Engine Controls - Tests W/codes - V6: Other Chevrolet Pickup K1500

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 TESTS W/O CODES article in this section.

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 SYSTEM/COMPONENT TESTS article in this section. Turn ignition on. Insert jumper wire from terminal "B" (diagnostic test terminal) to terminal "A" (ground) of DLC. (Scheme 303) 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 article in the ENGINE PERFORMANCE Section.
2. Malfunction Indicator Light (MIL) should flash codes. Each code is flashed 3 times. If codes DO NOT flash, perform DIAGNOSTIC CIRCUIT CHECK (GASOLINE) in BASIC TESTING article in this section. To exit diagnostic mode, turn ignition off and remove jumper wire from DLC.

Scheme 303

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 article in this section.

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 article in this section. 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 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

SUMMARY

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

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

ECM uses coolant temperature sensor inputs in determining control of fuel delivery, engine timing (EST), idle (IAC) and converter clutch (TCC). As engine warms, sensor resistance reduces. At normal operating temperature, voltage signal will be about .5-1.1 volts (4.3L Turbo) or 1.5-2.0 volts (all other models) at ECM 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 3 seconds on 4.3L Turbo or 6 seconds on all other models. 2) This simulates conditions for a Code 15. If ECM recognizes open circuit by displaying 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 .5-1.1 volts (4.3L Turbo) or 1.5-2.0 volts (all other models) at ECM coolant sensor signal terminal. If sensor signal circuit opens, ECM will see -40°F (-40°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, SYSTEM VOLTAGE HIGH/LOW 3.8L

Control module monitors battery voltage on battery feed circuit. If control module detects battery voltage greater than 17 volts or less than 9 volts for more than 10 seconds, it will set a Code 16 in memory.

Note. Test number refers to number on diagnostic chart. Starting engine with battery charger connected may set Code 16.

1. Test alternator output to determine proper operation of voltage regulator. Increase engine speed to moderate level. Measure voltage across battery terminals. If reading is more than 17 volts or less than 9 volts, service alternator.

CODE 16, TRANSMISSION OUTPUT SPEED 4.3L "S" & "T" SERIES UTILITY VEHICLES WITH M/T & 4.3L WITH 4L60E TRANSMISSION

VSS signal buffer supplies 2002 pulses per mile to PCM on vehicle speed signal circuit. Code 16 will set if this signal is lost due to an open or grounded signal line.

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

1. This tests for battery voltage at VSS buffer.
2. This tests for proper ground path for VSS buffer.
3. This tests for VSS buffer signal to PCM.
4. This test checks for faulty connections and a faulty VSS buffer.

CODE 17, RPM SIGNAL PROBLEM 3.8L

Note. Test numbers refer to numbers on diagnostic chart.

1. Verifies spark reference circuit is not shorted to ground or open in ignition jumper harness.
2. If a window on harmonic balancer is lined up with 18X Hall Effect switch, ignition module will ground spark reference signal. Starter may have to be bumped several times to obtain a voltage reading.
3. Voltage reading should be lower than reading obtained with engine not running, indicating a pulsed reference signal.

CODE 18, CAM/CRANK ERROR 3.8L

During cranking, ignition module monitors dual crank sensor sync signal. Sync signal is used to determine correct cylinder pair to spark first. After synch signal has been processed by ignition module, module sends a fuel control reference pulse to PCM. When PCM receives this pulse, it will command all 6 injectors to fire for one priming shot of fuel. After priming, all injectors are held off for 2 crankshaft revolutions to allow cylinder to use fuel from priming shot. After firing, PCM begins to operate injectors in sequential mode based upon true camshaft position. PCM expects to see 6 fuel control pulses for each cam pulse. If sequence of these pulses is incorrect 10 times consecutively, Code 18 will set.

Note. Test numbers refer to numbers on diagnostic chart.

1. Determines if conditions necessary to set code still exist.
2. If 5 volts are not present at PCM harness connector terminal, cam sensor may be interfacing with magnet in camshaft sprocket. Bumping starter should correct this problem.
3. If a failure is induced in fuel control reference circuit, 5 volts on circuit should change when faulty wiring or connection is manipulated.

