Engine Controls - Tests W/codes - V6: Other GMC Cab & Chassis K3500

Hard Failures

Hard failures cause SERVICE ENGINE SOON light 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 SERVICE ENGINE SOON light 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.

Field Service Mode Check

SERVICE ENGINE SOON light indicates operational mode of engine if ALDL 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.

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 BASIC TESTING article. If vehicle does not pass diagnostic circuit check, information received by Scan tester may be invalid.

The Scan Tester is a specialized tester which can diagnose on-board computer control systems by providing almost instant access to circuit voltage information without crawling under dash or hood to back-probe 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 ALDL 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 ALDL test terminal. If same codes are not flashed by SERVICE ENGINE SOON light as were indicated by Scan tester, tester cannot be used on vehicle and information obtained by it will not be guaranteed accurate.

SCAN 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). Data parameters are updated every 1 1/2 seconds. 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, go to TESTS W/O CODES article in this section for diagnosis by symptom (i.e. ROUGH IDLE, NO START, etc.), or intermittent diagnostic procedures.

Except 4.3L Turbo

When exhaust temperature is less than 600°F (316°C), the O2 sensor is open and produces no voltage. An open sensor circuit or cold sensor will not all system to entire closed loop.

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 1 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.

4.3L Turbo

When exhaust temperature is less than 600°F (316°C), the O2 sensor is open and produces no voltage. An open sensor circuit or cold sensor will not all system to entire closed loop.

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, O2 signal voltage is steady between .35-.55 volt and throttle position sensor signal is above idle. All conditions must be met for at least 1 minute.
2. This checks the oxygen sensor heating element. Resistance should be 3.5-14 ohms at 662°F (350°C).
3. This determines if fault is in O2 sensor, ECM or wiring.
4. 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.
5. If the A/C fuse is blown, check the A/C control circuit or generator for short circuits.

CODE 14, COOLANT SENSOR SIGNAL VOLTAGE LOW

The ECM uses coolant temperature sensor inputs in determining control of fuel delivery, engine timing (EST), idle (IAC) and converter clutch (TCC). As the engine warms, the 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 the ECM recognizes the open circuit by displaying a low temperature, the ECM and wiring are not at fault.

CODE 15, COOLANT SENSOR SIGNAL VOLTAGE HIGH

As the engine warms, the 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 the ECM recognizes the grounded circuit and displays a high temperature, the 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 21, TPS SIGNAL VOLTAGE HIGH

The 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. On models with non-adjustable TPS, 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 the ECM recognizes the low voltage signal and sets Code 22, the 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

The 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 the fault is a hard failure or an intermittent condition. Code 22 will set if engine is running, TPS voltage is less than .2 volt for 2-4 seconds.
2. This simulates Code 21. If the ECM recognizes a high voltage signal and sets Code 21, the ECM and wiring are not at fault.
3. On 2.8L, check and adjust TPS. On all others, replace TPS.
4. This simulates a high voltage signal to check for on open TPS signal circuit.

2.5L & 4.3L Turbo

The ECM supplies and monitors a voltage signal (4-6 volts) to the sensor. When temperatures are low, sensor resistance is high and the ECM will see a high-monitored voltage signal. As temperature increases, sensor resistance decreases and voltage sensed by the 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 the Scan tester displays a high temperature, the ECM and wiring are not at fault.
3. This checks for continuity of sensor signal and ground circuits.

Except 3.1L, C & K Series & Models W/4L80-E Transmission

The ECM applies and monitors 12 volts on circuit No. 437. Circuit No. 437 is connected to the Digital Ratio Adapter Controller (DRAC) which alternately grounds this circuit when it is receiving voltage pulses from the 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 the AC signal from the 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 aver age of 4-6 volts at about 20 MPH.

C & K Series Except With 4L80-E Transmission

The ECM applies and monitors 12 volts on circuit No. 437. Circuit No. 437 is connected to the Digital Ratio Adapter Controller (DRAC) which alternately grounds this circuit when it is receiving voltage pulses from the 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 10 seconds. These conditions are met during a road load deceleration.
2. 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 aver age of 4-6 volts at about 20 MPH.

C & K Series 2WD With 4L80-E Transmission

VSS output sensor circuit consists of a magnetic induction type speed sensor, Digital Ratio Adapter Controller (DRAC), and wiring. Gear teeth cut in the outside diameter of the rear internal gear induce an alternating current in the sensor when drive wheels are turning. Signal is transmitted to the instrument cluster DRAC where it is converted to a square wave signal which is passed on to the PCM, cruise control, speedometer, and anti-lock brake system. Code 24 will set if gear selector is not in Park or Neutral, input speed is at least 3000 RPM and output speed is less than 200 RPM for at least one second. 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.

