Engine Controls - Tests W/codes - 4.3L: Other Chevrolet Pickup K3500

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

Hard Failures

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 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 ECM 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 appropriate TESTS W/O CODES article in this section below.

1. H - TESTS W/O CODES (for Astro & Safari)
2. H - TESTS W/O CODES (for "S" & "T" Series)
3. H - TESTS W/O CODES - GASOLINE (for "C" & "K" Series)
4. H - TESTS W/O CODES - GASOLINE (For "G" & "P" Series)

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.

1. Turn ignition on with engine off. Malfunction Indicator Light (MIL) should glow. Locate Data Link Connector (DLC), attached to ECM 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 appropriate I - SYS/COMP TESTS article in this section below. Turn ignition on. Insert jumper wire from terminal "B" (diagnostic test terminal) to terminal "A" (ground) of DLC. (Scheme 85) I - SYSTEM/COMPONENT TESTS (for Astro & Safari) I - SYSTEM/COMPONENT TESTS (for "S" & "T" Series) I - SYSTEM/COMPONENT TESTS - GASOLINE (for "C" & "K" Series) I - SYSTEM/COMPONENT TESTS - GASOLINE (For "G" & "P" Series) 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 appropriate BASIC TESTING article in the ENGINE PERFORMANCE section below. F - BASIC TESTING (for Astro & Safari) F - BASIC TESTING (for "S" & "T" Series) F - BASIC TESTING - GASOLINE (for "C" & "K" Series) F - BASIC TESTING - GASOLINE (For "G" & "P" Series)
2. Malfunction Indicator Light (MIL) should flash codes. Each code is flashed 3 times. If codes do not flash, perform DIAGNOSTIC CIRCUIT CHECK in appropriate BASIC TESTING article in this section above. To exit diagnostic mode, turn ignition off and remove jumper wire from DLC.

Scheme 85

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 appropriate BASIC TESTING article in this section as listed below.

1. F - BASIC TESTING (for Astro & Safari)
2. F - BASIC TESTING (for "S" & "T" Series)
3. F - BASIC TESTING - GASOLINE (for "C" & "K" Series)
4. F - BASIC TESTING - GASOLINE (For "G" & "P" Series)

SCAN TESTER USAGE

Note. Before connecting scan tester, check diagnostic system and ensure accurate information is received by scan tester. Perform DIAGNOSTIC CIRCUIT CHECK in appropriate BASIC TESTING article in this section below. If vehicle does not pass diagnostic circuit check, information received by scan tester may be invalid.

1. F - BASIC TESTING (for Astro & Safari)
2. F - BASIC TESTING (for "S" & "T" Series)
3. F - BASIC TESTING - GASOLINE (for "C" & "K" Series)
4. F - BASIC TESTING - GASOLINE (For "G" & "P" Series)

The scan tester is a specialized tester which can diagnose on-board computer control systems by providing almost instant 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 table.

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 ECM. 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. 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 table.

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.

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

CODE 13, OPEN OXYGEN SENSOR CIRCUIT

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 4.3L engines, 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, ECM 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.

CODE 14, COOLANT SENSOR SIGNAL VOLTAGE LOW

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. Code 14 sets if signal voltage indicates a coolant temperature greater than 275°F (135°C) for more than 6 seconds.
2. This simulates conditions for a Code 15. If scan tester displays a low temperature, ECM and wiring are not at fault.

CODE 15, COOLANT SENSOR SIGNAL VOLTAGE HIGH

As engine warms, sensor resistance reduces and voltage drops. At normal operating temperature, voltage signal will be about 1.5-2.0 volts at ECM coolant sensor signal terminal. If sensor signal circuit opens, ECM 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. Code 15 will set if engine is running for more 50 seconds and signal voltage indicates a coolant temperature less than -22°F (-30°C) for more than 30 seconds.
2. This simulates conditions for a Code 14. If ECM recognizes grounded circuit and displays a high temperature, ECM and wiring are okay.
3. This determines if problem is ECM or wiring. There should be 5 volts present at sensor when measured with a DVOM.

CODE 16, VSS BUFFER FAULT - "G", "L", "M", "S" & "T" SERIES

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 PCM
4. Tests for faulty connections and a faulty VSS buffer

CODE 21, TPS SIGNAL VOLTAGE HIGH

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, ECM 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. Code 21 will set if TPS voltage is greater than 2.5 volts 2-10 seconds with engine running.
2. This test simulates conditions for Code 22. If ECM recognizes low voltage signal and sets Code 22, ECM and power and signal circuits are not at fault.
3. This step isolates a faulty sensor, ECM or an open ground circuit.

