operation. If monitored integrator counts do not change with EGR commanded, Code 32 will set. Linear EGR valve also incorporates an EGR pintle position sensor. If actual (feedback) pintle position signal does not match desired pintle position, Code 32 will set.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks status of EGR pintle.
2. Checks for correct operation of linear EGR valve.
3. Checks linear EGR control circuit.
4. Checks power feed to EGR valve connector.
5. Checks EGR control circuit connections and control module.
6. Checks pintle position signal circuit and control module.
7. Checks ground circuit for pintle position sensor and control module.
8. Checks for a grounded EGR control circuit.
9. Checks sensor ground circuit, 5-volt reference circuit and control module.
10. Checks sensor signal and circuit.
11. Checks control module connections, pintle position signal circuit, EGR valve connections, EGR valve and control module.

Before replacing control module, use a DVOM and check resistance of each control module-regulated solenoid and relay.

Code 32 Schematic ("L" & "M" Series EGR Circuit Error. Scheme 134

Code 32 Schematic ("S" & "T" Series) EGR Circuit Error. Scheme 135

Code 32 Flow Chart - 1 Of 2 (All Models) EGR Circuit Error. Scheme 136

Code 32 Flow Chart - 2 Of 2 (All Models) EGR Circuit Error. Scheme 137

CODE 32, EGR CIRCUIT ERROR - "C", "G" & "K" SERIES W/ LINEAR EGR

Control module regulates linear EGR valve to control exhaust gas recirculation by providing a ground control for internal pintle (solenoid). Pintle will pull away from its seat when energized. Control module controls linear EGR valve based upon coolant temperature and throttle position.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks pintle's ability to be commanded to desired position.
2. Checks for voltage to linear EGR valve to verify if problem is in ignition feed circuit.
3. Checks control module control circuit by jumpering across harness terminals with a test light and energizing EGR valve.

Before replacing control module, use an ohmmeter and check EGR valve resistances. Resistance between terminals "A" and "E" should be 7.8-8.6 ohms. Resistance between terminals "B" and "D" should be greater than 3000 ohms. Resistance between terminals "D" and "C" should start at .7 ohm and increase to about 4000 ohms as pintle is slowly moved inward.

Code 32 Schematic ("C" & "K" Series) EGR Circuit Error. Scheme 138

Code 32 Schematic ("G" Series 4.3L) EGR Circuit Error. Scheme 139

Code 32 Flow Chart (All Models) EGR Circuit Error. Scheme 140

CODE 33, MAP SENSOR SIGNAL VOLTAGE HIGH - ALL MODELS

Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). If MAP sensor fails, control module will substitute a fixed MAP value and use TPS input to control fuel delivery.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test confirms Code 33 and determines if it is a hard failure or an intermittent condition. Code 33 will set when voltage signal reading is too high and TPS voltage indicates throttle is closed.
2. This step simulates conditions for a Code 34. If control module recognizes and indicates low MAP signal, control module and 5-volt reference and MAP signal circuits are not at fault.

With ignition switch in ON position and engine off, manifold pressure is equal to atmospheric pressure and signal voltage is high. Comparing BARO readings from a known good vehicle using same sensor is a good way to check accuracy of suspected sensor. Readings should be within .4 volt of each other. Code 33 will also result if ground circuit is open or MAP signal circuit is shorted to voltage or to 5-volt reference circuit.

CODE 33 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 33 Schematic (4.3L) MAP Sensor Signal Voltage High. Scheme 141

Code 33 Flow Chart (4.3L) MAP Sensor Signal Voltage High. Scheme 142

CODE 34, MAP SENSOR SIGNAL VOLTAGE LOW - ALL MODELS

Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). If MAP sensor fails, control module will substitute a fixed MAP value and use TPS input to control fuel delivery.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This confirms Code 34 and determines if code was a hard failure or an intermittent condition. Code 34 will set when ignition is on and MAP signal voltage is low. On some systems, engine must be running to set code.
2. Jumpering harness 5-volt reference circuit and MAP signal circuit terminals will determine if problem is sensor, control module or wiring. If control module recognizes and indicates high MAP signal, control module and wiring are okay.
3. Scan tester may not display 12 volts. The important thing is that control module recognizes voltage as greater than 4 volts (high MAP voltage signal), indicating control module and MAP signal circuit are not at fault.

With ignition switch in ON position and engine off, manifold pressure is equal to atmospheric pressure and signal voltage will be high. Comparing BARO readings with a known good vehicle using the same sensor is a good way to check accuracy of suspected sensor. Readings should be within .4 volt of each other. A Code 34 will also result if 5-volt reference and MAP signal circuits are open or shorted to ground. If 5-volt reference circuit is not shorted to ground and a Code 22 is stored, check MAP signal circuit for short to ground.

CODE 34 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 34 Flow Chart (4.3L) MAP Sensor Signal Voltage Low. Scheme 143

CODE 35, IDLE SPEED ERROR - ALL MODELS

Code 35 will set when closed throttle engine speed is 150 RPM greater or less than correct idle speed for 20 seconds.

Note. Test numbers refer to test numbers on diagnostic chart.

1. IAC driver is used to extend and retract IAC valve. Movement is verified by changing engine speed. If no engine speed change occurs, valve can be retested when removed from throttle body.
2. Step checks IAC circuits. Each light on node light should flash Red and Green, while IAC valve is cycled. While color sequence is not important, if either light is off or does not flash Red and Green, check circuits beginning with poor terminal contacts.

IAC Valve Reset Procedure

Turn ignition off for 10 seconds. Start and run engine for 5 seconds. Turn ignition off another 10 seconds.

A slow, unstable idle may be caused by a system problem that cannot be overcome by IAC. Scan counts will be greater than 60 if too low, and zero counts if too high. If idle is too high, stop engine. With ignition on, ground ALDL test terminal "B". Wait 45 seconds for IAC to seat, then disconnect IAC. Start engine. If idle speed is greater than 800 RPM, inspect vehicle for vacuum leaks.

System Too Lean

If air/fuel ratio is too lean, idle speed may be either too high (check for vacuum leaks) or too low. Engine speed may vary and disconnecting IAC may not help. Scan tester and/or digital voltmeter (10 megohm) will read an oxygen sensor output less than 300 mv (.3 volt). Check for low fuel pressure or water in fuel.

System Too Rich

If air/fuel ratio is too rich, idle speed will be too low and scan tester counts will usually be greater than 80. The system may be obviously rich with Black smoke from tailpipe. Scan tester and/or voltmeter will read an oxygen sensor voltage signal fixed greater than 800 mv (.8 volt). Look for high fuel pressure or leaking/sticky injectors. Remove IAC and inspect bore for foreign material or evidence of IAC valve dragging bore. A silicone-contaminated oxygen sensor will produce lean air/fuel mixture. Oxygen sensor output would be fixed greater than 800 mv (.8 volt). This may also set Code 45.

Throttle Body

Remove IAC and inspect bore for evidence of IAC valve dragging.

IAC Valve Connections

Carefully inspect connections for looseness or corrosion.

PCV Valve

The wrong PCV valve may cause incorrect idle speed.

CODE 35 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 35 Schematic (4.3L) Idle Speed Error. Scheme 144

Code 35 Flow Chart (4.3L) Idle Speed Error. Scheme 145

CODE 36, IDLE SPEED ACTUATOR FAULT - "C", "G", "K" & "P" SERIES A/T

Control module grounds solenoid circuit when coolant temperature is within range to enable solenoid, allowing vacuum to retract solenoid actuator. This lowers idle speed to controlled IAC idle specification.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks if vacuum source is present.
2. Checks to see if actuator is commanded on.
3. Checks for power, ground and proper connections at actuator solenoid.
4. Checks for open ground circuit.
5. Checks for open driver circuit or faulty control module.

If BARO is low, a 96 second reset will result. If Code 36 is set, Code 35 will usually also be set. Repair actuator circuit first. If IAC counts are zero, idle speed control actuator will disengage and raise idle speed.

Code 36 Schematic ("C", "G", "K" & "P" Series A/T) Idle Speed Actuator Fault. Scheme 146

Code 36 Flow Chart ("C", "G", "K" & "P" Series A/T) Idle Speed Actuator Fault. Scheme 147

CODES 37/38, TCC BRAKE SWITCH ON/OFF - ALL MODELS A/T

The normally closed brake switch supplies battery voltage to control module. Signal voltage will drop to zero volts when brake pedal is applied.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks if voltage is available at brake switch.
2. Checks brake switch function.
3. Checks circuit from brake switch to control module.

If code is intermittent, check customer driving habits (driving with foot resting on brake pedal) or for unusual traffic conditions (stop and go expressway traffic).

Codes 37/38 Schematic ("C", "G", "K" & "P" Series) TCC Brake Switch On/Off. Scheme 148

Codes 37/38 Schematic ("L", "M", "S" & "T" Series) TCC Brake Switch On/Off. Scheme 149

Codes 37/38 Flow Chart (All Models) TCC Brake Switch On/Off. Scheme 150

CODE 39, TCC STUCK OFF - "C", "G", "K" & "P" SERIES WITH 4L80-E TRANSMISSION

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks mechanical status of TCC. When control module commands TCC solenoid off, TCC slip speed should increase.

If TCC is mechanically stuck on, vehicle speed is zero MPH, brakes are applied and D2 is selected, TCC fluid will mechanically apply TCC causing an engine stall.

Code 39 Schematic ("C", "G", "K" & "P" Series W/ 4L80-E Trans.) TCC Stuck Off. Scheme 151

Code 39 Flow Chart ("C", "G", "K" & "P" Series W/ 4L80-E Trans.) TCC Stuck Off. Scheme 152

CODE 42, IGNITION CONTROL - ALL MODELS

Code 42 indicates control module has seen an open or short to ground in Ignition Control (IC) or by-pass circuits.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test confirms Code 42 and determines if fault is a hard failure or intermittent condition.
2. This tests for a normal IC ground path through ignition module. If circuit is shorted to ground, reading will be less than 500 ohms.
3. As test light voltage touches by-pass circuit, module should switch. This causes ohmmeter to "over-range" with meter in 100-200 ohm range. A higher ohm range will indicate over 5000 ohms. This test assures module switched.
4. If module did not switch, this step tests for a short in IC circuit, an open in by-pass circuit and a faulty ignition module connection or module.
5. This step confirms Code 42 is a faulty control module and not an intermittent problem in IC and by-pass circuits.

The scan tester cannot help diagnose a Code 42 problem. See INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

Code 42 Schematic ("C", "K" & "P" Series) Ignition Control. Scheme 153

Code 42 Schematic ("G", "L" & "M", "S" & "T" Series) Ignition Control. Scheme 154

Code 42 Flow Chart (All Models) Ignition Control. Scheme 155

CODE 43, KNOCK SENSOR W/O SPARK MODULE (DUAL SENSORS)

Note. Test numbers refer to test numbers on diagnostic chart.

1. Tests for 5-volt signal applied by control module.
2. An improperly installed sensor can prevent knock sensor from grounding to block.

Control module applies and monitors a 5-volt DC signal to knock sensors. Internal knock sensor circuitry pulls this DC signal down to about 2.5 volts. When knock sensor detects detonation, it generates an AC signal which rides back on DC signal to control module. Knock signal intensity is dependent upon knock signal level.

Code 43 Schematic (4.3L) Knock Sensor Without Spark Module (Dual Sensors). Scheme 156

Code 43 Flow Chart (4.3L) Knock Sensor Without Spark Module (Dual Sensors). Scheme 157

CODE 43, KNOCK SENSOR WITHOUT SPARK MODULE (SINGLE SENSOR)

Note. Test numbers refer to test numbers on diagnostic chart.

1. Code 43 will set when vehicle reaches normal operating temperature (but not overheating), high engine load is indicated by MAP sensor and voltage on sensor signal circuit is greater than 3.5 volts DC or less than 1.5 volts DC. This step determines if system is functioning properly at current time.
2. This step determines state of 5-volt reference signal applied to sensor.
3. Checks knock sensor internal resistance.

Control module applies and monitors a 5-volt DC signal to knock sensor. Internal knock sensor circuitry pulls this DC signal down to about 2.5 volts. When knock sensor detects detonation, it generates an AC signal which rides back on DC signal to control module. Knock signal intensity is dependent upon knock signal level.

Code 43 Schematic (4.3L) Knock Sensor Without Spark Module (Single Sensor). Scheme 158

Code 43 Flow Chart (4.3L) Knock Sensor Without Spark Module (Single Sensor). Scheme 159

CODE 43, KNOCK SENSOR WITH SPARK CONTROL MODULE "C", "K" & "P" SERIES

Note. Test numbers refer to test numbers on diagnostic chart.

1. If conditions for a Code 43 exist, scan tester will display YES. A knock signal should exist at idle unless an internal or system problem exists.
2. Determines if system is functioning. Usually, a knock signal can be made by tapping on exhaust manifold. If knock signal is not made, try tapping on engine block near sensor. On models with automatic transmission, it may be necessary to place gear selector lever in Drive.
3. Because Code 43 sets when signal voltage on spark retard line remains low, this test should cause signal on that line to go high. The 12-volt signal should be seen by control module as a "no knock" signal if control module and wiring are okay.
4. This test determines if knock signal is detected on sensor-to-controller line or if ESC module is at fault.
5. If sensor line is routed too close to secondary ignition wires, ESC module may see interference as a knock signal.
6. This checks ground circuit to module. An open ground will cause voltage on monitored line to remain constant at about 12 volts. This would cause Code 43 functional test to fail.
7. This should generate a knock signal to controller. This determines if ESC controller is operating correctly.

Code 43 can be caused by a faulty knock sensor connection at ESC module or engine control module. Also, check controller-to-control module signal line for an open or short to ground.

Code 43 Schematic ("C" & "K" Series) Knock Sensor With Spark Control Module. Scheme 160

Code 43 Schematic ("P" Series) Knock Sensor With Spark Control Module. Scheme 161

Code 43 Flow Chart (All Models) Knock Sensor With Spark Control Module. Scheme 162

CODE 44, LEAN EXHAUST INDICATION - ALL MODELS

Sensor acts like an open sensor circuit and produces no voltage when exhaust temperature is less than 600°F (316°C). An open sensor circuit or cold sensor causes "open loop" operation.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Code 44 sets when O2 sensor signal remains low for a precalibrated period and system is operating in "closed loop".

Using scan tester, observe Block Learn Memory (BLM) value at different RPMs. If Code 44 conditions exist, block learn value will be around 150-172.

O2 Sensor Wire

Wire may be mispositioned and touching exhaust manifold. Check for ground between sensor and wire connector.

Fuel Contamination

Water, even small amounts, near in-tank fuel pump inlet can reach fuel injector, causing a lean exhaust and setting Code 44.

Fuel Pressure

System will be lean if fuel pressure is low. It may be necessary to monitor fuel pressure while driving vehicle. For fuel pressure checking procedure, see BASIC TESTING - 4.3L article.

Exhaust Leaks

If exhaust system has large leaks, exhaust system negative pressure pulses can cause outside air to be drawn into system and past O2 sensor. Vacuum or crankcase leaks can also cause a lean condition. If Code 44 is intermittent, see INTERMITTENTS in the TESTS W/O CODES - GASOLINE article.

CODE 44 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 44 Flow Chart (4.3L) Lean Exhaust Indication. Scheme 163

CODE 45, RICH EXHAUST INDICATION - ALL MODELS

Sensor acts like an open sensor circuit and produces no voltage when exhaust temperature is less than 600°F (316°C). An open sensor circuit or cold sensor causes "open loop" operation. Code 45 indicates a rich exhaust and diagnosis should begin with: fuel pressure, leaking injector, HEI shielding, canister purge saturation, coolant sensor, MAP sensor, O2 sensor contamination and TPS intermittent output.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Tests if O2 sensor is registering a rich condition. Code 45 is set when vehicle is at operating temperature (in "closed loop"), throttle angle is greater than 5 percent, O2 sensor signal at control module is greater than .75 volt for 60 seconds or more.

Code 45, rich exhaust, is most likely caused by one of the following

Fuel Pressure High

If fuel pressure is too high, air/fuel ratio will be rich. For fuel pressure checking procedure, see BASIC TESTING - 4.3L article. The control module can compensate for slight increases but if air/fuel ratio becomes too rich a Code 45 will be set.