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 4.5 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. On 2.8L, Code 21 may set if MAP sensor signal less than 2 volts.
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 4.5 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, MAT SENSOR TEMP. LOW - 3.8L & 4.3L CPI

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 -22°F (-30°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 23, MAT SENSOR TEMP. LOW - 4.3L TURBO

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. A scan tester cannot be used to diagnose this fault due to the ECM transmitting a default value when code is set. Code will set due to an open sensor, wire, or connections. This step determines if the wiring and ECM are okay.
2. If resistance is greater than 25 k/ohms, replace sensor.

CODE 23 - MAT Sensor Temp. Low Schematic (4.3L Turbo). Scheme 304

CODE 23 - MAT Sensor Temp. Low Flowchart (4.3L Turbo). Scheme 305

CODE 24, VEHICLE SPEED SENSOR - 4.3L TURBO & 4.3L ("C", "K" & "P" SERIES) WITH MANUAL TRANSMISSION

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 circuit No. 437. Circuit No. 437 is connected to Digital Ratio Adapter Controller (DRAC) 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. Code 24 will set if voltage on circuit No. 437 is constant, engine speed is greater than 1200 RPM, vehicle speed is less than 2 MPH, vehicle is not in Park or Neutral and all conditions have been met for at least 5 seconds. These conditions are met during a road load deceleration.
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, VEHICLE SPEED SENSOR (VSS) 3.1L

ECM applies and monitors a 12-volt signal on circuit No. 437 to Vehicle Speed Sensor (VSS). VSS is connected to speed sensor buffer which alternately grounds and opens circuit No. 437 when wheels are turning. This pulsing action takes place about 2000 times per mile. Voltage level and pulses increase with vehicle speed. ECM converts pulsing voltage to MPH. ECM uses VSS information in calculations to determine vehicle adjustments. Scan tester reading should closely match speedometer reading when wheels are turning.

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

1. A Code 24 sets when MPH is less than 2 MPH, transmission is not in Park or Neutral, engine speed is greater than 1400 RPM, TPS is greater than 5 percent, circuit No. 437 voltage is constant and all of these conditions are met for 30 seconds. These conditions are met during a road load deceleration.
2. A steady 8-12 volts at ECM connector indicates VSS circuit is open or speed sensor is faulty. A voltage of less than one volt at IP connector indicates circuit No. 437 wire is shorted to ground. Disconnect vehicle speed sensor connector. If voltage is now greater than 10 volts, vehicle speed sensor buffer is faulty. If voltage remains less than 8 volts, circuit is grounded. If circuit is not grounded, check for faulty ECM connector or ECM. Before replacing ECM, PROM should be checked for correct application.

CODE 24, VEHICLE SPEED SENSOR (VSS) 3.8L

Speed sensor, which is a Permanent Magnet (PM) generator, provides control module with vehicle speed information. PM generator, mounted in transmission, produces a pulsing AC voltage signal whenever vehicle speed is greater than 3 MPH. Voltage level and pulses increase with vehicle speed. Control module converts pulsing voltage to MPH, which is used by control module to calculate vehicle adjustments.

Note. Test numbers refer to numbers on diagnostic chart.

1. If Code 36 is stored along with Code 24, go to that chart first. PM generator only produces a voltage signal if drive wheels are turning greater than 3 MPH.
2. Before replacing control module, MEM-CAL should be checked for correct application.

CODE 24, VEHICLE SPEED SENSOR 4.3L "S" & "T" SERIES PICKUP WITH MAN. TRANS. (EXCEPT TURBO) & ALL 4WD WITH 4L80-E TRANSMISSION

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 200 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 verifies VSS signal at sensor.

CODE 24, VEHICLE SPEED SENSOR 4.3L "S" & "T" SERIES UTILITY VEHICLES WITH MANUAL TRANSMISSION, 4.3L, 5.0L & 5.7L WITH 4L60E TRANSMISSION & ALL 2WD WITH 4L80-E TRANSMISSION

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

VSS output sensor circuit consists of a magnetic induction type speed sensor, Digital Ratio Adapter Controller (DRAC), and wiring. Gear teeth pressed on outside diameter of output carrier assembly induce an alternating current in sensor when drive wheels are turning. Signal is transmitted to instrument cluster DRAC where it is converted to a square wave signal which is passed on to PCM, cruise control, speedometer, and anti-lock brake system. 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 200 RPM for at least 1.5 seconds. If input speed sensor is not operational at start-up, this can cause VSS to read zero.

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

Preliminary Steps

Disconnect VSS buffer connector. Connect test light between terminals C8 and C9. Turn ignition on. If test light does not light, repair power or ground circuit to VSS buffer.