CODE 22, TPS SIGNAL VOLTAGE LOW

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. Code 22 will set if engine is running and TPS voltage is less than .2 volt for 2-4 seconds.
2. This simulates Code 21. If ECM recognizes a high voltage signal and sets Code 21, ECM and wiring are not at fault. Replace TPS.
3. This simulates a high voltage signal to check for on open TPS signal circuit.

CODE 23, INTAKE AIR TEMPERATURE LOW

ECM supplies and monitors a voltage signal (4-6 volts) to sensor. When temperatures are low, sensor resistance is high and ECM will see a high-monitored voltage signal. As temperature increases, sensor resistance decreases and voltage sensed by ECM 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. Code 23 will set if engine is running for one minute, MAT sensor temperature is less than -31°F (-35°C) for 12 seconds and speed sensor signal is not present.
2. This simulates conditions for a Code 25. If scan tester displays a high temperature, ECM and wiring are not at fault.
3. This checks for continuity of sensor signal and ground circuits.

CODE 24, VSS - "P" SERIES M/T & "S" & "T" SERIES UTIL VEH M/T

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 PCM. 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 VSS voltage at PCM.
2. Test checks VSS circuit at buffer module.
3. Test verifies VSS signal at sensor.

CODE 24, VSS - 4.3L "T" SERIES PICKUP M/T

Note. The VSS buffer is an internal part of the Digital Ratio Adapter Controller (DRAC) and the terms are used interchangeably.

ECM applies and monitors 12 volts on VSS signal circuit. Circuit is connected to VSS buffer which alternately grounds this circuit when it is receiving voltage pulses from Vehicle Speed Sensor (VSS). Scan tester reading should closely match speedometer reading.

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

1. Checks to see if code is set as a result of a hard failure or an intermittent condition.
2. This determines if DRAC is receiving AC signal from VSS.
3. This test monitors DRAC voltage on circuit No. 437. With wheels turning, pulsing voltage should be present. Voltage variation will be greater at a low speed to an average of 4-6 volts at about 20 MPH.

CODE 24, VSS TRANSMISSION OUTPUT SIGNAL - "G", "L" & "M" SERIES, "P" SERIES M/T & "S" & "T" SERIES A/T

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

CODE 25, INTAKE AIR TEMPERATURE HIGH

ECM applies and monitors a voltage signal (4-6 volts) to MAT sensor. When manifold air is cold, sensor resistance is high and ECM sees a high signal voltage. As air warms, resistance decreases and voltage sensed by ECM 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. Code 25 will set if a VSS signal is present (2.5L) and monitored MAT sensor temperature is greater than 275°F (135°C).
2. This simulates conditions for a Code 23. If scan tester displays a low temperature, ECM and wiring are not at fault.

CODE 32, EGR CIRCUIT ERROR WITH LINEAR EGR

ECM 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. ECM 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 ECM control circuit by jumpering across harness terminals with a test light and grounding ALDL (DLC) test terminal "B" to command EGR on.

CODE 32, EGR CIRCUIT ERROR - "P" SERIES 4.3L M/T

ECM controls a solenoid regulating vacuum to EGR valve. Normally closed solenoid prevents vacuum from passing until it is energized by ECM. 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. When test terminal "B" of ALDL connector is grounded, EGR solenoid should be energized, allowing vacuum to EGR valve. Vacuum should hold.
2. When jumper wire is removed from terminal "B", 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.

CODE 32, EGR CIRCUIT ERROR - MODELS USING MANIFOLD VACUUM, 2-TERMINAL SOLENOID & BACKPRESSURE EGR

PCM controls a solenoid regulating vacuum to EGR valve. Normally closed solenoid prevents vacuum from passing until it is energized by PCM. 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.

CODE 32, EGR CKT ERROR MODELS USING PORTED VACUUM & EVRV SOL

PCM controls a solenoid that regulates vacuum to EGR valve. The normally closed solenoid prevents vacuum from passing until it is energized by PCM. 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.

ECM 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 (approximately 1/4 throttle) will produce a large and stable enough reading to determine if ECM is commanding system on.

CODE 33, MAP SENSOR SIGNAL VOLTAGE HIGH

Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). If MAP sensor fails, ECM 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 ECM recognizes and indicates low MAP signal, ECM and 5-volt reference and MAP signal circuits are not at fault.