Ignition Ground

If an open occurs at circuit No. 453, HEI induced electrical "noise" may result, causing simulated reference pulses picked up by control module on IC harness reference line. Additional pulses result in a higher than actual engine speed signal. The control module will increase injector pulse width ("on" time) to match increased RPM signal. Scan tester will show higher than actual RPM, which can help diagnose problem.

Fuel Canister

Charcoal canister fuel saturation will cause a rich air/fuel ratio. If full of fuel, check canister control and hoses.

MAP Sensor

If control module senses higher than normal manifold pressure (low vacuum) system can go rich. Disconnecting MAP sensor allows control module to substitute a fixed value for MAP sensor. If rich condition disappears, replace MAP sensor and continue testing.

TPS

An intermittent TPS output will cause system to operate rich due to a false indication of engine acceleration.

O2 Sensor Contamination

O2 sensor contamination, caused by silicone in certain fuels or use of improper RTV sealant, may cause a White-powdery coating to cover O2 sensor. The false high signal voltage produced (or low oxygen content sensed) is interpreted by control module as a rich mixture, causing control module to set Code 45.

EGR Problem

EGR valve sticking open at idle is usually accompanied by a rough idle and/or stalling. If Code 45 is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

CODE 45 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 45 Flow Chart (4.3L) Rich Exhaust Indication. Scheme 164

CODE 51, PROM ERROR (FAULTY OR INCORRECT PROM)

Ensure all pins are fully inserted in socket. If okay, replace PROM/MEM-CAL, clear memory and recheck. If Code 51 reappears, replace control module.

CODE 52, FAULTY CALPAK

Ensure all pins are fully inserted in socket. If okay, replace CALPAK, clear memory and recheck. If Code 51 reappears, replace control module.

CODE 52/53, SYSTEM VOLTAGE HIGH - ALL MODELS

Code 53 will set when ignition is on and control module 12-volt battery feed voltage is greater than 19.5 volts for about 2 seconds. During the time failure is present, force motor is turned off, transmission immediately shifts to 2nd gear, and TCC operation is inhibited. The setting of additional codes may result.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Normal voltage is 9-15 volts.
2. Checks if alternator is faulty under load.

Jump-starting engine or charging battery with a battery charger may set code. If code is set when an accessory is operated, check for poor connections or excessive current draw. Also, check for poor connections at starter solenoid or fusible link.

Code 52/53 Schematic ("C","K" & "P" Series) System Voltage High. Scheme 165

Code 52/53 Schematic ("G", "L" &"M" Series) System Voltage High. Scheme 166

Code 52/53 Schematic ("S" & "T" Series) System Voltage High. Scheme 167

Code 52/53 Flow Chart (All Models) System Voltage High. Scheme 168

CODE 54, FUEL PUMP CIRCUIT - ALL MODELS

The status of fuel pump signal is monitored by control module and is used to compensate fuel delivery based on system voltage. Signal is also used to store Code 54 if fuel pump relay is defective or if relay voltage is lost after engine is running. Voltage should be present at fuel pump signal terminal of control module the first 2 seconds after ignition is turned on and anytime reference (RPM) pulses are being received by control module.

Code 54 Schematic ("C" & "K" Series) Fuel Pump Circuit. Scheme 169

Code 54 Schematic ("G" Series) Fuel Pump Circuit. Scheme 170

Code 54 Schematic ("L" & "M" Series) Fuel Pump Circuit. Scheme 171

Code 54 Schematic ("P" Series) Fuel Pump Circuit. Scheme 172

Code 54 Schematic ("S" & "T" Series) Fuel Pump Circuit. Scheme 173

Code 54 Flow Chart (All Models) Fuel Pump Circuit. Scheme 174

CODE 55, ECM/PCM ERROR

Ensure control module grounds are good and MEM-CAL is properly latched. If okay, replace control module. Clear codes and confirm closed loop operation. Check operation of SERVICE ENGINE SOON light.

CODE 58, TRANSMISSION FLUID TEMPERATURE HIGH - ALL MODELS

Transmission fluid temperature sensor is a thermistor which controls signal voltage to control module. Control module applies and monitors voltage to sensor. When transmission fluid is cold, sensor resistance is high; therefore, control module will see high signal voltage. As transmission fluid warms, sensor resistance and voltage will drop. At normal transmission operating temperature of 212°F (100°C), voltage will be about 1.5-2.0 volts.

Note. Test numbers refer to numbers on diagnostic chart.

1. Code 58 will set if signal voltage indicates a transmission fluid temperature greater than 305°F (151°C) for one second.
2. This test determines if circuit is shorted to ground, which will result in conditions for Code 58.

Check harness routing for a potential short to ground in TFT signal circuit. Scan tester displays transmission fluid temperature in degrees Centigrade. After transmission is running, temperature display should rise steadily to about 100°C then stabilize. Test transmission sensor at various temperature levels to determine if sensor is out of calibration. See TRANSMISSION SENSOR - TEMP TO RESISTANCE chart. An out-of-calibration sensor could result in delayed shifts or TCC enabled complaint.

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

Code 58 Schematic ("C", "K" & "P" Series) Transmission Fluid Temperature High. Scheme 175

Code 58 Schematic ("G", "L", "M", "S" & "T" Series) Transmission Fluid Temperature High. Scheme 176

Code 58 Flow Chart (All Models) Transmission Fluid Temperature High. Scheme 177

CODE 59, TRANSMISSION FLUID TEMPERATURE LOW - ALL MODELS

Transmission fluid temperature sensor is a thermistor which controls signal voltage to control module. Control module applies and monitors 5 volts to sensor. When transmission fluid is cold, sensor resistance is high; therefore, control module will see high signal voltage. As transmission fluid temperature warms, sensor resistance and voltage drop. At normal transmission operating temperature of 212°F (100°C), voltage will be about 1.5-2.0 volts.

Note. Test numbers refer to numbers on diagnostic chart.

1. Code 59 will set if signal voltage indicates a transmission fluid temperature less than -34°F (-37°C) for one second.
2. This test simulates Code 58. If control module recognizes low signal voltage (high temperature) and scan tester reads 305°F (151°C) or greater, control module and wiring are okay.
3. This test determines if signal circuit is open. There should be 5 volts present at sensor connector if measuring with a DVOM.

Scan tester displays transmission fluid temperature in degrees Centigrade. After transmission is running, displayed temperature should rise steadily to about 100°C then stabilize. A faulty connection or an open in ground circuit or signal circuit will result in a Code 59. Test transmission sensor at various temperature levels to determine if sensor is out of calibration. See TRANSMISSION SENSOR - TEMP TO RESISTANCE chart. An out-of-calibration sensor could result in firm shifts or TCC enabled complaint. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

Code 59 Flow Chart (All Models) Transmission Fluid Temperature Low. Scheme 178

CODE 66, 3-2 CONTROL SOLENOID CKT FAULT 4L60-E TRANSMISSION

The 3-2 control solenoid hydraulically coordinates apply rate of 2-4 band with hydraulic release of 3-4 clutch during a 3-2 downshift. The 3-2 circuit duty cycle is continually monitored by control module depending one command state of circuit. When transmission is in 1st gear, duty cycle of solenoid is equal to zero. When transmission is in 2nd gear or higher, duty cycle of solenoid will be about 90 percent. When transmission downshifts 3-2, duty cycle of solenoid will be about 20 percent.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks 3-2 control solenoid and internal transmission harness for short circuits.
2. This test checks for power, from ignition through fuse, to 3-2 control solenoid.

Check all connections, especially those at transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in appropriate TESTS W/O CODES - GASOLINE article. Some slight TCC slippage is normal. The 3-2 control solenoid feedback normally oscillates on/off when duty cycle is applied.

Code 66 Schematic (4.3L) 3-2 Control Solenoid Circuit Fault 4L60-E Transmission. Scheme 179

Code 66 Flow Chart (4.3L) 3-2 Control Solenoid Circuit Fault 4L60-E Transmission. Scheme 180

CODE 67, TCC SOLENOID CIRCUIT CHECK - 4L60-E TRANSMISSION

TCC solenoid is a normally open exhaust valve. Control module will engage solenoid by grounding circuit with an internal quad-driver.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks ability of control module to control solenoid.
2. This test checks for power, from ignition through fuse, to TCC solenoid.

Check all connections, especially those at transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article. Some slight TCC slippage is normal.

Code 67 Schematic (4.3L) TCC Solenoid Circuit Check - 4L60-E Transmission. Scheme 181

Code 67 Flow Chart (4.3L) TCC Solenoid Circuit Check - 4L60-E Transmission. Scheme 182

CODE 68, OVERDRIVE RATIO ERROR - 4L80-E TRANSMISSION

Control module monitors the difference in engine RPM and input shaft RPM. With transmission in Drive, scan tester reading should show engine speed closely matching input speed.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks indicated range signal to selected range. A faulty switch could set this code.
2. Checks TCC for slippage while in a commanded lock-up state.

Check for spread connectors at pass-thru connector. Code 68 will set when going to default (2nd gear). If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

Code 68 Flow Chart (4.3L) Overdrive Ratio Error - 4L80-E Transmission. Scheme 183

CODE 69, TORQUE CONVERTER CLUTCH STUCK ON 4L60-E TRANSMISSION

Control module commands TCC PWM solenoid on by modulating signal fluid on converter clutch shift valve. TCC apply fluid applies torque converter clutch.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks mechanical state of TCC. When TCC is commanded off, slip speed should increase.

If TCC is mechanically stuck on, TCC fluid will apply TCC, causing an engine stall.

Code 69 Schematic (4.3L) Torque Converter Clutch Stuck On 4L60-E Transmission. Scheme 184

Code 69 Flow Chart (4.3L) Torque Converter Clutch Stuck On 4L60-E Transmission. Scheme 185

CODE 69, TORQUE CONVERTER CLUTCH STUCK ON 4L80-E TRANSMISSION

Control module commands TCC PWM solenoid on by modulating signal fluid on converter clutch shift valve. TCC apply fluid applies torque converter clutch.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks mechanical state of TCC. When TCC is commanded off, slip speed should increase.

If TCC is mechanically stuck on, TCC fluid will apply TCC, causing an engine stall.

Code 69 Flow Chart (4.3L) Torque Converter Clutch Stuck On 4L80-E Transmission. Scheme 186

CODE 72, VSS CIRCUIT LOSS TRANS OUTPUT SIGNAL - ALL MODELS

VSS output sensor is a magnetic induction type. Gear teeth pressed on outside diameter of output carrier assembly induce an alternating current in sensor when drive wheels are turning.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Test verifies VSS voltage at control module.
2. Test checks VSS buffer ground circuit.
3. Test checks VSS circuit at buffer module.
4. Test verifies VSS signal at sensor.

Code 72 will set when VSS signal is present and is lost. Code 24 will set when no vehicle speed is detected at vehicle start off. Check all connections, especially those at transmission pass-through connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

Code 72 Schematic ("C", "G", "K", "L", "M" & "P" Series) VSS Circuit Loss Transmission Output Signal. Scheme 187

Code 72 Schematic ("S" & "T" Series) VSS Circuit Loss Transmission Output Signal. Scheme 188

Code 72 Flow Chart (All Models) VSS Circuit Loss Transmission Output Signal. Scheme 189

CODE 73, PRESSURE CONTROL SOLENOID (CURRENT ERROR) - ALL MODELS

Note. This flow chart requires the use of a bidirectional (Tech 1) scan tester.

Pressure control solenoid is controlled by control module to regulate transmission line pressure. Control module looks at TPS voltage, engine RPM and other inputs to determine appropriate line pressure for a given load, then regulates pressure by applying a varying amperage. Applied amperage can vary from 1 to 1.1 amps. Control module then monitors amperage at return line.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks ability of control module to command solenoid.
2. Checks internal transmission harness and solenoid for incorrect resistance.

Check for poor connections at control module and transmission pass-thru connector.

Code 73 Schematic (4.3L) Pressure Control Solenoid (Current Error). Scheme 190

Code 73 Flow Chart (4.3L) Pressure Control Solenoid (Current Error). Scheme 191

CODE 74, TRANSMISSION INPUT SPEED ERROR 4L80-E TRANSMISSION

Sensor is a permanent magnet type. When rotating trigger teeth pass sensor, an AC voltage signal is produced. Signal voltage and frequency varies with forward rotational speed.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks entire circuit for continuity.
2. Checks output of input speed sensor.

Check for poor connections in related circuits.

Code 74 Schematic (4.3L) Transmission Input Speed Error 4L80-E Transmission. Scheme 192

Code 74 Flow Chart (4.3L) Transmission Input Speed Error 4L80-E Transmission. Scheme 193

CODE 75, SYSTEM VOLTAGE LOW - ALL MODELS

Code 75 will set when ignition is on and control module 12-volt battery feed voltage is less than 9 volts for about 4 seconds. During time failure is present, force motor, also referred to as pressure control solenoid, is turned off, maintaining only 2nd gear and inhibiting TCC operation.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks for normal battery voltage.
2. Checks if low voltage display is due to generator, battery voltage input circuit or control module.

Charging battery with a charger and jump starting engine may set this code. If code sets when an accessory is operated, check for poor connections or excessive current draw. Also, check for poor connections at starter solenoid or fusible link. Minimum voltage allowed for Code 75 to set is on a graduated scale and changes with temperature from a low of 7.3 volts at -40°F (-40°C) to a voltage of 11.7 volts at 304°F (150°C).

Code 75 Schematic (4.3L) System Voltage Low. Scheme 194

Code 75 Flow Chart (4.3L) System Voltage Low. Scheme 195

CODE 79, TRANSMISSION FLUID TEMPERATURE HIGH - ALL MODELS

Transmission fluid temperature sensor is a thermistor which controls signal voltage to control module. Control module applies and monitors voltage to sensor. When transmission fluid is cold, sensor resistance is high; therefore, control module will see high signal voltage. As transmission fluid warms, sensor resistance and voltage will drop. At normal transmission operating temperature of 212°F (100°C), voltage will be about 1.5-2.0 volts.

Note. Test numbers refer to numbers on diagnostic chart.

1. Code 79 will set if signal voltage indicates a transmission fluid temperature greater than 151°C for one second.
2. This test determines if circuit is shorted to ground, which will result in conditions for Code 79.

Check harness routing for a potential short to ground in signal circuit. Scan tester displays transmission fluid temperature in degrees Centigrade. After transmission is running, temperature display should rise steadily to about 100°C then stabilize. Test transmission sensor at various temperature levels to determine if sensor is out of calibration. See TRANSMISSION SENSOR - TEMP TO RESISTANCE chart. An out-of-calibration sensor could result in delayed shifts or TCC enabled complaint.

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

Code 79 Flow Chart (All Models) Transmission Fluid Temperature High. Scheme 196

CODE 81, TRANSMISSION 2-3 ERROR - ALL MODELS

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks function of 2-3 shift solenoid and internal transmission wiring.
2. Checks for power to 2-3 shift solenoid from ignition.

Check all connections, especially those at transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

Code 81 Schematic (4.3L) Transmission 2-3 Error. Scheme 197

Code 81 Flow Chart (4.3L) Transmission 2-3 Error. Scheme 198

CODE 82, TRANSMISSION 1-2 ERROR - ALL MODELS

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks 1-2 shift solenoid and internal transmission wiring harness for short circuits.
2. This test checks for power, from ignition through fuse, to shift solenoid.

Check all connections, especially at transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

Code 82 Schematic (4.3L) Transmission 1-2 Error. Scheme 199

Code 82 Flow Chart (4.3L) Transmission 1-2 Error. Scheme 200

CODE 83, TCC SOLENOID CIRCUIT FAULT - ALL MODELS

Control module continually monitors voltage on each circuit connected to quad-driver for either low or high voltage, depending on commanded state of device connected to it. Code 83 will set if control module detects an inappropriate reading on TCC circuit. For example, if TCC duty cycle is zero, but voltage on TCC circuit drops as if solenoid were on, then Code 83 will set. TCC solenoid, because of its large current draw, is connected to 2 terminals of a single quad-driver.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks if control module is commanding TCC solenoid on.
2. This test checks for voltage to solenoid.