1. Test verifies VSS voltage at PCM.
2. Test verifies VSS signal to DRAC.
3. Test verifies VSS signal at sensor.

CODE 25, MAT SENSOR TEMP. HIGH - 3.8L & 4.3L CPI

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 302°F (150°C) for 2.5L or 275°F (135°C) for others.
2. This simulates conditions for a Code 23. If scan tester displays a low temperature, ECM and wiring are not at fault.

CODE 25, MAT SENSOR TEMP. LOW - 4.3L TURBO

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. A scan tester cannot be used to diagnose this fault due to the ECM transmitting a default value when code is set. If voltage is greater than 4 volts, wiring and ECM are okay.
2. If resistance is less than 100 ohms, replace sensor.

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

CODE 25, Flowchart, MAT Sensor Temp. Low (4.3L Turbo). Scheme 306

CODE 26, QUAD-DRIVER CIRCUIT 3.8L

Each PCM Quad-Driver Module (QDM) has a fault line which is monitored by PCM. PCM compares voltage values of fault line with acceptable values in PCM memory. If PCM senses values other than accepted values, a Code 26 will set.

Some QDM circuits will normally cycle between high and low, such as when brake pedal is depressed. Some scan testers may set a false Code 26 if engine is running, tester is installed and brake pedal is depressed for more than 30 seconds.

Note. Test number refers to number on diagnostic chart.

1. PCM does not know which controlled circuit set Code 26, so this chart checks each circuit to determine which is at fault. This step tests SERVICE ENGINE SOON light driver and circuit.
2. This determines which circuit is out of specification.

Note. Test number refers to number on diagnostic chart.

This determines if problem is circuit or component. Factory-installed PCM has an internal fuse and is unlikely to need replacement.

CODE 26, Flowchart, Quad Driver Circuit (3.8L)(2 of 2). Scheme 307

CODE 26, QUAD-DRIVER NO. 1 CIRCUIT - 4.3L "S" & "T" SERIES PICKUP WITH MANUAL TRANSMISSION

Each PCM Quad-Driver Module (QDM) has a fault line which is monitored by PCM. PCM compares voltage values of fault line with acceptable values in PCM memory. If PCM senses values other than accepted values, a Code 26 will set.

CODE 27, QUAD-DRIVER NO. 2 CIRCUIT - 4.3L "S" & "T" SERIES PICKUP WITH MANUAL TRANSMISSION

Each PCM Quad-Driver Module (QDM) has a fault line which is monitored by PCM. PCM compares voltage values of fault line with acceptable values in PCM memory. If PCM senses values other than accepted values, a Code 27 will set.

CODE 31, PARK/NEUTRAL SWITCH CIRCUIT - 3.8L

Note. Complete diagnosis of Code 31 for 3.8L requires GM Tech 1 scan tester, which is capable of indicating status of all 4 park/neutral switch positions. This chart does not apply to 3.8L (VIN L) "C", "E" and "H" body vehicles. Schematics for these models are supplied for reference purposes only.

Park/neutral switch contacts are part of neutral start switch. Contacts close to ground in Park or Neutral and open in Drive. Code 31 will set if park/neutral signal circuit indicates an open for 3 consecutive starts or if conditions occur as follows with ignition on and engine off.

1. Vehicle speed is zero MPH.
2. Gear selector switch indicates a gear other than Park or Neutral.
3. Gear selector status indicates an incorrect combination of LO and HI for more than 10 seconds.

Code 31 does not turn on SERVICE ENGINE SOON light.

Note. Test numbers refer to numbers on diagnostic chart.

1. This tests for proper operation of gear selector switch.
2. This tests for proper operation of gear selector switch.
3. Tests for an open of grounded circuit.

CODE 31, TURBO OVERBOOST - 4.3L TURBO

On turbocharged engines, exhaust gases pass from exhaust manifold through turbocharger, turning turbine blades. Compressor side of turbocharger also turns, pulling air through air filter and pushing it into intake manifold under pressure.

Wastegate is normally closed, but will open to by-pass exhaust gases when an overboost condition exists. This is determined through monitored MAP sensor signals. Under normal driving conditions, ECM energizes wastegate control (solenoid) valve. This blocks off intake manifold pressure from wastegate actuator diaphragm. When overboost condition exists, ECM will de-energize wastegate control (solenoid) valve, allowing intake manifold pressure to act upon actuator diaphragm, opening wastegate.