CODE 34, MAP SENSOR SIGNAL VOLTAGE LOW

Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). If MAP sensor fails, ECM 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, ECM or wiring. If ECM recognizes and indicates high MAP signal, ECM and wiring are okay.
3. Scan tester may not display 12 volts. The important thing is that the ECM recognizes voltage as greater than 4 volts (high MAP voltage signal), indicating ECM and MAP signal circuit are not at fault.

CODE 35, IDLE SPEED ERROR

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. Checks IAC movement quality from step 1). Between 700-1500 RPM, engine speed should change smoothly with each tester light flash while extending or retracting. If IAC valve is retracted beyond control range (about 1500 RPM), it may take many flashes in extend position before engine speed reduces. This is normal on some engines. Fully extending IAC may cause engine to stall. This may be normal.
3. Steps 1) and 2) verified proper IAC valve operation. This 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.

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 86

Code 35 Flow Chart (All Models) Idle Speed Error. Scheme 87

CODE 36, IDLE SPD ACTUATOR FAULT - "C", "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.

CODE 42 - IGNITION CONTROL

Code 42 indicates ECM has seen an open or short to ground in the 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 ECM and not an intermittent problem in IC and by-pass circuits.

CODE 43, KNOCK SENSOR (DUAL SENSORS) - "S" & "T" SERIES

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

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

CODE 43, KNOCK SENSOR W/O SPARK MODULE (SINGLE SENSOR) - "C", "G", "K" & "P" SERIES A/T

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 circuit No. 496 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.

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 ECM as a "no knock" signal if ECM 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 44, LEAN EXHAUST INDICATION

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".

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 appropriate BASIC TESTING article in this section below.

1. F - BASIC TESTING (for Astro & Safari)
2. F - BASIC TESTING (for "S" & "T" Series)
3. F - BASIC TESTING - GASOLINE (for "C" & "K" Series)
4. F - BASIC TESTING - GASOLINE (For "G" & "P" Series)

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 appropriate TESTS W/O CODES article in this section below.

1. H - TESTS W/O CODES (for Astro & Safari)
2. H - TESTS W/O CODES (for "S" & "T" Series)
3. H - TESTS W/O CODES - GASOLINE (for "C" & "K" Series)
4. H - TESTS W/O CODES - GASOLINE (For "G" & "P" Series)

CODE 44 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 44 Flow Chart (4.3L) Lean Exhaust Indication. Scheme 88

Code 44 Flow Chart. Scheme 89

CODE 45, RICH EXHAUST INDICATION

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 ECM is greater than .75 volt for 60 seconds or more.

Fuel Pressure High

If fuel pressure is too high, air/fuel ratio will be rich. For fuel pressure checking procedure, see appropriate BASIC TESTING article in this section below. The ECM can compensate for slight increases but if air/fuel ratio becomes too rich a Code 45 will be set.

1. F - BASIC TESTING (for Astro & Safari)
2. F - BASIC TESTING (for "S" & "T" Series)
3. F - BASIC TESTING - GASOLINE (for "C" & "K" Series)
4. F - BASIC TESTING - GASOLINE (For "G" & "P" Series)

Ignition Ground

If an open occurs at circuit No. 453, HEI induced electrical "noise" may result, causing simulated reference pulses picked up by ECM on EST harness reference line. Additional pulses result in a higher than actual engine speed signal. The ECM 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 ECM senses higher than normal manifold pressure (low vacuum) system can go rich. Disconnecting MAP sensor allows ECM to substitute a fixed value for the 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 ECM as a rich mixture, causing ECM 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 appropriate TESTS W/O CODES article in this section below.

1. H - TESTS W/O CODES (for Astro & Safari)
2. H - TESTS W/O CODES (for "S" & "T" Series)
3. H - TESTS W/O CODES - GASOLINE (for "C" & "K" Series)
4. H - TESTS W/O CODES - GASOLINE (For "G" & "P" Series)

CODE 45 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 45 Flow Chart (4.3L) Rich Exhaust Indication. Scheme 90

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 ECM.

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 ECM.

CODE 53, SYSTEM OVERVOLTAGE

This code indicates a basic charging system problem. Code 53 will set when voltage at ECM terminal is greater than 17.1 volts for 2 seconds. Check and repair charging system.