1. Test verifies VSS voltage at PCM.
2. Test verifies operation of VSS.
3. Test checks circuits No. 821 and 822 up to the instrument cluster.

2WD With 4L80-E Trans. Except C & K Series

VSS output sensor circuit consists of a magnetic induction type speed sensor, Digital Ratio Adapter Controller (DRAC), and wiring. Gear teeth pressed on the outside diameter of the output carrier assembly induce an alternating current in the sensor when drive wheels are turning. Signal is transmitted to the instrument cluster DRAC where it is converted to a square wave signal which is passed on to the 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.

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

4WD With 4L80-E Transmission

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

3.1L Engine

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. The voltage level and pulses increase with vehicle speed. The ECM converts the pulsing voltage to MPH. The 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 the ECM, PROM should be checked for correct application.

The ECM applies and monitors a voltage signal (4-6 volts) to MAT sensor. When manifold air is cold, the sensor resistance is high and ECM sees a high signal voltage. As air warms, resistance decreases and voltage sensed by the 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 the 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 4.3L Turbo.

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 the wastegate control (solenoid) valve. This blocks off intake manifold pressure from wastegate actuator diaphragm. When overboost condition exists, ECM will de-energize the 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 the ability of the 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 the 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 the manifold to wastegate actuator.
4. This checks the electrical control portion of the system. With ignition on and engine not running, solenoid should not be energized.

Code 31 Schematic (4.3L Turbo) Turbo Overboost. Scheme 124

Code 31 Flow Chart (4.3L Turbo) Turbo Overboost. Scheme 125

4.3L C, G, K, & P Series W/4L80-E Transmission

PCM controls a solenoid regulating vacuum to the 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.

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

1. When test terminal "B" of the ALDL connector is grounded, EGR solenoid should be energized, allowing vacuum to the EGR valve. Vacuum should hold.
2. When jumper wire is removed from terminal "B", vacuum to the EGR valve should bleed through a vent in the 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 the 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.

All Engines With 4L80-E Trans. Except 4.3L

PCM controls a solenoid that regulates vacuum to the 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 indicate cruise condition and throttle position is 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 the ALDL connector is grounded, EGR solenoid should be energized, allowing vacuum to the 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.

All W/O 4L80-E Trans Except 3.1L, & 4.3L Turbo

ECM controls a solenoid that regulates vacuum to the 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 is 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 the ALDL connector is grounded, EGR solenoid should be energized, allowing vacuum to the 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 it does not this may indicate the 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.

ECM operates a Electronic Vacuum Regulator Valve (EVRV) to control vacuum to the EGR valve. Regulator normally allows only a small amount of vacuum to pass to EGR valve. When ECM grounds EVRV drive circuit, EVRV opens, allowing enough vacuum to pass through to open EGR valve. ECM monitors EGR effectiveness by periodically checking fuel integrator counts. This is done when vehicle speed is greater than 50 MPH, MAP sensor signal indicates cruise condition, throttle position is constant and all conditions have been met twice for 60 seconds.

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

1. With ignition on and engine off, EVRV should not be energized or allow enough vacuum through to open EGR valve. When test terminal "B" of the ALDL connector is grounded, EVRV will energize, allowing vacuum to the EGR valve.
2. Checks for plugged EGR passages. If passages are plugged, engine may have severe detonation on acceleration.

Code 32 Schematic (3.1L) EGR System Error. Scheme 126

Code 32 Flow Chart (3.1L) EGR System Error. Scheme 127

ECM controls a solenoid that regulates vacuum to the 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.

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

Diagnostic chart covers checks for the 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

The 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 the 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 the ECM recognizes and indicates low MAP signal, the ECM and 5-volt reference and MAP signal circuits are not at fault.

CODE 34, MAP SENSOR SIGNAL VOLTAGE LOW

The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). If the MAP sensor fails, the 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 terminals "B" to "C" 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 the ECM recognizes the voltage as greater than 4 volts (high MAP voltage signal), indicating the ECM and MAP signal circuit are not at fault.