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

1. Code 31 will set when Code 33 is not set and excessive manifold boost pressure is sensed for 2 seconds. This step tests ability of actuator to open wastegate. Code 31 may be caused by circuit No. 471 shorted to ground, a sticking wastegate or actuator, control (solenoid) valve stuck in closed position, a cut or pinched hose, or a faulty ECM.
2. After pressure is applied to valve and then removed, actuator should slowly close wastegate. If not, vent in control (solenoid) valve could be plugged.
3. With ignition on and ALDL test terminal "B" grounded, control (solenoid) valve should be energized. This closes off manifold to wastegate actuator.
4. This checks electrical control portion of system. With ignition on and engine not running, solenoid should not be energized.

CODE 31, Schematic, Turbo Overboost (4.3L Turbo). Scheme 308

CODE 31, Flowchart, Turbo Overboost (4.3L Turbo). Scheme 309

CODE 32, EGR SYSTEM ERROR - 3.1L

EGR vacuum is regulated by an ECM-controlled, normally closed solenoid. ECM will turn EGR on and off (duty cycle) by grounding and ungrounding solenoid drive circuit. Duty cycle is calculated by ECM based on coolant temperature, airflow and engine RPM. ECM will check EGR operation when vehicle speed is greater than 50 MPH, MAP is within calibrated range, throttle angle is 8-30 percent and throttle angle is constant. With ignition on and engine stopped, EGR solenoid is de-energized. Grounding ALDL test terminal should energize solenoid, allowing vacuum to pass through to EGR valve.

Note. Test numbers refer to numbers on diagnostic chart.

1. To check for plugged intake passage, turn ignition off. Remove EGR valve from intake. Plug exhaust side passage with a suitable stopper. With intake side open, attempt to start engine. If engine runs at a very high idle (up to 3000 RPM is possible) or starts and stalls, EGR intake passage is not plugged. If engine starts and idles normally, intake passage is plugged. To check for plugged exhaust passage, with EGR valve still removed, plug intake passage with a suitable stopper. Start engine. If no exhaust gas is present, passage is plugged.
2. By grounding ALDL test terminal "B", EGR solenoid should energize, allowing vacuum to pass through solenoid. Vacuum may slowly bleed off. What is important is that vacuum passes through solenoid.
3. When the ALDL test terminal "B" is ungrounded, vacuum should bleed off through a vent in the solenoid valve. What is important is that vacuum no longer exists on EGR side of solenoid.
4. This test will determine if the electrical control part of the system is at fault or if the connector or solenoid valve is at fault.
5. The remaining test checks the ability of the EGR valve to interact with exhaust system. This is a negative backpressure EGR valve which should hold vacuum with engine off.
6. When engine is started, vacuum should vent and EGR valve should seat.

CODE 32, Schematic, EGR System Error (3.1L). Scheme 310

CODE 32, Flowchart, EGR System Error (3.1L) (1 of 2). Scheme 311

CODE 32, Flowchart, EGR System Error (3.1L) (2 of 2). Scheme 312

CODE 32, EGR SYSTEM ERROR - 4.3L "G" SERIES, 4.3L "C", "K" & "M" SERIES (NON-CALIFORNIA) WITH 4L60E TRANSMISSION & 4.3L "C", "G", "K" & "P" SERIES WITH 4L80-E TRANSMISSION

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. 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 SYSTEM ERROR 4.3L "S" & "T" SERIES UTILITY VEHICLES WITH MANUAL TRANSMISSION (EXCEPT CPI)

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 test terminal "B" of ALDL connector is grounded, EGR solenoid should be energized, allowing vacuum 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 32, EGR SYSTEM ERROR - 4.3L "C", "K" & "P" SERIES) & 4.3L "S" & "T" SERIES PICKUP (EXCEPT TURBO) WITH MANUAL TRANSMISSION

ECM controls a solenoid that regulates vacuum to EGR valve. The 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.

ECM checks EGR operation when vehicle speed is greater than 50 MPH, engine vacuum indicates cruise and throttle position are steady.

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 test terminal "B" of ALDL connector is grounded, EGR solenoid should be energized, allowing vacuum to EGR valve. Vacuum should hold.
2. Checks for plugged EGR passages. If passages are plugged, engine may have severe detonation on acceleration.
3. SERVICE ENGINE SOON light should also flash while diagnostic terminal is grounded. If light does not flash, this may indicate Quad-Driver has been damaged by low TCC circuit resistance.
4. 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 32, EGR SYSTEM ERROR - 4.3L TURBO

ECM controls a solenoid that regulates vacuum to EGR valve. The 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.