CODE 55, ECM/PCM ERROR

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

CODE 54, FUEL PUMP CIRCUIT

The status of fuel pump signal is monitored by the 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 Exc. 5.0L/5.7L M/T Fuel Pump Circuit. Scheme 91

Code 54 Schematic ("G" Series) Fuel Pump Circuit. Scheme 92

Code 54 Schematic ("L" & "M" Series & "S" & "T" Series Utility Vehicles M/T Fuel Pump Circuit. Scheme 93

Code 54 Schematic ("P" Series A/T) Fuel Pump Circuit. Scheme 94

Code 54 Schematic ("P" Series M/T) Fuel Pump Circuit. Scheme 95

Code 54 Schematic ("S" & "T" Series Pickup A/T) Fuel Pump Circuit. Scheme 96

Code 54 Flow Chart Fuel Pump Circuit. Scheme 97

CODE P0107, MAP SENSOR CKT - 4.3L "S" & "T" SERIES PICKUP M/T

The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). The VCM 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 the MAP sensor fails, VCM 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 VCM or wiring.
3. Tech-1 may not display 5 volts. The important item is that VCM recognized voltage as more than 4 volts, indicating that VCM and circuit are okay.

CODE P0108, MAP SENSOR CKT - 4.3L "S" & "T" SERIES PICKUP M/T

The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). The VCM 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 the MAP sensor fails, VCM 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 VCM recognizes change, VCM and circuits are okay.

CODE P0117, COOLANT TEMP. SENS - "S" & "T" SERIES PICKUP M/T

ECT sensor uses a thermistor to control signal voltage to PCM. PCM applies and monitors a 5 volts to ECT sensor. When engine coolant is cold, sensor (thermistor) resistance is high and PCM will sense a high signal voltage. As engine coolant warms up, sensor resistance becomes less and PCM voltage drops. With Code P0117 set, PCM will turn cooling fans on and use a default engine coolant temperature value based on run time. PCM 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. Determines if circuit is shorted to ground.

CODE P0118, COOLANT TEMP. SENSOR - 4.3L "S" & "T" SERIES PICKUP M/T

ECT sensor uses a thermistor to control signal voltage to PCM. PCM applies and monitors a 5 volts to ECT sensor. When engine coolant is cold, sensor (thermistor) resistance is high and PCM will sense a high signal voltage. As engine coolant warms up, sensor resistance becomes less and PCM voltage drops. With Code P0118 set, PCM will turn cooling fans on and use a default engine coolant temperature value based on run time. PCM 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 P0118 exist.
2. This test will determine if signal circuit is shorted to ground which will cause conditions for Code P0118.
3. This test will determine if circuit is open. If circuits are okay, check connections at PCM and ECT sensor.

CODE P0122, THROTTLE POS SENS - 4.3L "S" & "T" SERIES P/U M/T

The 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 PCM for fuel control and for most of the PCM control outputs.

With Code P0122 set, Tech 1 will continue to display the actual throttle sensor voltage, but ignore TP signal and read mass air flow value for other PCM controlled functions, PCM will not allow 4th gear, TCC or cruise control operation, 1st, 2nd, and 3rd allowed. PCM will illuminate Malfunction Indicator Light (MIL).

Note. Test numbers refer to numbers on diagnostic chart.

1. Determines if conditions necessary to set Code P0122 exist.
2. Simulates Code P0123 (high voltage). If PCM recognizes high signal voltage and sets Code P0123, PCM and wiring are okay.
3. Simulates a high signal voltage. Checks signal circuit for an open.

CODE P0123, THROTTLE POS SENS - 4.3L "S" & "T" SERIES P/U M/T

The Throttle Position (TP) sensor provides a voltage signal that changes relative to throttle blade angle. Signal voltage will vary from .20-.74 volt or at idle to approximately 5 volts at Wide Open Throttle (WOT). TP sensor signal is one of the most important inputs used by PCM for fuel control and for most of the PCM control outputs. On non-adjustable TPS sensors, each time voltage drops below 1.25 volts and stops, the VCM assumes this value as 0 throttle angle and measures percent throttle from this point on.

On 4.3L, throttle position will default to 19 percent and Tech-1 will not indicate fault value. PCM will illuminate Malfunction Indicator Light (MIL).

Note. Test numbers refer to numbers on diagnostic chart.