Check all connections, especially those at transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

Code 83 Schematic (4.3L) TCC Solenoid Circuit Fault. Scheme 201

CODE 85, UNDEFINED GEAR RATIO - ALL MODELS

While in each gear, control module calculates actual gear ratio from input and output speed readings, also referred to as vehicle speed, then compares these to what gear ratio should be, taking into consideration selected gear range. This monitor includes reverse gear, but does not include overdrive gear.

Note. Test numbers refer to numbers on diagnostic chart.

1. An out-of-calibration transmission range pressure switch could falsely set Code 85.
2. This test verifies proper ratio.

Code will set when an unknown gear ratio is detected for any gear but 4th. Check all connections, especially those at transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article. Compare scan tester gear ratio reading to specifications in GEAR RATIO table.

GEAR RATIO

Code 85 Schematic (4.3L) Undefined Gear Ratio. Scheme 202

Code 85 Flow Chart (4.3L) Undefined Gear Ratio. Scheme 203

CODE 86, LOW GEAR RATIO ERROR - ALL MODELS

Control module calculates ratio based on transmission input speed and output speed sensor readings. Control module compares known ratio to calculated ratio.

Note. Test numbers refer to numbers on diagnostic chart.

1. An out-of-calibration transmission range pressure switch could falsely indicate actual transmission range.
2. This test verifies proper ratio.

If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

Code 86 Flow Chart (4.3L) Low Gear Ratio Error. Scheme 204

CODE 87, HIGH GEAR RATIO ERROR - ALL MODELS

Control module calculates ratio based on transmission input speed and output speed sensor readings. Control module compares known ratio to calculated ratio.

1. An out-of-calibration transmission range pressure switch could falsely indicate actual transmission range.
2. This test verifies proper ratio.

Check all connections, especially those at transmission pass-thru connector. Fault may be an internal transmission problem.

Code 87 Flow Chart (4.3L) High Gear Ratio Error. Scheme 205

CODE P0106, MAP SENSOR RANGE PROBLEM - "S" & "T" SERIES 4.3L (VIN W)

Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). Control module receives this information as a signal voltage that varies from 1-1.5 volts at idle to 4-4.6 volts at full throttle. Scan tool displays manifold pressure in volts. Low pressure (high vacuum) reads a low voltage, while high pressure (low vacuum) reads a high voltage. If MAP sensor fails, control module will substitute a fixed MAP value and use throttle position sensor to control fuel delivery.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks MAP sensor output voltage to control module. This voltage represents a barometric reading to control module.
2. Applying 10 inches of vacuum to sensor should cause voltage to change. The value obtained by subtracting second reading from first reading should be greater than 1.5 volts. Change should be instantaneous.
3. Remove sensor from bracket and gently twist sensor to check for intermittents. Voltage changes of greater than 0.1 volt indicate a bad connector or connection.

Check MAP sensor vacuum source for leaks or restriction. Ensure MAP sensor mounting screws are tight.

Code P0106 Schematic ("S" & "T" Series 4.3L - VIN W) MAP Sensor Range Problem. Scheme 206

Code P0106 Flow Chart ("S" & "T" Series 4.3L - VIN W) MAP Sensor Range Problem. Scheme 207

CODE P0107, MAP SENSOR CKT - "S" & "T" SERIES 4.3L (VIN W)

Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). Control module receives this information as a signal voltage that varies from 1-1.5 volts at idle to 4-4.6 volts at full throttle. Scan tool displays manifold pressure in volts. Low pressure (high vacuum) reads a low voltage, while high pressure (low vacuum) reads a high voltage. If MAP sensor fails, control module will substitute a fixed MAP value and use throttle position sensor to control fuel delivery.

Note. Test numbers refer to numbers on diagnostic chart.

1. Determines if code is set because of a hard failure or intermittent condition.
2. Jumping circuits determines if sensor is at fault or if problem exists in control module or wiring.
3. Tech 1 may not display 5 volts. The important item is that control module recognized voltage as more than 4 volts, indicating that control module and circuit are okay.

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside insulation. Check for

Intermittents

An intermittent or open in Gray or Light Green wire circuits

will set Code P0107.

Sensor Accuracy

With ignition on and engine off, pressure signal is equal to atmospheric pressure with signal high. This information is used by control module as an indication of altitude. Comparison of this reading with a known good vehicle with same sensor is a way to check accuracy of suspect sensor. Reading should be the same within 0.4 volt.

1. Disconnect sensor from bracket and twist sensor by hand to check for intermittents. Output changes greater than 0.1 volt indicate a bad sensor or connection.

Code P0107 Flow Chart ("S" & "T" Series 4.3L - VIN W) MAP Sensor Circuit. Scheme 208

CODE P0108, MAP SENSOR CKT - "S" & "T" SERIES 4.3L (VIN W)

Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). Control module receives this information as a signal voltage that varies from 1-1.5 volts at idle to 4-4.6 volts at full throttle. If MAP sensor fails, control module will substitute a fixed MAP value and use throttle position sensor to control fuel delivery.

Note. Test numbers refer to numbers on diagnostic chart.

1. Rough, unstable or incorrect idle and low manifold vacuum must be corrected before using chart.
2. This simulates conditions for Code P0108. If control module recognizes change, control module and circuits are okay.

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside insulation. Check for

An intermittent or open in wire circuits will set Code P0108.

With ignition on and engine off, pressure signal is equal to atmospheric pressure with signal high. This information is used by control module as an indication of altitude. Comparison of this reading with a known good vehicle with same sensor is a way to check accuracy of suspect sensor. Reading should be the same within 0.4 volt.

1. Disconnect sensor from bracket and twist sensor by hand to check for intermittents. Output changes greater than 0.1 volt indicate a bad sensor or connection.

CODE P0112, IAT SENSOR CIRCUIT LOW INPUT - "S" & "T" SERIES 4.3L (VIN W)

Intake Air Temperature (IAT) sensor is a thermistor. Control module applies and monitors 5 volts to sensor. When air is cold, sensor resistance is high and control module will measure a high signal voltage. If air is warm, sensor resistance is low causing control module to measure low voltage.

With Code P0112 set, control module will substitute a default value for intake air temperature. Code P0112 does not illuminate Malfunction Indicator Light (MIL).

Note. Test numbers refer to numbers on diagnostic chart.

1. An open sensor, wire or connection will cause Code P0112 to set. An open circuit would cause Tech 1 to display about -40°F (-40°C). This test determines if wiring and control module are okay.

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside insulation. Check for

Poor Connection Or Damaged Harness

Inspect control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.

Intermittent Test

Observe IAT on Tech 1 while moving related connectors and wiring harness with warm engine running. If failure is induced, IAT display will change to a -40° temperature reading. This may assist in isolating location of malfunction.

Code P0112 Schematic ("S" & "T" Series 4.3L - VIN W) IAT Sensor Circuit Low Input. Scheme 209

CODE P0113, IAT SENSOR CIRCUIT HIGH INPUT - "S" & "T" SERIES 4.3L (VIN W)

Intake Air Temperature (IAT) sensor is a thermistor. Control module applies and monitors 5 volts on circuit to sensor. When air is cold, sensor resistance is high and control module will measure a high signal voltage. If air is warm, sensor resistance is low causing control module to measure low voltage. With Code P0113 set, control module will use a default value of about 63°F (17°C) for intake air temperature. Code P0113 does not illuminate Malfunction Indicator Light (MIL).

Note. Test numbers refer to numbers on diagnostic chart.

1. Tech 1 may be used to diagnose this fault since control module transmits actual values when fault is present. A grounded circuit will cause Tech 1 to display a temperature of about 297°F (147°C).
2. If Tech 1 displays -40° with IAT sensor disconnected, control module and wiring are okay. Replace IAT sensor.

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside insulation. Check for

1. Poor Connection Or Damaged Harness Inspect control module harness for short to ground in circuit, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
2. Intermittent Test Observe IAT on Tech 1 while moving related connectors and wiring harness with warm engine running. If failure is induced, IAT display will change to a 297°F (147°C) temperature reading. This may assist in isolating location of malfunction.

CODE P0117, COOLANT TEMPERATURE SENSOR TEMPERATURE LOW "S" & "T" SERIES 4.3L (VIN W)

Coolant sensor uses a thermistor to control signal voltage to control module. Control module applies and monitors a 5 volts to sensor. When engine coolant is cold, sensor (thermistor) resistance is high and control module will sense a high signal voltage. As engine coolant warms, sensor resistance becomes less and control module voltage drops. With Code P0117 set, control module will turn cooling fans on and use a default engine coolant temperature value based on run time. Control module will illuminate Malfunction Indicator Light (MIL).

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if conditions necessary to set Code P0117 exist.
2. Simulates Code P0118. If control module recognizes high signal voltage, control module and wiring are okay.
3. Determines if 5-volt reference is open. This determines if there is a wiring problem or faulty control module.

Tech 1 displays engine temperature in degrees celsius (°C) and fahrenheit (°F). After engine is started, temperature should rise steadily to 194°F (90°C), then stabilize when thermostat opens. An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside insulation. Check for

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
2. Intermittent Test Using Tech 1, monitor engine coolant temperature while moving related connectors and wiring harness. If failure is induced, display will change. This may assist in isolating location of malfunction.
3. Shifted Sensor See TEMPERATURE-TO-RESISTANCE VALUES table to test ECT sensor at various temperature levels to evaluate the possibility of a shifted (mis-scaled) sensor which may result in driveability complaints.
4. A faulty connection, or an open in sensor circuits will cause Code P0117 to set.

TEMPERATURE-TO-RESISTANCE VALUES (1)

Code P0117 Flow Chart (4.3L "S" & "T" Series - VIN W) Coolant Temperature Sensor Temperature Low. Scheme 210

CODE P0118, COOLANT TEMPERATURE SENSOR TEMPERATURE HIGH "S" & "T" SERIES 4.3L (VIN W)

Coolant sensor uses a thermistor to control signal voltage to control module. Control module applies and monitors a 5 volts to sensor. When engine coolant is cold, sensor (thermistor) resistance is high and control module will sense a high signal voltage. As engine coolant warms, sensor resistance becomes less and control module voltage drops. With Code P0118 set, control module will turn cooling fans on and use a default engine coolant temperature value based on run time. Control module will illuminate Malfunction Indicator Light (MIL).

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if conditions necessary to set code exist.
2. Simulates Code P0117. If control module recognizes low voltage, control module and wiring are okay.

Tech 1 displays engine temperature in degrees celsius (°C) and fahrenheit (°F). After engine is started, temperature should rise steadily to 194°F (90°C), then stabilize when thermostat opens. An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside insulation. Check the following for

1. A short to ground in 5-volt reference circuit.
2. Intermittent Test

Using Tech 1, monitor engine coolant temperature while moving related connectors and wiring harness. If failure is induced, display will change. This may assist in isolating location of malfunction.

1. Shifted Sensor See TEMPERATURE-TO-RESISTANCE VALUES table to test ECT sensor at various temperature levels to evaluate the possibility of a shifted (mis-scaled) sensor which may result in driveability complaints.

TEMPERATURE-TO-RESISTANCE VALUES (1)

Code P0118 Schematic ("S" & "T" Series 4.3L - VIN W) Coolant Temperature Sensor Temperature High. Scheme 211

Code P0118 Flow Chart ("S" & "T" Series 4.3L - VIN W) Coolant Temperature Sensor Temperature High. Scheme 212

CODE P0121, TP SENSOR RANGE/PERFORMANCE PROBLEM "S" & "T" SERIES 4.3L (VIN W)

Throttle Position (TP) sensor provides a voltage signal that changes relative to throttle blade angle. Signal voltage will vary from .6 volt at idle to about 4.8 volts at Wide Open Throttle (WOT). TP sensor signal is one of the most important inputs used by control module for fuel control and for most other control module control outputs.

Note. Test numbers refer to numbers on diagnostic chart.

1. TP sensor has a self-zeroing feature. If voltage is about .2-.9 volt, control module will use that value at closed throttle. If reading is out or range at idle, check for binding throttle linkage.
2. With TP sensor unplugged, voltage should go low if control module and wiring are okay.

Tech 1 displays throttle position in volts. Voltage should increase steadily as throttle is moved toward WOT.

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
2. Intermittent Test Monitor TP sensor voltage display on Tech 1 while moving related connectors and wiring harness. If failure is induced, display will change. This may assist in isolating location of malfunction.
3. TP Sensor Scaling Observe TP sensor voltage display while depressing accelerator pedal with engine stopped and ignition on. Display should vary from closed throttle TP sensor voltage when throttle is closed (less than 1.25 volts) to greater than 4.5 volts when throttle is held at WOT position.

Code P0121 Schematic ("S" & "T" Series 4.3L - VIN W) Throttle Position Sensor Range/Performance Problem. Scheme 213

Code P0121 Flow Chart ("S" & "T" Series 4.3L - VIN W) Throttle Position Sensor Range/Performance Problem. Scheme 214

CODE P0122, THROTTLE POSITION SENSOR SIGNAL LOW "S" & "T" SERIES 4.3L (VIN W)

Throttle Position (TP) sensor provides a voltage signal that changes relative to throttle blade angle. Signal voltage will vary from .20-.74 volt at idle to greater than 4 volts at Wide Open Throttle (WOT). TP sensor signal is one of the most important inputs used by control module for fuel control and for most other control module control outputs.

Note. Test numbers refer to numbers on diagnostic chart.

1. Determines if conditions necessary to set code exist.
2. Simulates Code P0123 (high voltage). If control module recognizes high signal voltage, control module and wiring are okay.
3. Simulates a high signal voltage. Checks signal circuit for an open.

Tech 1 displays throttle position in volts. Voltage should increase steadily as throttle is moved toward WOT. An open or short to ground in reference and signal circuits will cause code to set. Check for

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
2. Intermittent Test Monitor TP sensor voltage display on Tech 1 while moving related connectors and wiring harness. If failure is induced, display will change. This may assist in isolating location of malfunction.
3. TP Sensor Scaling Observe TP sensor voltage display while depressing accelerator pedal with engine stopped and ignition on. Display should vary from closed throttle TP sensor voltage when throttle is closed (less than 1.25 volts) to greater than 4.5 volts when throttle is held at WOT position.

CODE P0123, THROTTLE POSITION SENSOR SIGNAL HIGH "S" & "T" SERIES 4.3L (VIN W)

Throttle Position (TP) sensor provides a voltage signal that changes relative to throttle blade angle. Signal voltage will vary from about .5 volt or at idle to about 4 volts at Wide Open Throttle (WOT). TP sensor signal is one of the most important inputs used by control module for fuel control and for most other control module control outputs. On non-adjustable TPS sensors, each time voltage drops below 1.25 volts and stops, control module assumes this value as 0 throttle angle and measures percent throttle from this point on. With Code P0123 set, throttle position will default to 19 percent and Tech 1 will not indicate fault value. Control module will illuminate Malfunction Indicator Light (MIL).

Note. Test numbers refer to numbers on diagnostic chart.

1. If TP signal is greater than 4.8 volts, fault is present.
2. With TP sensor disconnected, TP sensor voltage should decrease. This test verifies that control module and wiring are okay.
3. Probing ground circuit with a test light checks sensor ground circuit. This isolates a faulty sensor, control module or open ground circuit.

Tech 1 displays throttle position in volts. With closed throttle, ignition on, or at idle, voltage should be .45-.85 volt. If voltage is not as specified, replace TP sensor. Check for

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
2. Intermittent Test Monitor TP sensor voltage display on Tech 1 while moving related connectors and wiring harness. If failure is induced, display will change. This may assist in isolating location of malfunction.
3. TP Sensor Scaling Observe TP sensor voltage display while depressing accelerator pedal with engine stopped and ignition on. Display should vary from closed throttle TP sensor voltage when throttle is closed (less than 1.25 volts) to greater than 4.5 volts when throttle is held at WOT position.