Diagnostic chart covers checks for entire EGR system. If no trouble is found while performing chart routine, fault is an intermittent electrical problem or a sticky EGR valve.

CODE 32, EGR SYSTEM ERROR - 4.3L 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 circuits No. 435 and 439 with a test light and grounding ALDL test terminal "B" to command EGR 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 34, MAF SENSOR SIGNAL VOLTAGE LOW - 3.8L

Code 34 is set when engine is running without MAF sensor signal for greater than 4 seconds. If Code 34 is set, control module will substitute a value for MAF based upon RPM, TPS and IAC monitored parameters.

Note. Test numbers refer to numbers on diagnostic chart.

1. Determines if code is result of an intermittent or hard failure.
2. Voltage reading at sensor harness terminal "A" of less than 4 volts or more than 6 volts indicates fault in circuit No. 492 or poor connections.
3. Verifies both ignition voltage and a good ground are available.

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 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION

CODE 35, Schematic, Idle Speed Error. Scheme 313

CODE 35, Flowchart, Idle Speed Error. Scheme 314

CODE 36, SHIFT CONTROL PROBLEM - 3.8L

4T60E transaxle is electronically shifted. Within transaxle are 4 solenoids. Solenoid "A" is used for 1st and 4th gear operation only. Solenoid "B" is used for 1st and 2nd gear operation. Remaining 2 solenoids are for TCC operation only. All PRNDL indications are ignored as far as transaxle shifting is concerned except manual low. Code 36 will set if solenoid "B" failed in OFF position, which will cause transaxle to be in 4th gear, and desired gear is 1st, TPS is greater than 40 percent, engine speed is greater than 2000 RPM, VSS is 3-20 MPH and Codes 21, 22 and 24 are not present. Code will also set if solenoid "B" failed in ON position, which will cause transaxle to be in 1st gear, and desired gear is 4th, PRNDL is in 3rd or 4th, TPS is greater than 10 percent and Codes 31, 21 and 22 are not present. Code 36 does not turn on SERVICE ENGINE SOON light.

CODE 38, BRAKE SWITCH CIRCUIT - 3.8L

Code 38 will set if Code 24 is not present, status at brake input terminal of control module has not changed from high to low and vehicle speed has been greater than 35 MPH and back to zero MPH a precalibrated number of times.

Note. Test numbers refer to numbers on diagnostic chart.

1. Jumpering brake switch determines if ECM and wiring for brake switch are okay.
2. Determines if brake switch is out of adjustment or is faulty.
3. Verifies voltage to brake switch.

CODE 38, KNOCK SENSOR CIRCUIT OPEN - 4.3L "S" & "T" SERIES PICKUP WITH MANUAL TRANSMISSION

Note. Test numbers refer to numbers on diagnostic chart.

1. VCM applies and monitors 5 volts on knock signal circuit. This voltage should be present on sensor leads with sensors disconnected.
2. Determines if wiring or knock sensor portion of the VCM's internal PROM (MEM-CAL) is faulty
3. An improperly installed knock sensor can prevent knock sensor from grounding to block.

CODE 39, TCC CIRCUIT - 3.8L

Code 39 will set when TCC is commanded on, vehicle speed is greater than 35 MPH, throttle angle is greater than 10 percent, vehicle is in 3rd or 4th gear, Engine RPM-to-vehicle speed indicates TCC is not locked and all conditions have existed for more than 10 seconds. Code 39 does not turn on SERVICE ENGINE SOON light.

Note. Test numbers refer to numbers on diagnostic chart.

1. With ALDL test terminal grounded, PCM provides a ground for solenoid. This step tests for power to solenoid.
2. Tests to see if problem is faulty solenoid, power supply or driver circuit.
3. Tests TCC solenoid driver circuit at PCM.

CODE 39, KNOCK SENSOR CIRCUIT SHORTED - 4.3L "S" & "T" SERIES PICKUP WITH MANUAL TRANSMISSION

Note. Test numbers refer to numbers on diagnostic chart.

1. VCM applies and monitors 5 volts on knock signal circuit. This voltage should be present on sensor leads with sensors disconnected.
2. Determines if wiring or knock sensor portion of the VCM's internal PROM (MEM-CAL) is faulty
3. An improperly installed knock sensor can prevent knock sensor from grounding to block.