1. On 4.3L, if TP signal is greater than 1.25 volts, fault is present.
2. With TP sensor disconnected, TP sensor voltage should decrease and Code P0122 will set. This test verifies that PCM and wiring are okay.
3. Probing ground circuit with a test light check sensor ground circuit. A faulty sensor ground circuit will cause Code P0123 to set.

CODE P0139, HEATED O2 SENS OPEN CKT - 4.3L "S" & "T" SERIES PICKUP M/T

The VCM applies a bias voltage of about 450 mV between Purple and Tan wire terminals. The heated oxygen sensor varies the voltage about 1000 mV if exhaust is rich, to about 100 mV if exhaust is lean. Sensor is like an open circuit and applies no voltage when below 600°F (315°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 open or closed loop operation.
2. This checks oxygen sensor heating element.
3. This will determine if sensor is at fault.
4. For this test only use a high impedance DVM. This checks continuity of circuits.

CODE P0171, LEAN EXHAUST - 4.3L "S" & "T" SERIES PICKUP M/T

The VCM applies a bias voltage of about 450 mV between Purple and Tan wire terminals. The heated oxygen sensor varies the voltage about 1000 mV if exhaust is rich, to about 100 mV 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.

CODE P0172, RICH EXHAUST - 4.3L "S" & "T" SERIES PICKUP M/T

The VCM applies a bias voltage of about 4.5 volts between Purple and Tan wire terminals. The heated oxygen sensor varies the voltage about 1 volt if exhaust is rich, to about 0.1 volt if exhaust is lean. Code P0172 will set if oxygen sensor voltage remains high too long.

Note. Test numbers refer to numbers on diagnostic chart.

1. Oxygen sensor voltage above 0.69 volt indicates fault is still present.

CODE P0320, IGN CNTRL ERROR - 4.3L "S/T" SERIES PICKUP - M/T

When system is running on ignition control module, when no voltage is on the bypass line, the DI module grounds the ignition control signal. The VCM expects to sense no voltage on the IC line during this condition. If it senses voltage it sets a Code P0320 and will not go into the IC mode.

When RPM for IC is reached and bypass voltage is applied, the IC will no longer be grounded in the DI module, so IC voltage will vary. VCM will disable the IC mode if short test fails or default to bypass 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 DI module. If IC Tan/Black wire is shorted to ground, it will also read less than 500 ohms. This will be checked later.
3. As test light voltage touches Tan/Black wire circuit, the 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 the module switched.
4. If module did not switch, this step checks for IC circuit shorted to ground, bypass circuit open or faulty ignition module or connection.
5. This step confirms that code is set because of a faulty VCM and not an intermittent in circuitry.

CODE P0327, KNOCK SENSOR GROUNDED - 4.3L "S/T" PICKUP - M/T

Knock sensor circuit consists of 2 knock sensors connected in parallel and a single wire to VCM. Each knock sensor has a resistance value of about 8600 ohms. VCM 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 if generated by the sensors on top of the existing 2.5 volts. An AC voltage monitor in VCM will detect this voltage and provide a signal to begin retarding spark until knock diminishes.

CODE P0328, KNOCK SENSOR OPEN - 4.3L "S/T" PICKUP - M/T

Knock sensor circuit consists of 2 knock sensors connected in parallel and a single wire to VCM. Each knock sensor has a resistance value of about 8600 ohms. VCM 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 if generated by the sensors on top of the existing 2.5 volts. An AC voltage monitor in VCM 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. VCM 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 VCM.
3. Improperly installed sensor can prevent sensor from grounding to block.

CODE P0400, (EGR) ERROR - 4.3L "S" & "T" SERIES PICKUP M/T

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

The VCM 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. Vehicle must be driven during this test to ensure sufficient load to operate EGR.

CODE P0500, VSS CIRCUIT - 4.3L "S" & "T" SERIES PICKUP M/T

The 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 VCM. This pulsing action happens about 2000 times per mile and VCM will 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 VCM and VSS.

CODE P1222, FUEL PUMP VLTGE LOW - 4.3L "S/T" PICKUP - M/T

The status of the fuel pump circuit is monitored by the VCM and is used to compensate fuel delivery based on system voltage. Signal is also used to store a Code P1222 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 VCM.

SUMMARY

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

1. H - TESTS W/O CODES (for Astro & Safari)
2. H - TESTS W/O CODES (for "S" & "T" Series)
3. H - TESTS W/O CODES - GASOLINE (for "C" & "K" Series)
4. H - TESTS W/O CODES - GASOLINE (For "G" & "P" Series)