Code P0123 Flow Chart (4.3L "S" & "T" Series - VIN W) Throttle Position Sensor Signal High. Scheme 215

CODE P0123, THROTTLE POSITION SENSOR SIGNAL HIGH 4.3L "S" & "T" SERIES (VIN Z)

Throttle Position (TP) sensor provides a voltage signal that changes relative to throttle blade angle. Signal voltage will vary from about .5 volt or at idle to about 4 volts at Wide Open Throttle (WOT). TP sensor signal is one of the most important inputs used by control module for fuel control and for most other control module control outputs. On non-adjustable TPS sensors, each time voltage drops below 1.25 volts and stops, control module assumes this value as 0 throttle angle and measures percent throttle from this point on. With Code P0123 set, throttle position will default to 19 percent and Tech 1 will not indicate fault value. Control module will illuminate Malfunction Indicator Light (MIL).

Note. Test numbers refer to numbers on diagnostic chart.

1. If TP signal is greater than 1.25 volts, fault is present.
2. With TP sensor disconnected, TP sensor voltage should decrease. This test verifies that control module and wiring are okay.
3. Probing ground circuit with a test light checks sensor ground circuit. This isolates a faulty sensor, control module or open ground circuit.

Tech 1 displays throttle position in volts. With closed throttle, ignition on, or at idle, voltage should be .45-.85 volt. If voltage is not as specified, replace TP sensor. Check for

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
2. Intermittent Test Monitor TP sensor voltage display on Tech 1 while moving related connectors and wiring harness. If failure is induced, display will change. This may assist in isolating location of malfunction.
3. TP Sensor Scaling Observe TP sensor voltage display while depressing accelerator pedal with engine stopped and ignition on. Display should vary from closed throttle TP sensor voltage when throttle is closed (less than 1.25 volts) to greater than 4.5 volts when throttle is held at WOT position.

Code P0123 Schematic (4.3L "S" & "T" Series - VIN Z) Throttle Position Sensor Signal High. Scheme 216

Code P0123 Flow Chart (4.3L "S" & "T" Series - VIN Z) Throttle Position Sensor Signal High. Scheme 217

CODE P0125, COOLANT SENSOR EXCESSIVE TIME TO CLOSED LOOP "S" & "T" SERIES 4.3L (VIN W)

While engine is warming, control module monitors coolant sensor to determine how long it takes engine to reach temperature for closed loop operation. Control module checks for code conditions during a cold start and only once per cold start.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks engine warm-up rate.
2. Checks coolant sensor reading on Tech 1 against actual coolant temperature.

Code indicates a skewed coolant sensor or thermostat that is not operating properly. Check for

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
2. Intermittent Test Monitor voltage by backprobing coolant sensor terminals at control module. While moving related connectors and wiring harness, if failure is induced, display will change. This may assist in isolating location of malfunction.

Code P0125 Flow Chart ("S" & "T" Series 4.3L - VIN W) Coolant Sensor Excessive Time To Closed Loop. Scheme 218

CODE P0131, HEATED OXYGEN SENSOR LOW SIGNAL VOLTAGE (LEAN) "S" & "T" SERIES 4.3L (VIN W)

Control module supplies and monitors a voltage of about .45 volt between control module HO2S low reference circuit and HO2S signal circuit. If read with a 10-megohm DVOM, voltage may read as low as .32 volt. HO2S varies voltage within a range of about 1.0 volt (rich) to .10 volt (lean). Sensor is like an open circuit and produces no voltage when less than about 600°F (360°C). Open loop operation is caused by Codes P0131 or P0132, an open sensor circuit, or a cold sensor. Code P0131 will cause system to operate in open loop. Control module will illuminate Malfunction Indicator Light (MIL).

Note. Test numbers refer to numbers on diagnostic chart.

1. Running engine at 1200 RPM keeps oxygen sensor hot to ensure oxygen sensor remains active and can read exhaust oxygen content accurately.
2. Opening HO2S signal circuit and grounding HO2S low circuit should result in a displayed voltage of .35-.55 volt. If display is still fixed less than .35 volt, fault is a short to ground in signal circuit, or faulty control module.

Using Tech 1, observe LT fuel trim values at different RPM and air flow conditions. Tech 1 also displays fuel trim cells allowing LT fuel trim values to be checked in each cell, determining when Code P0131 may have been set. LT fuel trim values will be around 158 or greater if conditions exist for Code P0131 to set.

1. Heated Oxygen Sensor Wire Sensor pigtail may be mispositioned and contacting exhaust manifold.
2. Check for intermittent ground in wire between connector and sensor.
3. Poor control module to engine block ground.
4. MAF Sensor System will go lean when MAF sensor output causes control module to sense a lower than normal air flow. Disconnect MAF sensor. If lean condition no longer exists, replace MAF sensor.
5. Lean Injectors Perform injector balance test.
6. Fuel Contamination Water near in-tank fuel pump inlet causing fuel contamination in fuel delivered to injectors. Water causes a lean exhaust and can set Code P0131.
7. Load Conditions
8. Exhaust Leaks If an exhaust leak above oxygen sensor is present, outside air may be allowed to be pulled into exhaust stream and flow past sensor.
9. Vacuum Or Crankcase Leak Can cause a lean condition and/or possibly a high idle.
10. If all components/systems are okay, heated oxygen sensor is faulty.
11. Faulty HO2S Heater or Heater Circuit Refer to DIAGNOSTICS AIDS in Code P0134 chart.

Code P0131 Schematic ("S" & "T" Series 4.3L - VIN W) Heated Oxygen Sensor Low Signal Voltage (Lean). Scheme 219

Code P0131 Flow Chart ("S" & "T" Series 4.3L - VIN W) Heated Oxygen Sensor Low Signal Voltage (Lean). Scheme 220

CODE P0132, HEATED O2 SENSOR HIGH SIGNAL VOLTAGE (RICH) "S" & "T" SERIES 4.3L (VIN W) BANK 1, SENSOR 1

Control module supplies and monitors a voltage of about .45 volt between control module HO2S low reference circuit and HO2S signal circuit. If read with a 10-megohm DVOM, voltage may read as low as .32 volt. HO2S varies voltage within a range of about 1.0 volt (rich) to .10 volt (lean). Sensor is like an open circuit and produces no voltage when less than about 600°F (360°C). Open loop operation is caused by Codes P0131, P0132, or an open or cold sensor. Code P0132 will cause system to operate in open loop. Control module will illuminate Malfunction Indicator Light (MIL).

Note. Test numbers refer to numbers on diagnostic chart.

1. Determines if conditions necessary to set Code P0132 exist
2. When grounding circuits, Tech 1 should read less than .35 volt. If display is not as specified, check for short to voltage in signal circuit.

1. Fuel Pressure If pressure is too high, system will be rich. Control module can compensate for some increase. However, Code P0132 will set if pressure gets too high.
2. Leaking Injectors Check for fuel contaminated oil.
3. EVAP Canister Purge Check for fuel saturation. If full of fuel, check canister control and hoses.
4. TP Sensor An intermittent TP sensor output will cause system to go rich due to a false indication of throttle moving.
5. False rich indication due to silicon contamination of heated oxygen sensor. This will be indicated by Code P0131 accompanied by lean driveability conditions and a powdery White deposit on sensor.
6. Heated Oxygen Sensor (HO2S) If HO2S is internally shorted, HO2S voltage displayed on Tech 1 will be greater than 1 volt. Disconnect HO2S with ignition on and engine running. If displayed voltage decreases from greater than 1.0 volt to about .45 volt, replace HO2S.
7. Faulty HO2S Heater Or Heater Circuit Refer to DIAGNOSTICS AIDS in Code P0134 chart.

Code P0132 Flow Chart ("S" & "T" Series 4.3L - VIN W) Heated O2 Sensor High Signal Voltage (Rich). Scheme 221

CODE P0133, HEATED OXYGEN SENSOR SLOW RESPONSE

"S" & "T" SERIES 4.3L (VIN W) BANK 1, SENSOR 1

Control module supplies and monitors a voltage of about .45 volt between control module HO2S low reference circuit and HO2S signal circuit. If read with a 10-megohm DVOM, voltage may read as low as .32 volt. HO2S varies voltage within a range of about 1.0 volt (rich) to .10 volt (lean). Sensor is like an open circuit and produces no voltage when less than about 600°F (360°C). Open loop operation is caused by Codes P0131, P0132, or an open or cold sensor. Code P0132 will cause system to operate in open loop. Control module will illuminate Malfunction Indicator Light (MIL).

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks for other codes.
2. Checks for physical damage to engine components and converter.
3. Checks fuel injector and isolates sensor.

Check sensor heater operation.

Code P0133 Flow Chart ("S" & "T" Series 4.3L (VIN W) Bank 1, Sensor 1) Heated Oxygen Sensor Slow Response. Scheme 222

CODE P0134, HEATED OXYGEN SENSOR OPEN CIRCUIT - "S" & "T" SERIES 4.3L (VIN W) BANK 1, SENSOR 1

Control module supplies and monitors voltage of about .45 volt between control module HO2S low reference circuit and HO2S signal circuit. If read with a 10 megohm DVOM, voltage may read as low as .32 volt. HO2S varies voltage within a range of about 1 volt (rich) to .10 volt (lean). Sensor is like an open circuit and produces no voltage when less than about 600°F (360°C). Open loop operation is caused by open oxygen sensor circuit or a cold sensor. Code P0134 will cause system to operate in open loop. Control module will illuminate Malfunction Indicator Light (MIL).

Note. Test numbers refer to numbers on diagnostic chart.

1. If conditions necessary to set Code P0132 exist, system will not go into closed loop.
2. This will determine if sensor or wiring is cause of Code P0134.

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside insulation. Check for

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
2. Faulty HO2S Heater Or Heater Circuit With ignition on and engine off, HO2S voltage displayed on Tech 1 should gradually decrease to less than .25 volt, indicating heater is functioning properly. If display does not read as specified, disconnect HO2S and connect test light between power supply and ground terminals. If test light does not light, repair open in suspect wire. If test light lights, replace HO2S.
3. Intermittent Test Using Tech 1, monitor HO2S signal voltage while moving related connectors and wiring harness with engine running at part throttle in closed loop. If failure is induced, HO2S signal voltage reading will change from normal fluctuating voltage (greater than .6 volt and less than .30 volt) to a fixed value of about .45 volt. This may assist in isolating location of malfunction.

Code P0134 Flow Chart ("S" & "T" Series 4.3L (VIN W) Bank 1, Sensor 1) Heated Oxygen Sensor Open Circuit. Scheme 223

CODE P0135, HEATED OXYGEN SENSOR HEATER CIRCUIT MALFUNCTION "S" & "T" SERIES 4.3L (VIN W) BANK 1, SENSOR 1

Control module supplies and monitors voltage of about .45 volt between control module HO2S low reference circuit and HO2S signal circuit. If read with a 10 megohm DVOM, voltage may read as low as .32 volt. HO2S varies voltage within a range of about 1 volt (rich) to .10 volt (lean). Sensor is like an open circuit and produces no voltage when less than about 600°F (360°C).

Note. Test numbers refer to numbers on diagnostic chart.

1. Determines if code is result of a hard failure or an intermittent condition.
2. Verifies voltage is present at heater.
3. If voltage is available at connector, it is a good voltage source to check for a good sensor ground.
4. Determines if voltage is not available at sensor due to an open fuse or open in ignition feed circuit.

Check for

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.

Code P0135 Flow Chart ("S" & "T" Series 4.3L (VIN W) Bank 1, Sensor 1) Heated Oxygen Sensor Heater Circuit Malfunction. Scheme 224

CODE P0137, HEATED OXYGEN SENSOR LOW SIGNAL VOLTAGE (LEAN) "S" & "T" SERIES 4.3L (VIN W) BANK 1, SENSOR 2

Control module supplies and monitors a voltage of about .45 volt between control module HO2S low reference circuit and HO2S signal circuit. If read with a 10-megohm DVOM, voltage may read as low as .32 volt. HO2S varies voltage within a range of about 1.0 volt (rich) to .10 volt (lean). Sensor is like an open circuit and produces no voltage when less than about 600°F (360°C). Open loop operation is caused by Codes P0131 or P0132, an open sensor circuit, or a cold sensor. Code P0131 will cause system to operate in open loop. Control module will illuminate Malfunction Indicator Light (MIL).

Note. Test numbers refer to numbers on diagnostic chart.

1. Determines if conditions necessary to set code are present.
2. Tech 1 should display .35-.55 volt. If signal voltage is still low, sensor low is shorted to ground or control module is faulty.

Check for

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
2. Intermittent Test Using Tech 1, monitor HO2S signal voltage while moving related connectors and wiring harness with engine running at part throttle in closed loop. If failure is induced, HO2S signal voltage reading will change from normal fluctuating voltage (greater than .6 volt and less than .30 volt) to a fixed value of about .45 volt. This may assist in isolating location of malfunction.

Code P0137 Schematic ("S" & "T" Series 4.3L (VIN W) Bank 1, Sensor 2) Heated Oxygen Sensor Low Signal Voltage (Lean). Scheme 225

Code P0137 Flow Chart ("S" & "T" Series 4.3L (VIN W) Bank 1, Sensor 2) Heated Oxygen Sensor Low Signal Voltage (Lean). Scheme 226

CODE P0138, HEATED OXYGEN SENSOR HIGH SIGNAL VOLTAGE (RICH) "S" & "T" SERIES 4.3L (VIN W) BANK 1, SENSOR 2

Control module supplies and monitors a voltage of about .45 volt between control module HO2S low reference circuit and HO2S signal circuit. If read with a 10-megohm DVOM, voltage may read as low as .32 volt. HO2S varies voltage within a range of about 1.0 volt (rich) to .10 volt (lean). Sensor is like an open circuit and produces no voltage when less than about 600°F (360°C). Open loop operation is caused by Codes P0131, P0132, or an open or cold sensor. Code P0132 will cause system to operate in open loop. Control module will illuminate Malfunction Indicator Light (MIL).

Note. Test numbers refer to numbers on diagnostic chart.

1. Determines if conditions necessary to set code exist
2. When grounding circuits, Tech 1 should read less than .35 volt. If display is not as specified, check for short to voltage in signal circuit.

Check for

1. Heated Oxygen Sensor (HO2S) If HO2S is internally shorted, HO2S voltage displayed on Tech 1 will be greater than one volt. Disconnect HO2S with ignition on and engine running. If displayed voltage decreases from greater than 1.0 volt to about .45 volt, replace HO2S.
2. Intermittent Test Using Tech 1, monitor HO2S signal voltage while moving related connectors and wiring harness with engine running at part throttle in closed loop. If failure is induced, HO2S signal voltage reading will change from normal fluctuating voltage (greater than .6 volt and less than .30 volt) to a fixed value of about .45 volt. This may assist in isolating location of malfunction.
3. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.

Code P0138 Flow Chart ("S" & "T" Series 4.3L (VIN W) Bank 1, Sensor 2) Heated Oxygen Sensor High Signal Voltage (Rich). Scheme 227

CODE P0139, HEATED OXYGEN SENSOR OPEN CIRCUIT "S" & "T" SERIES 4.3L (VIN Z) BANK 1, SENSOR 2

Control module applies a bias voltage of about 450 mV between sensor high and low circuits. Heated oxygen sensor varies voltage about 1.0 volt if exhaust is rich, to about 0.1 volt if exhaust is lean. Sensor is like an open circuit and applies no voltage when below 600°F (360°C). An open sensor circuit or cold sensor causes open loop operation.

Note. Test numbers refer to numbers on diagnostic chart.

1. This will determine if sensor is at fault.
2. For this test only use a high impedance DVM. This checks continuity of circuits.
3. If fuse was open, check complete control module wiring circuit.

Sensor heater resistance should be 3.5-14 ohms. If oxygen sensor heater is not operating properly, system may go into open loop operation after extended idle. Tech 1 voltage varies between 0.1-1.0 volt in closed loop. Code sets in 1 minute if voltage is fixed at 0.35-0.55 volt. System will go into open loop in about 15 seconds. Oxygen sample in sensor is needed for proper operation. Sample is directed through sensor wiring. All sensor wiring and connectors should be examined for breaks or contamination which may prevent reference oxygen from reaching sensor.