CODE 41, CAM SENSOR CIRCUIT - 3.8L

Code 41 will set if engine is running and a cam sensor signal is not received by ECM for last 5 seconds.

Note. Test numbers refer to numbers on diagnostic chart.

1. Verifies proper operation of circuits No. 633, 644 and 645.
2. Tests circuit No. 630 from C(3)I module to ECM.
3. If camshaft gear magnet is interfacing with cam sensor, voltage reading will be zero. Bumping engine should correct this condition.
4. If voltage reading is constantly varying around a midpoint of 4.6 volts and connections are good, PCM is faulty.

CODE 42, ELECTRONIC SPARK TIMING (EXCEPT 3.8L)

Code 42 indicates ECM has seen an open or short to ground in High Energy Ignition Electronic Spark Timing (HEI EST) system 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 EST ground path through ignition module. If circuit No. 423 is shorted to ground, reading will be less than 500 ohms.
3. As test light voltage touches circuit No. 424, 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 circuit No. 423, an open in circuit No. 424 and a faulty ignition module connection or module.
5. This step confirms Code 42 is a faulty ECM and not an intermittent problem in circuits No. 423 and 424.

CODE 42, ELECTRONIC SPARK TIMING - 3.8L

Code 42 will set if EST or by-pass circuit is open or grounded at time of engine start-up.

Note. Test numbers refer to numbers on diagnostic chart.

1. Tests if ECM recognizes a problem. If ECM does not set Code 42 at this point, problem is intermittent. Check for a loose connection.
2. With ECM disconnected, digital volt-ohmmeter should indicate less than 200 ohms. This is normal EST circuit resistance through ignition module. A higher resistance would indicate a fault in circuit No. 423, a poor ignition module connection or a faulty ignition module.
3. If test light was on when connected from 12 volts to ECM harness by-pass circuit, either circuit No. 424 is shorted to ground or ignition module is faulty.
4. Tests if ignition module switches when by-pass circuit is energized by 12 volts through test light. If ignition module switches, resistance reading should switch from less than 200 ohms to more than 6000 ohms.
5. Disconnecting ignition module should make ohmmeter indicate as if it were monitoring an open circuit (infinite reading). Otherwise, circuit No. 423 is shorted to ground.

CODE 43, ELECTRONIC SPARK CONTROL WITH SPARK CONTROL MODULE

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 43 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION

CODE 43 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION

CODE 43, ELECTRONIC SPARK CONTROL WITHOUT ESC MODULE (SINGLE SENSOR) - EXCEPT 4.3L "S" & "T" SERIES PICKUP WITH M/T & 4.3L CPI

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, ELECTRONIC SPARK CONTROL 4.3L "S" & "T" SERIES WITH M/T & "L" & "M" SERIES WITH 4.3L CPI

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

1. Tests to determine if system is currently functioning properly.
2. An improperly installed sensor can prevent knock sensor from grounding to block.

CODE 44, LEAN EXHAUST INDICATION (EXCEPT 4.3L TURBO)

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 BASIC TESTING article in this section.

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 TESTS W/O CODES article in this section.

CODE 44 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION

CODE 44, Flowchart, Lean Exhaust Indication(Exc. 4.3L Turbo). Scheme 315

CODE 44, LEAN EXHAUST INDICATION - 4.3L TURBO

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. This tests oxygen sensor heating element. Heating element resistance should be 3.5 ohms at 68°F (20°C) and 14 ohms at 562°F (350°C).
2. Code 44 sets when O2 sensor signal at ECM is less than .2 volt for at least 2 minutes and system is operating in "closed loop".

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

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

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 article in this section.

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 TESTS W/O CODES article in this section.

CODE 44, Schematic, Lean Exhaust Indication (4.3L Turbo). Scheme 316

CODE 45, RICH EXHAUST INDICATION (EXCEPT 4.3L TURBO)

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 BASIC TESTING article in this section. The ECM 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 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 TESTS W/O CODES article in this section.

CODE 45 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION

CODE 45, Flowchart, Rich Exhaust Indication(Exc. 4.3L Turbo). Scheme 317

CODE 45, RICH EXHAUST INDICATION - 4.3L TURBO

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 50 seconds or more.

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

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.

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

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 MAP sensor. If rich condition disappears, replace MAP sensor and continue testing.

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

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 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 article in this section.