Code P0139 Schematic ("S" & "T" Series 4.3L (VIN Z) Bank 1, Sensor 2) Heated Oxygen Sensor Open Circuit. Scheme 228

Code P0139 Flow Chart ("S" & "T" Series 4.3L (VIN Z) Bank 1, Sensor 2) Heated Oxygen Sensor Open Circuit. Scheme 229

CODE P0140, HEATED OXYGEN SENSOR OPEN CIRCUIT "S" & "T" SERIES 4.3L (VIN W) BANK 1, SENSOR 2

Control module supplies and monitors voltage of about .45 volt between control module HO2S low reference circuit and HO2S signal circuit. If read with a 10 megohm DVOM, voltage may read as low as .32 volt. HO2S varies voltage within a range of about 1 volt (rich) to .10 volt (lean). Sensor is like an open circuit and produces no voltage when less than about 600°F (360°C). Open loop operation is caused by open oxygen sensor circuit or a cold sensor. Code P0134 will cause system to operate in open loop. Control module will illuminate Malfunction Indicator Light (MIL).

Note. Test numbers refer to numbers on diagnostic chart.

1. If conditions necessary to set code exist, system will not go into closed loop.
2. This will determine if sensor or wiring is cause of code.

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside insulation. Check for

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
2. Faulty HO2S Heater Or Heater Circuit With ignition on and engine off, HO2S voltage displayed on Tech 1 should gradually decrease to less than .25 volt, indicating heater is functioning properly. If display does not read as specified, disconnect HO2S and connect test light between power supply and ground terminals. If test light does not light, repair open in suspect wire. If test light lights, replace HO2S.
3. Intermittent Test Using Tech 1, monitor HO2S signal voltage while moving related connectors and wiring harness with engine running at part throttle in closed loop. If failure is induced, HO2S signal voltage reading will change from normal fluctuating voltage (greater than .6 volt and less than .30 volt) to a fixed value of about .45 volt. This may assist in isolating location of malfunction.

CODE P0141, HEATED OXYGEN SENSOR HEATER CIRCUIT MALFUNCTION "S" & "T" SERIES 4.3L (VIN W) BANK 1, SENSOR 2

Control module supplies and monitors voltage of about .45 volt between control module HO2S low reference circuit and HO2S signal circuit. If read with a 10 megohm DVOM, voltage may read as low as .32 volt. HO2S varies voltage within a range of about 1 volt (rich) to .10 volt (lean). Sensor is like an open circuit and produces no voltage when less than about 600°F (360°C).

Note. Test numbers refer to numbers on diagnostic chart.

1. Determines if code is result of a hard failure or an intermittent condition.
2. Verifies voltage is present at heater.
3. If voltage is available at connector, it is a good voltage source to check for a good sensor ground.
4. Determines if voltage is not available at sensor due to an open fuse or open in ignition feed circuit.

Check for

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.

Code P0141 Flow Chart ("S" & "T" Series 4.3L (VIN W) Bank 1, Sensor 2) Heated Oxygen Sensor Heater Circuit Malfunction. Scheme 230

CODE P0171, FUEL TRIM LEAN - "S" & "T" SERIES 4.3L (VIN W) BANK 1, SENSOR 1

A closed loop air/fuel metering system is used to provide the best possible combination of driveability, fuel economy and emission control. While in closed loop, control module monitors oxygen sensor signal voltage and adjusts fuel delivery based on signal voltage. Tech 1 monitors fuel trim values, and will indicate a change made to fuel delivery by the long and short term fuel trim values.

Ideal fuel trim values are about 128. If oxygen sensor signal indicates a lean condition, control module will add fuel, resulting in a fuel trim value greater than 128. If a rich condition is detected, fuel trim values will be less than 128, indicating control module is reducing amount of fuel delivered. Control module will set code if an excessively lean condition is detected.

Note. Test numbers refer to numbers on diagnostic chart.

1. Visually/physically checking components/systems which may cause a lean condition may determine cause of code.
2. Checks for proper fuel delivery system operation.
3. Check for faults in EVAP purge system that may cause Code to set.

If problem can not be isolated using diagnostic charts, monitor LT fuel trim and fuel trim cell while operating vehicle under various loads. This may isolate condition which caused code to set.

Code P0171 Flow Chart - 1 Of 2 ("S" & "T" Series 4.3L (VIN W) Bank 1, Sensor 1) Fuel Trim Lean. Scheme 231

Code P0171 Flow Chart - 2 Of 2 ("S" & "T" Series 4.3L (VIN W) Bank 1, Sensor 1) Fuel Trim Lean. Scheme 232

CODE P0171, LEAN EXHAUST - "S" & "T" SERIES 4.3L (VIN Z)

Control module applies a bias voltage of about 450 mV between sensor high and low circuits. The heated oxygen sensor varies voltage about 1.0 volt if exhaust is rich, to about 0.1 volt if exhaust is lean.

Note. Test numbers refer to numbers on diagnostic chart.

1. Oxygen sensor voltage below 0.35 volt indicates fault is still present.

Using Tech 1, observe long term fuel trim values at different RPM and airflow conditions. If conditions for code exist, long term fuel trim values will be around 150. Check for

1. Oxygen Sensor Wiring Reference oxygen is supplied through sensor wiring. Check sensor wiring for breaks or contamination that may prevent reference oxygen from reaching sensor.
2. Intermittent Ground Check for an intermittent ground in wire between sensor and connector.
3. Fuel Contamination Water near in-tank fuel pump can be delivered to injectors, causing a lean exhaust and setting code.
4. Fuel Pressure If fuel pressure is too low, system will be lean. Monitor fuel pressure while driving at various speeds and loads to confirm.
5. Exhaust Leaks If an exhaust leak exists, engine may pull outside air into exhaust and past sensor. Vacuum or crankcase leaks can cause a lean condition.
6. Sensor Ground Circuit If Tan wire circuit is open, voltage at signal terminal may be about 0.45 volt. This may also cause a Code P0139 to be set.
7. Oxygen Sensor If all checks are okay, heated oxygen sensor is faulty.

Code P0171 Flow Chart ("S" & "T" Series 4.3L - VIN Z) Lean Exhaust. Scheme 233

CODE P0172, FUEL TRIM RICH - "S" & "T" SERIES 4.3L (VIN W) BANK 1, SENSOR 1

A closed loop air/fuel metering system is used to provide the best possible combination of driveability, fuel economy and emission control. While in closed loop, control module monitors oxygen sensor signal voltage and adjusts fuel delivery based on signal voltage. Tech 1 monitors fuel trim values, and will indicate a change made to fuel delivery by the long and short term fuel trim values.

Ideal fuel trim values are about 128. If oxygen sensor signal indicates a lean condition, control module will add fuel, resulting in a fuel trim value greater than 128. If a rich condition is detected, fuel trim values will be less than 128, indicating control module is reducing amount of fuel delivered. Control module will set code if an excessively rich condition is detected.

Note. Test numbers refer to numbers on diagnostic chart.

1. Visually/physically checking components/systems which may cause a rich condition may determine cause of code.
2. Checks fuel system operation.
3. Silicon contamination of Heated Oxygen Sensor HO2S can cause a false rich indication that will be sensed by control module. Control module will then reduce amount of fuel delivered, possibly causing severe driveability symptoms.

If problem can not be isolated using diagnostic charts, monitor LT fuel trim and fuel trim cell while operating vehicle under various loads. This may isolate condition which caused code to set.

Code P0172 Flow Chart - 1 Of 2 ("S" & "T" Series 4.3L (VIN W) Bank 1, Sensor 1) Fuel Trim Rich. Scheme 234

Code P0172 Flow Chart - 2 Of 2 ("S" & "T" Series 4.3L (VIN W) Bank 1, Sensor 1) Fuel Trim Rich. Scheme 235

CODE P0172, RICH EXHAUST - "S" & "T" SERIES 4.3L (VIN Z)

Control module applies a bias voltage of about 4.5 volts between sensor high and low circuits. Heated oxygen sensor varies voltage about 1 volt if exhaust is rich, to about 0.1 volt if exhaust is lean. Code will set if oxygen sensor voltage remains high too long.

Note. Test numbers refer to numbers on diagnostic chart.

1. Oxygen sensor voltage greater than about 0.6 volt indicates fault is still present.

Using Tech 1, observe long term fuel trim values at different RPM and airflow conditions. If conditions for code exist, long term fuel trim values will be around 115. Check for

1. Oxygen Sensor Wiring Reference oxygen is supplied through sensor wiring. Check sensor wiring for breaks or contamination that may prevent reference oxygen from reaching sensor.
2. Fuel Pressure System will go rich is pressure is too high. Control module will compensate for some increase, but code will set if pressure gets too high.
3. Fuel Contamination Check for oil contaminated fuel.
4. HEI Shielding An open ground in ignition system reference low circuit may result in electrical interference. Control module looks at this at reference pulses. Pulses result in a higher than actual engine speed signal. Control module then delivers too much fuel, causing system to go rich. Tachometer will also show higher than actual engine speed, which can help in diagnosing problem.
5. MAP Sensor If an output causes control module to sense high manifold pressure (low vacuum), system will go rich. Disconnecting MAP sensor will allow control module to set a fixed MAP value. Substitute a different MAP sensor if rich condition is gone when sensor is disconnected.
6. Pressure Regulator Check for leaking fuel in pressure regulator diaphragm by checking for liquid fuel.
7. Throttle Position Sensor An intermittent TP sensor output will cause system to go rich due to false indication of engine accelerating.
8. Engine Coolant Temperature Sensor Check for shifted sensor that could cause a rich exhaust, but set Code P0117.

Code P0172 Flow Chart ("S" & "T" Series 4.3L - VIN Z) Rich Exhaust. Scheme 236

CODE P0300, RANDOM MISFIRE DETECTED - "S" & "T" SERIES 4.3L (VIN W)

Misfire is detected using cam and crankshaft sensors. Control module monitors both sensors. When control module senses crankshaft deceleration not associated with normal engine speed reduction, camshaft position is used to determine misfiring cylinder. Control module determines misfires on each cylinder and can thus evaluate a random misfire and set code.

Note. Test numbers refer to numbers on diagnostic chart.

1. If other codes are set, they may be responsible for misfires.
2. It may be necessary to test drive vehicle to duplicate misfire conditions.
3. When checking spark, spark should be consistent. A few sparks and then nothing constitute a no spark.
4. At this point, ignition is okay. Problem may be fuel related.
5. Determines if fuel injector is functioning properly.
6. Determines if poppet nozzles are functioning properly.
7. Both fuel and ignition are okay, problem is mechanical in nature.

An intermittent misfire may be caused by faulty plug/coil wires, contaminated fuel, low fuel level, misaligned (loose or cocked) crankshaft sensor. Code may also be set by tires which are severely out of balance.

Code P0300 Schematic ("S" & "T" Series 4.3L - VIN W) Random Misfire Detected. Scheme 237

Code P0300 Flow Chart ("S" & "T" Series 4.3L - VIN W) Random Misfire Detected. Scheme 238

CODE P0301-306, CYLINDERS 1-6 MISFIRING - "S" & "T" SERIES 4.3L (VIN W)

Misfire is detected using cam and crankshaft sensors. Control module monitors both sensors. When control module senses crankshaft deceleration not associated with normal engine speed reduction, camshaft position is used to determine misfiring cylinder and set code.

1. Code P0301=Cylinder No. 1
2. Code P0302=Cylinder No. 2
3. Code P0303=Cylinder No. 3
4. Code P0304=Cylinder No. 4
5. Code P0305=Cylinder No. 5
6. Code P0306=Cylinder No. 6 NOTE: Test numbers refer to numbers on diagnostic chart.

1. It may be necessary to test drive vehicle to duplicate misfire conditions.
2. When checking spark, spark should be consistent. A few sparks and then nothing constitute a no spark.
3. Inspects spark plugs.
4. At this point, ignition is okay. Problem may be fuel related.
5. Both fuel and ignition are okay, problem is mechanical in nature.

An intermittent misfire may be caused by faulty plug/coil wires, contaminated fuel, low fuel level, misaligned (loose or cocked) crankshaft sensor.

Code P0301-306 Flow Chart ("S" & "T" Series 4.3L - VIN W) Cylinders 1-6 Misfiring. Scheme 239

CODE P0320, IGNITION CONTROL ERROR - "S" & "T" SERIES 4.3L (VIN Z)

When system is running on ignition control module, when no voltage is on by-pass line, IC module grounds ignition control signal. Control module expects to sense no voltage on IC line during this condition. If it senses voltage, it sets a code and will not go into IC mode.

When RPM for IC is reached and bypass voltage is applied, IC will no longer be grounded in ignition module, so IC voltage will vary. Control module will disable IC mode if short test fails or default to by-pass mode if long test fails.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks to see if code is an intermittent.
2. Checks for a normal IC ground path through module. If IC by-pass wire is shorted to ground, it will also read less than 500 ohms. This will be checked later.
3. As test light voltage touches by-pass wire circuit, module should switch, causing ohmmeter to over-range if meter is in 100-200 range. With meter in 10-20,000 range, position will be above 5000 ohms. The important thing is that module switched.
4. If module did not switch, this step checks for IC circuit shorted to ground, by-pass circuit open or faulty ignition module or connection.
5. This step confirms code is set because of a faulty control module and not an intermittent in circuitry.

Tech 1 tool does not have ability to help diagnose code problems. Check for intermittents. An intermittent may be caused by a poor connection, rubbed through wire insulation, or a broken wire inside insulation. Check for backed out connector terminals or broken down insulation.

Code P0320 Schematic ("S" & "T" Series 4.3L - VIN Z) Ignition Control Error. Scheme 240

Code P0320 Flow Chart ("S" & "T" Series 4.3L - VIN Z) Ignition Control Error. Scheme 241

CODE P0325/326, KNOCK SENSOR OPEN OR SHORTED "S" & "T" SERIES 4.3L (VIN W)

Knock sensor circuit consists of 2 knock sensors connected in parallel and a single wire to control module. Each knock sensor has a resistance value of about 8200 ohms. Control module supplies a 5 volt reference to sensors, but because of sensor resistance, it is cut to about 2.5 volts. When a knock occurs, a small AC voltage is generated by sensors on top of existing 2.5 volts DC. An AC voltage monitor in control module detects this voltage and provides a signal to begin retarding spark until knock diminishes.

Note. Test numbers refer to numbers on diagnostic chart.

1. When sensors are disconnected, 5 volts should be present on sensor wire.
2. By checking sensor resistance, it can be determined if problem lies with connector, sensor installation or control module.
3. Knock sensor has been determined faulty.

Check sensor connections for proper fit. Also check for proper installation of PROM/MEM-CAL.

Code P0325/326 Schematic ("S" & "T" Series 4.3L - VIN W) Knock Sensor Open Or Shorted. Scheme 242

Code P0325/326 Flow Chart ("S" & "T" Series 4.3L - VIN W) Knock Sensor Open Or Shorted. Scheme 243

CODE P0327, KNOCK SENSOR GROUNDED - "S" & "T" SERIES 4.3L (VIN Z)

Knock sensor circuit consists of 2 knock sensors connected in parallel and a single wire to control module. Each knock sensor has a resistance value of about 8600 ohms. Control module supplies a 5 volt reference to sensors, but because of sensor resistance, it is cut to about 2.5 volts. When a knock occurs, a small AC voltage is generated by sensors on top of existing 2.5 volts DC. An AC voltage monitor in control module detects this voltage and provides a signal to begin retarding spark until knock diminishes.

Note. Test numbers refer to numbers on diagnostic chart.

1. When sensors are disconnected, 5 volts should be present on sensor wire.
2. By checking sensor resistance, it can be determined if problem lies with connector, sensor installation or control module.
3. Knock sensor has been determined faulty.

Check sensor connections for proper fit. Also check for proper installation of PROM/MEM-CAL.

Code P0327 Schematic ("S" & "T" Series 4.3L - VIN Z) Knock Sensor Grounded. Scheme 244

Code P0327 Flow Chart ("S" & "T" Series 4.3L - VIN Z) Knock Sensor Grounded. Scheme 245

CODE P0328, KNOCK SENSOR OPEN - "S" & "T" SERIES 4.3L (VIN Z)

Knock sensor circuit consists of 2 knock sensors connected in parallel and a single wire to control module. Each knock sensor has a resistance value of about 8600 ohms. Control module supplies a 5 volt reference to sensors, but because of sensor resistance, it is cut to about 2.5 volts. When a knock occurs, a small AC voltage is generated by sensors on top of existing 2.5 volts. An AC voltage monitor in control module will detect this voltage and provide a signal to begin retarding spark until knock diminishes.

Note. Test numbers refer to numbers on diagnostic chart.

1. Control module applies 5 volts on sensor circuit which should be present at connectors when sensors are disconnected.
2. By verifying sensor resistance, problem can be narrowed to connector, sensor installation or control module.
3. Improperly installed sensor can prevent sensor from grounding to block.

Check sensor connections for proper fit. Also check for proper installation of PROM/MEM-CAL.

Code P0328 Flow Chart ("S" & "T" Series 4.3L - VIN Z) Knock Sensor Open. Scheme 246

CODE P0337/338, CRANK SENSOR INPUT - "S" & "T" SERIES 4.3L (VIN W)

Crankshaft position sensor sends a toggling (0-5 volt) voltage signal to control module to indicate crankshaft position and RPM. Control module uses this signal to determine when pulse ignition coil and control timing. Code will set if duty cycle ratio is low (Code P0337) or high (Code P0338).

Note. Test numbers refer to numbers on diagnostic chart.

1. Determines if code is hard or intermittent.
2. Checks for low duty cycle.
3. Determines if voltage is available to sensor through control module.
4. If feed circuit is shorted to ground, test light will be on. This determines if circuit is open or shorted to ground.
5. Checks crankshaft sensor, signal circuit and control module.

An intermittent may be caused by a poor connection, rubbed through wire insulation, or a broken wire inside insulation. Check for

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out cam signal circuit terminal, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.

Code P0337/338 Schematic ("S" & "T" Series 4.3L - VIN W) Crank Sensor Input. Scheme 247

Code P0337/338 Flow Chart ("S" & "T" Series 4.3L - VIN W) Crank Sensor Input. Scheme 248

CODE P0340, CAM SENSOR MALFUNCTION - "S" & "T" SERIES 4.3L (VIN W)

Camshaft sensor is used to indicate camshaft position so that control module can determine which cylinder is misfiring. It is also used to check for a properly installed distributor.

Note. Test numbers refer to numbers on diagnostic chart.

1. Determines if code is hard or intermittent.
2. Determines if voltage is available to sensor through control module.
3. If feed circuit is shorted to ground, test light will be on. This determines if circuit is open or shorted to ground.

An intermittent may be caused by a poor connection, rubbed through wire insulation, or a broken wire inside insulation. Check for

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out cam signal circuit terminal, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.

Code P0340 Schematic ("S" & "T" Series 4.3L - VIN W) CAM Sensor Malfunction. Scheme 249

Code P0340 Flow Chart ("S" & "T" Series 4.3L - VIN W) CAM Sensor Malfunction. Scheme 250

CODE P0341, CAM SENSOR RANGE - "S" & "T" SERIES 4.3L (VIN W)

Code will set if cam pulses are not in proper ratio to crank pulses. If code is set, check for intermittent short to voltage or electromagnetic interference on cam sensor wires such as wires run alongside spark plug wires of high power transmitters (like mobile radios operating in the vicinity).

An intermittent may be caused by a poor connection, rubbed through wire insulation, or a broken wire inside insulation. Check for

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out cam signal circuit terminal, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.

CODE P0400, EGR ERROR - "S" & "T" SERIES 4.3L (VIN Z)

Control module operates an EVRV solenoid, that is pulse width modulated to control EGR valve. Solenoid is normally closed. By providing a ground path, control module energizes solenoid which allows vacuum to pass to EGR valve. Control module can vary solenoid on time from zero to 100% depending on load conditions.

Control module monitors EGR effectiveness by de-energizing EGR control solenoid, thereby shutting off vacuum to EGR diaphragm. With EGR valve closed and oxygen sensor operating normally, short fuel trim count will be greater than during normal EGR operation. If test passed, it will be run only once during ignition cycle. If test failed, it must fail twice before Code P0400 will be set.

Note. Test numbers refer to numbers on diagnostic chart.

1. With ignition on and engine stopped, solenoid should not be energized and vacuum should not pass to EGR valve. Grounding diagnostic terminal will energize solenoid and allow vacuum to pass.
2. Checks for plugged EGR passages. If passages are plugged, engine may have severe detonation on acceleration.
3. MIL should flash while diagnostic terminal is grounded. If light does not flash, it may indicate that quad-driver module has been damaged.
4. Vehicle must be driven during this test to ensure sufficient load to operate EGR.

Before replacing control module, check resistance of each controlled relay and solenoid coil, except EVRV. EVRV is solid state. Replace any solenoid when resistance measures less than 2 ohms. An intermittent may be caused by a broken wire inside insulation. Check for

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out EGR control circuit terminal, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.

Code P0400 Schematic ("S" & "T" Series 4.3L - VIN Z) EGR Error. Scheme 251

Code P0400 Flow Chart ("S" & "T" Series 4.3L - VIN Z) EGR Error. Scheme 252

CODE P0401, EGR FLOW TEST FAILURE - "S" & "T" SERIES 4.3L (VIN W)

Control module tests exhaust gas recirculation valve by momentarily commanding valve on while monitoring engine RPM. Control module will illuminate Malfunction Indicator Light (MIL) and store Code P0401 if expected drop is not seen for a calibrated number of tests.

Note. Test numbers refer to numbers on diagnostic chart.

1. Ensures EGR valve is allowing EGR flow and control module is capable of controlling EGR valve.
2. Ensures control circuit is not shorted to ground.
3. Ensures control circuit is not open and that control module quad-driver for circuit is properly functioning.

An intermittent may be caused by a broken wire inside insulation. Check for

1. Poor Connection Or Damaged Harness - Inspect control module harness connectors for backed out EGR control circuit terminal, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
2. Intermittent Test - If connections and harness are okay, monitor a DVOM connected between control module EGR control circuit terminal and ground while moving related connectors and wiring harness. If failure is induced, voltage reading will change.

Note. Check exhaust system for blockage (possibly a plugged converter) if EGR valve shows signs of excessive heat. Repair cause of restricted exhaust system as necessary. Injector stuck open may be due to stuck injector pintle, grounded driver circuit or possible faulty control module. If conditions are not present, oil should be checked for possible fuel contamination.

Code P0401 Schematic ("S" & "T" Series 4.3L - VIN W) EGR Flow Test Failure. Scheme 253

Code P0401 Flow Chart ("S" & "T" Series 4.3L - VIN W) EGR Flow Test Failure. Scheme 254

CODE P0420, CATALYST EFFICIENCY LOW (BANK 1)

"S" & "T" SERIES 4.3L (VIN W)

Check the following

1. Ensure 3-way catalyst is factory original for application. Check converter for dents, severe discoloration, holes, loose internal (rattle) materials, etc.
2. Check exhaust for leaks or missing hardware.
3. Ensure oxygen sensor is secure and that sensor pigtail is not damaged or touching hot exhaust.
4. Check all control module and sensor grounds. Check sensor high and low circuits for intermittent open or short to ground.

If no problems are found, catalytic converter is bad. If converter needs to be replaced, ensure that conditions which may damage new converter do not exist i.e. misfire, high oil or coolant consumption or retarded or weak spark.

Code P0420 Schematic ("S" & "T" Series 4.3L - VIN W) Catalyst Efficiency Low (Bank 1). Scheme 255

CODE P0441, INCORRECT EVAP PURGE - "S" & "T" SERIES 4.3L (VIN W)

Canister purge is controlled by a solenoid valve which allows ported vacuum to purge canister when energized by control module. A vacuum switch in purge line is used to detect when system is being purged. Control module supplies and monitors a 5-volt reference to switch. If switch is closed (no purge detected-low monitored voltage) when control module is commanding purge, code will be set.

Note. Test numbers refer to numbers on diagnostic chart.

1. Solenoid valve will click when commanded on and off. This determines if control module can control solenoid.
2. Ported (off-idle) vacuum is used at solenoid. Vacuum will only be present at off idle.
3. If voltage is not available to solenoid, control module cannot energize solenoid.
4. Simulates a purge signal to control module.
5. Determines if solenoid cannot be energized due to a circuit problem or faulty solenoid.

Make a visual inspection of all vacuum hoses. Check throttle body for damage which may affect vacuum signal. Check engine for possible mechanical problem.

Code P0441 Schematic ("S" & "T" Series 4.3L - VIN W) Incorrect Evap Purge. Scheme 256

Code P0441 Flow Chart ("S" & "T" Series 4.3L - VIN W) Incorrect Evap Purge. Scheme 257

CODE P0500, VSS CIRCUIT - "S" & "T" SERIES

Vehicle Speed Sensor (VSS) is magnetic induction type. Gear teeth pressed on outside of output shaft induce an AC current in sensor as shaft rotates. Signal goes directly to control module. Control module uses pulsed signal to calculate vehicle speed based on time between pulses.

Note. Test numbers refer to numbers on diagnostic chart.

1. This determines if code is set as result of a hard failure or an intermittent.
2. This checks wiring, connections and control module and VSS.

Check that proper calibration is in control module for vehicle speedometer.

Code P0500 Schematic ("S" & "T" Series) VSS Circuit. Scheme 258

Code P0500 Flow Chart ("S" & "T" Series) VSS Circuit. Scheme 259

CODES P0502/P0503, VSS CIRCUIT LOW/HIGH - "S" & "T" SERIES 4.3L (VIN W)

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks VSS circuit.
2. Checks integrity of sensor.
3. Checks 5-volt and ground circuits to control module.

Code P0502 will set if no vehicle speed is detected at start off. Code P0503 will set when vehicle speed has been detected and lost. Inspect all harnesses and related wiring, including control module connectors.

Codes P0502/P0503 Schematic ("S" & "T" Series 4.3L - VIN W) VSS Circuit Low/High. Scheme 260

Codes P0502/P0503 Flow Chart ("S" & "T" Series 4.3L - VIN W) VSS Circuit Low/High. Scheme 261

CODE, P0506/507 IAC COUNTS LOW/HIGH - "S" & "T" SERIES 4.3L (VIN W)

Note. Test numbers refer to numbers on diagnostic chart.

1. Ensures no other codes are set.
2. Determines if problem is engine mechanical related.

An IAC valve which is stopped and cannot respond to control module, a throttle stop screw which has been tampered with, or a damaged throttle linkage could cause code to set. A slow, unstable, or fast idle may be caused by a non-IAC valve system problem that cannot be overcome by IAC. An out-of-range control IAC will scan above 60 counts if idle is too low, and zero if idle is too high. Check the following

1. Fuel system too rich or too lean
2. Foreign material in/on throttle body ports
3. Vacuum leaks.

IAC reset is performed using Tech 1 in miscellaneous test mode.

Code P0506/507 Schematic ("S" & "T" Series 4.3L) IAC Counts Low/High. Scheme 262

Code P0506/507 Flow Chart ("S" & "T" Series 4.3L) IAC Counts Low/High. Scheme 263

CODE 560, SYSTEM VOLTAGE MALFUNCTION - "S" & "T" SERIES 4.3L (VIN W)

Note. Test numbers refer to numbers on diagnostic chart.

1. Verifies normal charging system voltage is between 13.0 and 15.8 volts.
2. Checks actual battery voltage against voltage at control module.

Charging battery with a battery charger and jump-starting engine may set this code. If code sets when an accessory is operated, check for faulty connections or excessive current draw. Check for faulty connections at starter solenoid and fusible link. Check for loose or damaged terminals at generator. Check belt tension and wear.

Code 560 Schematic ("S" & "T" Series 4.3L - VIN W) System Voltage Malfunction. Scheme 264

Code 560 Flow Chart ("S" & "T" Series 4.3L - VIN W) System Voltage Malfunction. Scheme 265

CODE P0703 - TCC BRAKE INPUT CIRCUIT "S" & "T" SERIES 4.3L (VIN W)

TCC brake switch input is used by control module to determine when to energize TCC solenoid.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks for voltage at brake switch.
2. Checks brake switch.
3. Checks TCC brake switch signal at control module.

Check customer driving habits and/or unusual traffic conditions, i.e. stop and go freeway traffic. Check brake switch adjustment. Check for most current control module calibration software.

Code P0703 Schematic ("S" & "T" Series 4.3L - VIN W) TCC Brake Input Circuit. Scheme 266

Code P0703 Flow Chart ("S" & "T" Series 4.3L - VIN W) TCC Brake Input Circuit. Scheme 267

CODE P0712, TRANSAXLE TEMPERATURE SENSOR SIGNAL VOLTAGE LOW "S" & "T" SERIES 4.3L (VIN W)

Control module sends a voltage signal to sensor and monitors return voltage. Transaxle temperature sensor varies resistance as temperature of transaxle fluid changes. Temperature sensor resistance is high when transaxle fluid is cold. As fluid temperature increases, sensor resistance decreases.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for short to ground or skewed sensor.
2. Checks for an internal fault within transmission by creating an open.

After engine is started, temperature should rise steadily to about 212°F (100°C). Check sensor for shifted calibration by using sensor TEMPERATURE-TO-RESISTANCE VALUES table. An intermittent may be caused by a poor connection, chaffed wire insulation or a broken wire. Monitor voltage of each terminal while moving related harness connectors. If failure is induced, voltage reading will change.

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

Code P0712 Schematic ("S" & "T" Series 4.3L - VIN W) Transaxle Temperature Sensor Signal Voltage Low. Scheme 268

Code P0712 Flow Chart ("S" & "T" Series 4.3L - VIN W) Transaxle Temperature Sensor Signal Voltage Low. Scheme 269

CODE P0713, TRANSAXLE TEMPERATURE SENSOR SIGNAL VOLTAGE HIGH "S" & "T" SERIES 4.3L (VIN W)

Control module sends a voltage signal to sensor and monitors return voltage. Transaxle temperature sensor varies resistance as temperature of transaxle fluid changes. Temperature sensor resistance is high when transaxle fluid is cold. As fluid temperature increases, sensor resistance decreases.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Verifies a problem in sensor circuit.
2. Simulates Code P0712. If control module recognizes low signal voltage, control module and wiring are okay.
3. Checks 5-volt reference for open. There should be 5 volts present at sensor connector.

After engine is started, temperature should rise steadily to about 212°F (100°C). Check sensor for shifted calibration by using sensor TEMPERATURE-TO-RESISTANCE VALUES table. An intermittent may be caused by a poor connection, chaffed wire insulation or a broken wire. Monitor voltage of each terminal while moving related harness connectors. If failure is induced, voltage reading will change.

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

Code P0713 Flow Chart ("S" & "T" Series 4.3L - VIN W) Transaxle Temperature Sensor Signal Voltage High. Scheme 270

CODE P0742, TCC STUCK ON - "S" & "T" SERIES 4.3L (VIN W)

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks mechanical state of TCC when control module commands TCC solenoid off. TCC slip speed should increase.

Check for proper TP sensor operation. If TCC is mechanically stuck on, vehicle speed is zero, brakes are applied, and D2 is selected, TCC fluid will mechanically apply TCC, causing an engine stall.

Code P0742 Schematic ("S" & "T" Series 4.3L - VIN W) TCC Stuck On. Scheme 271

Code P0742 Flow Chart ("S" & "T" Series 4.3L - VIN W) TCC Stuck On. Scheme 272

CODE P0748, PRESSURE CONTROL SOLENOID - "S" & "T" SERIES 4.3L (VIN W)

Pressure control solenoid is used to regulate transmission line pressure. Control module compares TPS voltage, engine RPM and other inputs to determine proper line pressure for a given load. Control module regulates pressure by applying a varying amperage to pressure control solenoid. Amperage varies from 0.1 to 1.1 amps.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks ability of control module to command pressure control solenoid.
2. Checks for voltage at control module.

Check for poor connections at control module and at transmission connector.

Code P0748 Flow Chart ("S" & "T" Series 4.3L - VIN W) Pressure Control Solenoid. Scheme 273

CODE P0753, 1-2 SHIFT SOLENOID - "S" & "T" SERIES 4.3L (VIN W)

Voltage is supplied directly to solenoid. Control module energizes solenoid by providing a ground circuit for solenoid.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks 1-2 shift solenoid and internal transmission for shorts.
2. Checks for power to 1-2 shift solenoid from ignition through fuse.