CODE 45, Flowchart, Rich Exhaust Indication (4.3L Turbo). Scheme 318

CODE 51, FAULTY PROM/MEM-CAL

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 (EXCEPT 3.8L)

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.

CODES 53, 54 & 55, EGR FAULT - 3.8L

Codes 53, 54 and 55 are EGR flow test failures. PCM tests medium and large EGR solenoid valves by cycling each of them on momentarily while monitoring engine RPM. When a solenoid is turned on, PCM expects to see a drop in engine RPM. If expected drop is not detected for EGR solenoid No. 1 for 8 out of 12 tests, Code 53 will set. If expected drop is not detected for medium EGR solenoid (EGR No. 2) for 6 out of 8 tests, Code 54 will set. If expected drop is not detected for large EGR solenoid (EGR No. 3) for 3 out of 5 times, Code 55 will set.

PCM runs EGR flow tests when coolant temperature is greater than 183°F (84°C), vehicle is in a coast-down mode (but not in a fuel-cut situation), O2 sensor voltage is greater than .57 volt, transmission is in 1st or 2nd gear, A/C clutch is not engaged, and vehicle speed is greater than 25 MPH. EGR No. 1 engine test speed range is 800-1000 RPM. EGR No. 2 engine test speed range is 825-1025 RPM. EGR No. 3 engine test speed range is 850-1050 RPM.

Note. Test numbers refer to numbers on diagnostic chart.

1. A noticeable change in engine speed should occur as each solenoid is cycled on at idle.
2. If test light glows, PCM and wiring are okay.

CODE 54, FUEL PUMP CIRCUIT

Code 54 will set if ECM does not see 12 volts on fuel pump signal voltage monitor during the first 2 seconds after ignition is turned on.

CODE 54, Schematic, Fuel Pump Circuit (4.3L Turbo). Scheme 319

CODE 54, Schematic, Fuel Pump Circuit (3.1L & 4.3L C, K & P Series W/ M/T). Scheme 320

CODE 54, Schematic, Fuel Pump Circuit (3.1L). Scheme 321

CODE 54, Schematic, Fuel Pump Circuit (4.3L S & T Series P/U W/ M/T). Scheme 322

CODE 54, Schematic, Fuel Pump Circuit (4.3L S & T Series Util. W/ M/T & 4.3L L & M Series). Scheme 323

CODE 54, Schematic, Fuel Pump Circuit (4.3L C & K Series W/4L60E Trans.). Scheme 324

CODE 54, Schematic, Fuel Pump Circuit (4.3L G Series W/4L60E Trans.). Scheme 325

CODE 54, Schematic, Fuel Pump Circuit (All Models W/ 4L80E Trans.). Scheme 326

CODE 54, Flowchart, Fuel Pump Circuit (Exc. 3.8L). Scheme 327

CODE 56, QUAD-DRIVER "B" FAULT 3.8L

PCM uses Quad-Driver Modules (QDMs) to control several devices. When PCM is commanding a component on, voltage of output circuit will be low (near zero volts). When PCM is commanding component off, voltage of output circuit will be high (near battery voltage). Primary function of quad-driver module is to control ground circuit for component being controlled. PCM has an internal fault line for each quad-driver module. Fault line status can be displayed on a scan tester. If PCM detects an output voltage other than what is expected on fault line, PCM will set Code 56.

Note. Test numbers refer to numbers on diagnostic chart.

1. PCM does not know which controlled circuit caused Code 56 to set. This chart will check each circuit to determine which is at fault. If other QDM-related codes are present, use those charts first.
2. If QDM "B" related symptoms are present, checks on Code 56 chart should isolate cause of fault.
3. These steps help determine which circuit is out of specification.
4. This step helps determine if problem is circuit or component.

CODE 61, CRUISE VENT SOLENOID - 3.8L

Cruise switch within turn signal lever receives ignition voltage from a 15-amp fuse on circuit No. 639. Cruise switches are inputs to cruise control portion of PCM. Cruise servo and vent and vacuum solenoids are output lines and are controlled by high side drivers in PCM. With ignition on, PCM looks at these output lines and will set a code depending upon actual status of these lines compared with commanded status. In this manner, PCM can detect opens or shorts to ground or voltage. Code 61 does not turn on SERVICE ENGINE SOON light.