Check all connections at transmission. An open in ignition feed circuit can cause multiple codes to set.

Code P0753 Schematic ("S" & "T" Series 4.3L - VIN W) 1-2 Shift Solenoid. Scheme 274

Code P0753 Flow Chart ("S" & "T" Series 4.3L - VIN W) 1-2 Shift Solenoid. Scheme 275

CODE P0756, 2-3 SHIFT SOLENOID PERFORMANCE - "S" & "T" SERIES 4.3L (VIN W)

Voltage is supplied directly to solenoid. Control module energizes solenoid by providing a ground circuit for solenoid.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks function of transmission range switch.
2. Checks for proper selected gear ratio.

Check all related wiring, including control module connectors. A 4WD switch/wiring problem may set this code.

Code P0756 Schematic ("S" & "T" Series 4.3L - VIN W) 2-3 Shift Solenoid Performance. Scheme 276

Code P0756 Flow Chart ("S" & "T" Series 4.3L - VIN W) 2-3 Shift Solenoid Performance. Scheme 277

CODE P0758, 2-3 SHIFT SOLENOID ELECTRICAL "S" & "T" SERIES 4.3L (VIN W)

Voltage is supplied directly to solenoid. Control module energizes solenoid by providing a ground circuit for solenoid.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks 2-3 shift solenoid and internal transmission for shorts.
2. Checks for power to 2-3 shift solenoid from ignition through fuse.
3. Checks ability of control module and wiring to control ground (driver) circuit.

Check all related wiring, including control module connectors. An open ignition feed circuit may set this code.

Code P0758 Flow Chart ("S" & "T" Series 4.3L - VIN W) 2-3 Shift Solenoid Electrical. Scheme 278

CODE P1106, MAP INTERMITTENT HIGH - "S" & "T" SERIES 4.3L (VIN W)

Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). Control module receives this information as a signal voltage that varies from 1-1.5 volts at idle to 4-4.6 volts at full throttle. Scan tool displays manifold pressure in volts. Low pressure (high vacuum) reads a low voltage, while high pressure (low vacuum) reads a high voltage. If MAP sensor fails, control module will substitute a fixed MAP value and use throttle position sensor to control fuel delivery.

Note. Test numbers refer to numbers on diagnostic chart.

1. With engine idling, transmission in Neutral and all accessories off, MAP voltage should be 2.0-2.5 volts.
2. If unplugging MAP sensor does not produce a reading of less than one volt, problem may lie somewhere in signal circuit.
3. If test light does not light, sensor ground may be open.

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside insulation. Comparing key on, engine off MAP voltage reading with that of a known good vehicle will help to check accuracy of MAP sensor. Readings should be within .4 volt of each other. Disconnect sensor from bracket and twist sensor by hand to check for intermittents. Output changes greater than 0.1 volt indicate a bad sensor or connection.

Code P1106 Flow Chart ("S" & "T" Series 4.3L - VIN W) MAP Intermittent High. Scheme 279

CODE P1107, MAP INTERMITTENT LOW - "S" & "T" SERIES 4.3L (VIN W)

Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). Control module receives this information as a signal voltage that varies from 1-1.5 volts at idle to 4-4.6 volts at full throttle. Scan tool displays manifold pressure in volts. Low pressure (high vacuum) reads a low voltage, while high pressure (low vacuum) reads a high voltage. If MAP sensor fails, control module will substitute a fixed MAP value and use throttle position sensor to control fuel delivery.

Note. Test numbers refer to numbers on diagnostic chart.

1. Determines if code is set because of a hard failure or intermittent condition.
2. Jumping circuits determines if sensor is at fault or if problem exists in control module or wiring.
3. Tech 1 may not display 5 volts. What is important is that control module recognized voltage as more than 4 volts, indicating that control module and circuit are okay.
4. Determines if 5 volts are present at sensor.

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside insulation. Comparing key on, engine off MAP voltage reading with that of a known good vehicle will help to check accuracy of MAP sensor. Readings should be within .4 volt of each other. Disconnect sensor from bracket and twist sensor by hand to check for intermittents. Output changes greater than 0.1 volt indicate a bad sensor or connection.

Code P1107 Flow Chart ("S" & "T" Series 4.3L - VIN W) MAP Intermittent Low. Scheme 280

CODE P1111, IAT INTERMITTENT HIGH - "S" & "T" SERIES 4.3L (VIN W)

Intake Air Temperature (IAT) sensor is a thermistor. Control module applies and monitors 5 volts on circuit to sensor. When air is cold, sensor resistance is high and control module will measure a high signal voltage. If air is warm, sensor resistance is low causing control module to measure low voltage.

Note. Test numbers refer to numbers on diagnostic chart.

1. Tech 1 may be used to diagnose this fault since control module transmits actual values when fault is present. A grounded circuit will cause Tech 1 to display a temperature of about 297°F (147°C).
2. If Tech 1 displays -40° with IAT sensor disconnected, control module and wiring are okay. Replace IAT sensor.

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside insulation. Check for

1. Poor Connection Or Damaged Harness Inspect control module harness for short to ground in 5-volt reference circuit, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
2. Intermittent Test Observe IAT on Tech 1 while moving related connectors and wiring harness with warm engine running. If failure is induced, IAT display will change to a 297°F (147°C) temperature reading. This may assist in isolating location of malfunction.

CODE P1112, IAT INTERMITTENT LOW - "S" & "T" SERIES 4.3L (VIN W)

Intake Air Temperature (IAT) sensor is a thermistor. Control module applies and monitors 5 volts on circuit to sensor. When air is cold, sensor resistance is high and control module will measure a high signal voltage. If air is warm, sensor resistance is low causing control module to measure low voltage.

Note. Test numbers refer to numbers on diagnostic chart.

1. An open sensor, wire or connection will cause code to set. An open circuit would cause Tech 1 to display about -40°F (-40°C). This test determines if wiring and control module are okay.

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside insulation. Check for

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
2. Intermittent Test Observe IAT on Tech 1 while moving related connectors and wiring harness with warm engine running. If failure is induced, IAT display will change to a -40° temperature reading. This may assist in isolating location of malfunction.

CODE P1114, ECT INTERMITTENT LOW - "S" & "T" SERIES 4.3L (VIN W)

ECT sensor uses a thermistor to control signal voltage to control module. Control module applies and monitors a 5 volts to ECT sensor. When engine coolant is cold, sensor (thermistor) resistance is high and control module will sense a high signal voltage. As engine coolant warms up, sensor resistance becomes less and control module voltage drops. With code set, control module will turn cooling fans on and use a default engine coolant temperature value based on run time. Control module will illuminate Malfunction Indicator Light (MIL).

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if conditions necessary to set code exist.
2. Simulates Code P0118. If control module recognizes high signal voltage, control module and wiring are okay.
3. Determines if 5-volt reference is open. This determines if there is a wiring problem or faulty control module.

Tech 1 displays engine temperature in degrees celsius (°C) and fahrenheit (°F). After engine is started, temperature should rise steadily to 194°F (90°C), then stabilize when thermostat opens. An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside insulation. Check for

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
2. Intermittent Test Using Tech 1, monitor engine coolant temperature while moving related connectors and wiring harness. If failure is induced, display will change. This may assist in isolating location of malfunction.
3. Shifted Sensor See TEMPERATURE-TO-RESISTANCE VALUES table to test ECT sensor at various temperature levels to evaluate the possibility of a shifted (mis-scaled) sensor which may result in driveability complaints.
4. A faulty connection, or an open in sensor circuits will cause Code P0117 to set.

TEMPERATURE-TO-RESISTANCE VALUES (1)

CODE P1115, ECT INTERMITTENT HIGH - "S" & "T" SERIES 4.3L (VIN W)

ECT sensor uses a thermistor to control signal voltage to control module. Control module applies and monitors a 5 volts to ECT sensor. When engine coolant is cold, sensor (thermistor) resistance is high and control module will sense a high signal voltage. As engine coolant warms up, sensor resistance becomes less and control module voltage drops. With code set, control module will turn cooling fans on and use a default engine coolant temperature value based on run time. Control module will illuminate Malfunction Indicator Light (MIL).

Note. Test numbers refer to numbers on diagnostic chart.

1. This step determines if conditions necessary to set code exist.
2. Simulates Code P0117. If control module recognizes low voltage, control module and wiring are okay.
3. Checks for open or grounded signal circuit. Also checks sensor ground circuit and control module.

Tech 1 displays engine temperature in degrees celsius (°C) and fahrenheit (°F). After engine is started, temperature should rise steadily to 194°F (90°C), then stabilize when thermostat opens. An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside insulation. Check the following for

1. A short to ground in 5-volt reference circuit.
2. Intermittent Test Using Tech 1, monitor engine coolant temperature while moving related connectors and wiring harness. If failure is induced, display will change. This may assist in isolating location of malfunction.
3. Shifted Sensor See TEMPERATURE-TO-RESISTANCE VALUES table to test ECT sensor at various temperature levels to evaluate the possibility of a shifted (mis-scaled) sensor which may result in driveability complaints.

TEMPERATURE-TO-RESISTANCE VALUES (1)

CODE P1121, TP SENSOR INTERMITTENT HIGH - "S" & "T" SERIES 4.3L (VIN W)

Throttle Position (TP) sensor provides a voltage signal that changes relative to throttle blade angle. Signal voltage will vary from about .5 volt or at idle to about 4 volts at Wide Open Throttle (WOT). TP sensor signal is one of the most important inputs used by control module for fuel control and for most other control module control outputs. On non-adjustable TPS sensors, each time voltage drops below 1.25 volts and stops, control module assumes this value as 0 throttle angle and measures percent throttle from this point on.

Note. Test numbers refer to numbers on diagnostic chart.

1. If TP signal is greater than 4.8 volts, fault is present.
2. With TP sensor disconnected, TP sensor voltage should decrease. This test verifies that control module and wiring are okay.
3. Probing ground circuit with a test light checks sensor ground circuit. This isolates a faulty sensor, control module or open ground circuit.

Tech 1 displays throttle position in volts. With closed throttle, ignition on, or at idle, voltage should be .45-.85 volt. If voltage is not as specified, replace TP sensor. Check for

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
2. Intermittent Test Monitor TP sensor voltage display on Tech 1 while moving related connectors and wiring harness. If failure is induced, display will change. This may assist in isolating location of malfunction.
3. TP Sensor Scaling Observe TP sensor voltage display while depressing accelerator pedal with engine stopped and ignition on. Display should vary from closed throttle TP sensor voltage when throttle is closed (less than 1.25 volts) to greater than 4.5 volts when throttle is held at WOT position.

Code P1121 Flow Chart ("S" & "T" Series 4.3L - VIN W) TP Sensor Intermittent High. Scheme 281

CODE P1122, TP SENSOR INTERMITTENT LOW - "S" & "T" SERIES 4.3L (VIN W)

Throttle Position (TP) sensor provides a voltage signal that changes relative to throttle blade angle. Signal voltage will vary from about .5 volt or at idle to about 4 volts at Wide Open Throttle (WOT). TP sensor signal is one of the most important inputs used by control module for fuel control and for most other control module control outputs. On non-adjustable TPS sensors, each time voltage drops below 1.25 volts and stops, control module assumes this value as 0 throttle angle and measures percent throttle from this point on.

Note. Test numbers refer to numbers on diagnostic chart.

1. Determines if conditions necessary to set code exist.
2. Simulates Code P0123 (high voltage). If control module recognizes high signal voltage, control module and wiring are okay.
3. Simulates a high signal voltage. Checks signal circuit for an open.

Tech 1 displays throttle position in volts. With closed throttle, ignition on, or at idle, voltage should be .45-.85 volt. If voltage is not as specified, replace TP sensor. Check for

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
2. Intermittent Test Monitor TP sensor voltage display on Tech 1 while moving related connectors and wiring harness. If failure is induced, display will change. This may assist in isolating location of malfunction.
3. TP Sensor Scaling Observe TP sensor voltage display while depressing accelerator pedal with engine stopped and ignition on. Display should vary from closed throttle TP sensor voltage when throttle is closed (less than 1.25 volts) to greater than 4.5 volts when throttle is held at WOT position.

CODE P1133, OXYGEN SENSOR INSUFFICIENT ACTIVITY "S" & "T" SERIES 4.3L (VIN W) BANK 1, SENSOR 1

Control module supplies and monitors a voltage of about .45 volt between control module HO2S low reference circuit and HO2S signal circuit. If read with a 10-megohm DVOM, voltage may read as low as .32 volt. HO2S varies voltage within a range of about 1.0 volt (rich) to .10 volt (lean). Sensor is like an open circuit and produces no voltage when less than about 600°F (360°C). Open loop operation is caused by Codes P0131, P0132, or an open or cold sensor. Code P0132 will cause system to operate in open loop. Control module will illuminate Malfunction Indicator Light (MIL).

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks for other codes.
2. Checks for physical damage to engine components and converter.
3. Checks fuel injector and isolates sensor.

Check sensor heater operation.

Code P1133 Flow Chart ("S" & "T" Series 4.3L (VIN W) Bank 1, Sensor 1) Oxygen Sensor Insufficient Activity. Scheme 282

CODE P1221, FUEL PUMP SECONDARY SYSTEM CIRCUIT LOW "S" & "T" SERIES 4.3L (VIN W)

Status of fuel pump circuit is monitored by control module and is used to compensate fuel delivery based on system voltage. Signal is also used to store a code if fuel pump relay is defective or fuel pump voltage is lost while engine is running. There should be about 12 volts on fuel pump signal circuit for at least 2 seconds after ignition is turned on, or when reference pulses are being received by control module.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks to see if code was set as the result of a hard or intermittent condition.
2. Checks fuel pump relay.
3. Checks relay power and ground supply circuit.
4. Checks fuel pump relay control circuit connections at relay and control module.
5. Checks oil pressure switch and connections.

Fuel pump should run for 2 seconds and then turn off if control module senses no reference (RPM) pulses. Fuel pump relay is located in glove box.

Code P1221 Schematic ("S" & "T" Series 4.3L - VIN W) Fuel Pump Secondary System Circuit Low. Scheme 283

Code P1221 Flow Chart ("S" & "T" Series 4.3L - VIN W) Fuel Pump Secondary System Circuit Low. Scheme 284

CODE P1222, FUEL PUMP VOLTAGE LOW - "S" & "T" SERIES 4.3L (VIN Z)

Status of fuel pump circuit is monitored by control module and is used to compensate fuel delivery based on system voltage. Signal is also used to store a code if fuel pump relay is defective or fuel pump voltage is lost while engine is running. There should be about 12 volts on fuel pump signal circuit for at least 2 seconds after ignition is turned on, or when reference pulses are being received by control module.

Fuel pump should run for 2 seconds and then turn off if control module senses no reference (RPM) pulses. Fuel pump relay is located in convenience center.

Code P1222 Schematic ("S" & "T" Series 4.3L - VIN Z) Fuel Pump Voltage Low. Scheme 285

CODE P1345, CRANKSHAFT/CAMSHAFT SENSOR CORRELATION ERROR "S" & "T" SERIES 4.3L (VIN W)

Connect scan tester to vehicle. Start engine and observe engine RPM. If RPM fluctuate up to twice desired RPM, replace faulty crankshaft sensor and retest. If RPM does not fluctuate up to twice desired RPM, check for proper installation of distributor.

CODE P1351, EST OUTPUT HIGH - "S" & "T" SERIES 4.3L (VIN W)

Note. Test numbers refer to numbers on diagnostic chart.

1. Determines if code is intermittent.
2. With engine not running or cranking, there should be no voltage on IC line.
3. Determines if IC signal from engine control module is available at ignition coil module.
4. Remaining tests begin to check that coil module circuitry is okay.

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside insulation. Check for

1. Poor Connection Or Damaged Harness Inspect engine and ignition control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection and damaged harness.
2. Intermittent Test Monitor a DVOM connected between affected terminal and ground while moving related connectors and wiring harness. If failure is induced, voltage reading will change.

Code P1351 Schematic ("S" & "T" Series 4.3L - VIN W) EST Output High. Scheme 286

Code P1351 Flow Chart ("S" & "T" Series 4.3L - VIN W) EST Output High. Scheme 287

CODE P1361, EST NOT TOGGLING - "S" & "T" SERIES 4.3L (VIN W)

Note. Test numbers refer to numbers on diagnostic chart.

1. If engine starts, code is intermittent.
2. Determines if IC signal from engine control module is available at ignition coil module.
3. Remaining tests begin to check that coil module circuitry is okay.

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside insulation. Check for

1. Poor Connection Or Damaged Harness Inspect engine and ignition control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection and damaged harness.
2. Intermittent Test Monitor a DVOM connected between affected terminal and ground while moving related connectors and wiring harness. If failure is induced, voltage reading will change.

Code P1361 Flow Chart ("S" & "T" Series 4.3L - VIN W) EST Not Toggling. Scheme 288

CODE P1406, EGR VALVE PINTLE POSITION - "S" & "T" SERIES 4.3L (VIN W) (1 OF 2)

Control module constantly monitors linear EGR valve pintle position sensor to ensure valve is responding properly to commands from control module.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks for an EGR valve sticking partially open or an incorrect pintle position sensor feedback signal.
2. Ensures control module is capable of controlling EGR pintle position and is reading pintle position feedback signal properly.
3. Checks linear EGR high and low circuits.

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside insulation. Check for

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection and damaged harness.
2. Intermittent Test If connections and harness are okay, monitor a DVOM connected between affected terminal and ground while moving related connectors and wiring harness. If failure is induced, voltage reading will change.

Code P1406 Flow Chart 1 Of 2 ("S" & "T" Series 4.3L - VIN W) EGR Valve Pintle Position. Scheme 289

CODE P1406, EGR VALVE PINTLE POSITION - "S" & "T" SERIES 4.3L (VIN W) (2 OF 2)

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks will diagnose an incorrect EGR pintle position sensor input signal. This test verifies a 5 volts reference at EGR valve.
2. Ensures position signal circuit is not shorted to ground.
3. Checks position signal circuit for an open.

Code P1406 Flow Chart 2 Of 2 ("S" & "T" Series 4.3L - VIN W) EGR Valve Pintle Position. Scheme 290

CODE P1441, EVAPORATIVE SYSTEM CONTINUOUS PURGE "S" & "T" SERIES 4.3L (VIN W)

Canister purge is controlled by a solenoid valve which allows ported vacuum to purge canister when energized by control module. A vacuum switch in purge line is used to detect when system is being purged. Control module supplies and monitors a 5-volt reference to switch. If switch is closed (no purge detected-low monitored voltage) when control module is commanding purge, code will be set.

Note. Test numbers refer to numbers on diagnostic chart.

1. A clicking noise from solenoid confirms ability of control module to energize solenoid.
2. With ignition on, engine off, no purge vacuum should exist.
3. Checks for faulty solenoid, wiring or control module.
4. Determines if vacuum switch, wiring or control module is at fault.
5. Determines if wiring or control module is faulty.
6. If 5-volt reference was not available to vacuum switch and no other codes are set, connection is faulty or circuit is open.

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside insulation. Check for

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection and damaged harness.
2. Intermittent Test If connections and harness are okay, monitor a DVOM connected between affected terminal and ground while moving related connectors and wiring harness. If failure is induced, voltage reading will change.

Code P1441 Flow Chart ("S" & "T" Series 4.3L - VIN W) Evaporative System Continuous Purge. Scheme 291

CODE P1442, EVAPORATIVE SYSTEM VACUUM SWITCH STATIC TEST MALFUNCTION - "S" & "T" SERIES 4.3L (VIN W)

Note. Test numbers refer to numbers on diagnostic chart.

1. Verifies proper purge switch operation.
2. Checks purge switch, diagnostic switch circuit and control module.
3. Checks switch ground circuit and control module.

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside insulation. Check for

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection and damaged harness.
2. Intermittent Test If connections and harness are okay, monitor a DVOM connected between affected terminal and ground while moving related connectors and wiring harness. If failure is induced, voltage reading will change.

Code P1442 Flow Chart ("S" & "T" Series 4.3L - VIN W) Evaporative System Vacuum Switch Static Test Malfunction. Scheme 292

CODE P1508/1509, IDLE SPEED LOW/HIGH - "S" & "T" SERIES 4.3L (VIN W)

Note. Test numbers refer to test numbers on diagnostic chart.

1. IAC driver is used to extend and retract IAC valve. Movement is verified by changing engine speed. If no engine speed change occurs, valve can be retested when removed from throttle body.
2. Step checks IAC circuits. Each light on node light should flash Red and Green, while IAC valve is cycled. While color sequence is not important, if either light is off or does not flash Red and Green, check circuits beginning with poor terminal contacts.

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside insulation. Check for

1. Poor Connection Or Damaged Harness Inspect control module harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection and damaged harness.
2. Intermittent Test If connections and harness are okay, monitor a DVOM connected between affected terminal and ground while moving related connectors and wiring harness. If failure is induced, voltage reading will change.

CODE P1632, SYSTEM VOLTAGE HIGH - "S" & "T" SERIES 4.3L (VIN Z)

Note. Test numbers refer to numbers on diagnostic chart.

1. Normal battery voltage is 9.6-16 volts.
2. Checks if high voltage reading is due to generator or control module.
3. Checks to see if generator is faulty under load condition.

During time failure is present, all control module outputs will be disengaged to protect hardware.

Code P1632 Schematic ("S" & "T" Series 4.3L - VIN Z) System Voltage High. Scheme 293

Code P1632 Flow Chart ("S" & "T" Series 4.3L - VIN Z) System Voltage High. Scheme 294

CODE P1810, TRANSMISSION RANGE SWITCH ASSEMBLY CIRCUIT "S" & "T" SERIES 4.3L (VIN W)

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks indicated range to actual manual valve position.
2. Checks for correct voltage from control module to transmission external connector.
3. Checks for a short to ground or an open in any one of the 3 circuits.

Check scan tester accompanying value combinations chart. Check all wiring connectors for proper connections and tension.

VALUE COMBINATION CHART

Code P1810 Flow Chart ("S" & "T" Series 4.3L - VIN W) Transmission Range Switch Assembly Circuit. Scheme 295

CODE P1812, TRANS FLUID OVERTEMP - "S" & "T" SERIES 4.3L (VIN W)

Transmission fluid temperature sensor is a thermistor which controls signal voltage to control module. Control module applies and monitors voltage to sensor. When transmission fluid is cold, sensor resistance is high; therefore, control module will see high signal voltage. As transmission fluid warms, sensor resistance and voltage will drop. At normal transmission operating temperature of 212°F (100°C), voltage will be about 1.5-2.0 volts.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks for skewed sensor or shorted circuit.
2. Simulates Code P0713.

Check harness routing for a potential short to ground in signal circuit. Scan tester displays transmission fluid temperature in degrees Centigrade. After transmission is running, temperature display should rise steadily to about 100°C then stabilize. Test transmission sensor at various temperature levels to determine if sensor is out of calibration. See TRANSMISSION SENSOR - TEMP TO RESISTANCE chart. An out-of-calibration sensor could result in delayed shifts or TCC enabled complaint.

TEMPERATURE-TO-RESISTANCE VALUES (1)

Code P1812 Flow Chart ("S" & "T" Series 4.3L - VIN W) Transmission Fluid Overtemp. Scheme 296

CODE P1860, TCC PWM SOLENOID ELECTRICAL CIRCUIT "S" & "T" SERIES 4.3L (VIN W)

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks if control module is commanding solenoid on.
2. This test checks for voltage to solenoid.

Check all connections, especially those at transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

Code P1860 Schematic ("S" & "T" Series 4.3L - VIN W) TCC PWM Solenoid Electrical Circuit. Scheme 297

Code P1860 Flow Chart ("S" & "T" Series 4.3L - VIN W) TCC PWM Solenoid Electrical Circuit. Scheme 298

CODE P1864, TCC SOLENOID ELECTRICAL CIRCUIT "S" & "T" SERIES 4.3L (VIN W)

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks control module's ability to control solenoid.
2. Checks power supply to TCC solenoid.

Some slight TCC slip is normal. An open in ignition feed circuit will cause multiple codes to set. A short to ground in TCC circuit may also cause Code P0742 to set.

Code P1864 Schematic ("S" & "T" Series 4.3L - VIN W) TCC Solenoid Electrical Circuit. Scheme 299

Code P1864 Flow Chart ("S" & "T" Series 4.3L - VIN W) TCC Solenoid Electrical Circuit. Scheme 300

CODE P1870, TRANSMISSION COMPONENT SLIPPING "S" & "T" SERIES 4.3L (VIN W)

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks indicated range signal to actual selected range. A faulty switch could set this code.
2. Checks TCC for slippage while in a commanded lock-up state.

An intermittent incorrect engine speed signal lasting more than 10 seconds could set this code. A 1-2 (A) shift solenoid stuck off or a 2-3 (B) shift solenoid stuck on could set this code.

Code P1870 Flow Chart ("S" & "T" Series 4.3L - VIN W) Transmission Component Slipping. Scheme 301

CODE P1886, 3-2 SOLENOID ELECTRICAL CIRCUIT "S" & "T" SERIES 4.3L (VIN W)

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks function of 3-2 control solenoid and internal transmission harness.
2. Checks power to 3-2 solenoid from ignition through fuse.

Check all connections at transmission connector. An open ignition feed circuit will cause multiple codes to set.

Code P1886 Schematic ("S" & "T" Series 4.3L - VIN W) 3-2 Solenoid Electrical Circuit. Scheme 302

Code P1886 Flow Chart ("S" & "T" Series 4.3L - VIN W) 3-2 Solenoid Electrical Circuit. Scheme 303

SUMMARY

If hard fault codes are not present and driveability symptoms or intermittent codes exist, proceed to TESTS W/O CODES - GASOLINE article for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.), or intermittent diagnostic procedures.

Wiring Diagram (Astro & Safari 4.3L VIN W - 1 Of 2). Scheme 304

Wiring Diagram (Astro & Safari 4.3L VIN W - 2 Of 2). Scheme 305

Wiring Diagram ("C" & "K" Series Pickup, Sierra, Suburban, Tahoe & Yukon - 4.3L With A/T - 1 Of 3). Scheme 306

Wiring Diagram ("C" & "K" Series Pickup, Sierra, Suburban, Tahoe & Yukon - 4.3L With A/T - 2 Of 3). Scheme 307

Wiring Diagram ("C" & "K" Series Pickup, Sierra, Suburban, Tahoe & Yukon - 4.3L With A/T - 3 Of 3). Scheme 308

Wiring Diagram ("C" & "K" Series Pickup & Sierra - 4.3L With M/T - 1 Of 3). Scheme 309

Wiring Diagram ("C" & "K" Series Pickup & Sierra - 4.3L With M/T - 2 Of 3). Scheme 310

Wiring Diagram ("C" & "K" Series Pickup & Sierra - 4.3L With M/T - 3 Of 3). Scheme 311

Wiring Diagram ("P" Series - 4.3L With M/T - 1 Of 2). Scheme 312

Wiring Diagram ("P" Series - 4.3L With M/T - 2 Of 2). Scheme 313

Wiring Diagram ("P" Series - Commercial Van/Motorhome - 4.3L With 4L80-E - 1 Of 3). Scheme 314

Wiring Diagram ("P" Series - Commercial Van/Motorhome - 4.3L With 4L80-E - 2 Of 3). Scheme 315

Wiring Diagram ("P" Series - Commercial Van/Motorhome - 4.3L With 4L80-E - 3 Of 3). Scheme 316

Wiring Diagram ("S" & "T" Blazer, Jimmy, Pickup & Sonoma - 4.3L VIN W W/PCM - 1 Of 2). Scheme 317

Wiring Diagram ("S" & "T" Blazer, Jimmy, Pickup & Sonoma - 4.3L VIN W W/PCM - 2 Of 2). Scheme 318

Wiring Diagram ("S" & "T" Blazer, Jimmy, Pickup & Sonoma - 4.3L VIN W W/VCM - 1 Of 3). Scheme 319

Wiring Diagram ("S" & "T" Blazer, Jimmy, Pickup & Sonoma - 4.3L VIN W W/VCM - 2 Of 3). Scheme 320

Wiring Diagram ("S" & "T" Blazer, Jimmy, Pickup & Sonoma - 4.3L VIN W W/VCM - 3 Of 3). Scheme 321

Wiring Diagram ("S" & "T" Blazer, Jimmy, Pickup & Sonoma - 4.3L VIN Z W/PCM - 1 Of 2). Scheme 322

Wiring Diagram ("S" & "T" Blazer, Jimmy, Pickup & Sonoma - 4.3L VIN Z W/PCM - 2 Of 2). Scheme 323

Wiring Diagram ("S" & "T" Blazer, Jimmy, Pickup & Sonoma - 4.3L VIN Z W/VCM - 1 Of 2). Scheme 324

Wiring Diagram ("S" & "T" Blazer, Jimmy, Pickup & Sonoma - 4.3L VIN Z W/VCM - 2 Of 2). Scheme 325

Wiring Diagram (Van - 4.3L With 4L60-E - 1 Of 3). Scheme 326

Wiring Diagram (Van - 4.3L With 4L60-E - 2 Of 3). Scheme 327

Wiring Diagram (Van - 4.3L With 4L60-E - 3 Of 3). Scheme 328

Wiring Diagram (Van - 4.3L With 4L80-E - 1 Of 3). Scheme 329

Wiring Diagram (Van - 4.3L With 4L80-E - 2 Of 3). Scheme 330

Wiring Diagram (Van - 4.3L With 4L80-E - 3 Of 3). Scheme 331

See also:
• BASIC TESTING - 4.3L
• TESTS W/O CODES - GASOLINE
• SYSTEM/COMPONENT TESTS - 4.3L
• MODEL IDENTIFICATION
• READING TROUBLE CODES
• RETRIEVING CODES (NON-SCAN)
• SCAN TESTER USAGE
• CODE 13
• CODE 14
• CODE 15
• CODE 16
• CODE 21
• CODE 22
• CODE 23
• CODE 24
• CODE 25
• CODE 28
• CODE 32
• CODE 32
• CODE 32
• CODE 33
• CODE 34
• CODE 35
• CODE 36
• CODES 37/38
• CODE 39
• CODE 42
• CODE 43
• CODE 43
• CODE 43
• CODE 44
• CODE 45
• CODE 51
• CODE 52
• CODE 52/53
• CODE 54
• CODE 55
• CODE 58
• CODE 59
• CODE 66
• CODE 67
• CODE 68
• CODE 69
• CODE 72
• CODE 73
• CODE 74
• CODE 75
• CODE 79
• CODE 81
• CODE 82
• CODE 83
• CODE 85
• CODE 86
• CODE 87
• CODE P0106
• CODE P0107
• CODE P0108
• CODE P0112
• CODE P0113
• CODE P0117
• CODE P0118
• CODE P0121
• CODE P0122
• CODE P0123
• CODE P0125
• CODE P0131
• CODE P0132
• CODE P0133
• CODE P0134
• CODE P0135
• CODE P0137
• CODE P0138
• CODE P0139
• CODE P0140
• CODE P0141
• CODE P0171
• CODE P0171
• CODE P0172
• CODE P0172
• CODE P0300
• CODE P0301-306
• CODE P0320
• CODE P0325/0326
• CODE P0327
• CODE P0328
• CODE P0337/0338
• CODE P0340
• CODE P0400
• CODE P0401
• CODE P0420
• CODE P0441
• CODE P0500
• CODES P0502/0503
• CODE P0506/0507
• CODE P0560
• CODE P0703
• CODE P0712
• CODE P0713
• CODE P0742
• CODE P0748
• CODE P0753
• CODE P0756
• CODE P0758
• CODE P1106
• CODE P1107
• CODE P1111
• CODE P1112
• CODE P1114
• CODE P1115
• CODE P1121
• CODE P1122
• CODE P1133
• CODE P1221
• CODE P1222
• CODE P1345
• CODE P1351
• CODE P1361
• CODE P1406
• CODE P1441
• CODE P1442
• CODE P1508/1509
• CODE P1632
• CODE P1810
• CODE P1812
• CODE P1860
• CODE P1864
• CODE P1870
• CODE P1886
• SCAN TESTER DATA