CODE 61, Schematic, Cruise Vent Solenoid (3.8L). Scheme 328

CODE 61, Flowchart, Cruise Vent Solenoid (3.8L). Scheme 329

CODE 62, CRUISE VACUUM SOLENOID - 3.8L

Cruise switch within turn signal lever receives ignition voltage from a 15-amp fuse on circuit No. 639. Cruise switches are inputs to cruise control portion of PCM. Cruise servo, vent and vacuum solenoids are output lines and are controlled by high side drivers in PCM. With ignition on, PCM looks at these output lines and will set a code depending upon actual status of these lines compared with commanded status. In this manner, PCM can detect opens or shorts to ground or voltage. Code 62 does not turn on SERVICE ENGINE SOON light.

CODE 62, Flowchart, Cruise Vacuum Solenoid (3.8L). Scheme 330

CODE 63, CRUISE SYSTEM PROBLEM - 3.8L

PCM monitors cruise control system servo position to ensure desired cruise position and actual cruise position are equal to each other. If servo position is low when maximum servo position is commanded, PCM will set Code 63. Code 63 does not turn on SERVICE ENGINE SOON light. If Code 61, 62 or 65 is set with Code 63, repair other code before using this flow chart.

CODE 63, Flowchart, Cruise System Problem (3.8L). Scheme 331

CODE 65, CRUISE SERVO POSITION - 3.8L

PCM supplies 5 volts to cruise control servo position sensor. Depending upon actual servo position, voltage on servo position sensor circuit will indicate to PCM position of servo. Code 65 will set if circuit No. 399 is open or if circuit No. 398 is open or shorted to ground or voltage. Code 65 does not turn on SERVICE ENGINE SOON light.

CODE 65, Flowchart, Cruise Servo Position (3.8L). Scheme 332

CODE 66, LOW A/C REFRIGERANT CHARGE - 3.8L

PCM monitors A/C request and completes ground for A/C relay when an A/C mode is selected at control head and refrigerant pressure is sufficient to close pressure cycling switch. If A/C pressure is low and clutch cycles too often, PCM will protect compressor by disabling A/C relay and setting Code 66. Relay will be disabled until next ignition cycle. If Code 66 is set during 3 consecutive ignition cycles, A/C relay will be disabled until Code 66 is cleared from memory. Code 66 does not illuminate SERVICE ENGINE SOON light. Code 66 will set if A/C request signal lasts less than 1.5 seconds for 10 or more consecutive compressor "on" cycles within a 15 minute period.

CODE 66, Schematic, Low A/C Refrigerant Charge (3.8L). Scheme 333

CODE 66, Flowchart, Low A/C Refrigerant Charge (3.8L). Scheme 334

CODE 67, CRUISE ENGAGE SWITCHES - 3.8L

Code 67 will set if circuit No. 397 or 86 is open, set/coast or resume/accel switch is closed for an extended amount of time or any cruise engage switch circuit is shorted to voltage. Code 67 does not illuminate SERVICE ENGINE SOON light.

CODE 67, Schematic, Cruise Engage Switches (3.8L). Scheme 335

CODE 67, Flowchart, Cruise Engage Switches (3.8L). Scheme 336

CODE 68, CRUISE SYSTEM PROBLEM USING TECH 1 - 3.8L

PCM-integrated cruise control system is designed to monitor itself to ensure desired cruise position and actual cruise position are equal to each other. Code 68 sets when actual servo position sensor signal is 15 percent greater than desired servo position sensor signal for .6 second. Code 68 does not illuminate SERVICE ENGINE SOON light. If Code 68 is current, vehicle will operate in "power management" mode. During this mode, PCM will shut of fuel to 3 cylinders to avoid over-revving engine in case of throttle being held open by cruise system malfunction. Power management mode may be perceived as a severe engine miss of lack of power.

Note. Test number refers to number on diagnostic chart.

1. Most VAC solenoids leak a small amount of vacuum when closed. However, they should not leak enough to allow WOT in 15 seconds with vent closed.
2. When VAC solenoid is forced on, vacuum will be vented to atmosphere unless vent valve is stuck closed.

CODE 69, A/C HEAD PRESSURE SWITCH - 3.8L

A/C head pressure switch opens ground at 210 psi (14.77 kg/cm 2 ). When switch is open, fans should run at high speed. If Code 69 is set, engine cooling fans will run at high speed anytime A/C is requested.

CODE 69, Schematic, A/C Head Pressure Switch (3.8L). Scheme 337

CODE 69, Flowchart, A/C Head Pressure Switch (3.8L). Scheme 338