CODE 35, IDLE SPEED ERROR 2.5L & 4.3L TURBO

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 the extend position before engine speed reduces. This is normal on some engines. Fully extending the 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 the node light should flash Red and Green, while the IAC valve is cycled. While the 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, the idle speed may be either too high (check for vacuum leaks) or too low. Engine speed may vary and disconnecting the IAC may not help. Scan 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 the 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 the 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 ID (2.5L)

CODE 35 ECM TERMINAL & CIRCUIT WIRING ID (4.3L TURBO)

Code 35 Schematic (2.5L & 4.3L Turbo) Idle Speed Error. Scheme 128

Code 35 Flow Chart (2.5L & 4.3L Turbo) Idle Speed Error. Scheme 129

CODE 42, ELECTRONIC SPARK TIMING (EST)

Code 42 indicates ECM has seen an open or short to ground in HEI EST 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 the 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, the 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 the 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.

Knock Sensor W/O Spark Module (Dual Sensors) - "L", "M", "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.

All Except 4.3L Turbo & Models With 4L80-E Transmission

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

1. If conditions for a Code 34 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 the 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 the signal voltage on the spark retard line remains low, this test should cause the signal on that line to go high. The 12-volt signal should be seen by the ECM as a no knock signal if the ECM and wiring are okay.
4. This test determines if the knock signal is detected on the sensor-to-controller line or if the ESC module is at fault.
5. If sensor line is routed too close to secondary ignition wires, the ESC module may see the interference as a knock signal.
6. This checks ground circuit to module. An open ground will cause the voltage on the monitored line to remain constant at about 12 volts. This would cause the Code 43 functional test to fail.
7. This should generate a knock signal to the controller. This determines if the ESC controller is operating correctly.

4.3L Turbo & Models With 4L80-E Transmission

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

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 the current time.
2. This step determines the state of the 5-volt reference signal applied to the sensor.
3. Checks knock sensor internal resistance.

Code 43 Schematic (4.3L Turbo & Models W/4L80-E Trans.) Electronic Spark Control. Scheme 130

Code 43 Flow Chart (4.3L Turbo & Models W/4L80-E Trans.) Electronic Spark Control. Scheme 131

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 at ECM is less than .2 volt for 50 seconds or more and system is operating in "closed loop".

O2 Sensor Wire

O2 sensor wire may be mispositioned and touching exhaust manifold. Check for ground between sensor and wire connector.

Fuel Contamination

Water, even small amounts, near the 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.

Exhaust Leaks

If the exhaust system has large leaks, exhaust system negative pressure pulses can cause outside air to be drawn into the system and past the O2 sensor. Vacuum or crankcase leaks can also cause a lean condition.

If Code 44 is intermittent, see INTERMITTENTS in the article TESTS W/O CODES .

CODE 44 ECM TERMINAL & CIRCUIT WIRING ID (ALL W/4L80-E A/T)

CODE 44 ECM TERMINAL & CIRCUIT WIRING ID (ALL OTHERS)

Code 44 Flow Chart (Except 4.3L Turbo) Lean Exhaust Indication. Scheme 132

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 at ECM is less than .2 volt for 50 seconds or more and system is operating in "closed loop".

O2 sensor wire may be mispositioned and touching exhaust manifold. Check for ground between sensor and wire connector.

Water, even small amounts, near the 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.

If the exhaust system has large leaks, exhaust system negative pressure pulses can cause outside air to be drawn into the system and past the O2 sensor. Vacuum or crankcase leaks can also cause a lean condition.

If Code 44 is intermittent, see INTERMITTENTS in the article TESTS W/O CODES .

Code 44 Flow Chart (4.3L Turbo) Lean Exhaust Indication. Scheme 133

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 .7 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. 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 the 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) the system can go rich. Disconnecting the 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 the 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 the ECM as a rich mixture, causing the 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.

CODE 45 ECM TERMINAL & CIRCUIT WIRING ID (ALL W/4L80-E A/T)

CODE 45 ECM TERMINAL & CIRCUIT WIRING ID (ALL OTHERS)

Code 45 Flow Chart (Except 4.3L Turbo) Rich Exhaust Indication. Scheme 134

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 .7 volt for 60 seconds or more.

If fuel pressure is too high, air/fuel ratio will be rich. For fuel pressure checking procedure, see TESTS W/CODES - DIESEL (for Diesel engines) or TESTS W/CODES - V6 (for Gas engines) and the article TESTS W/CODES - TRANSMISSION (if transmission codes are displayed). 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 the 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) the system can go rich. Disconnecting the MAP sensor allows ECM to substitute a fixed value for the MAP sensor. If rich condition disappears, replace MAP sensor and continue testing.

An intermittent TPS output will cause the 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 the ECM as a rich mixture, causing the 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.

Code 45 Flow Chart (4.3L Turbo) Rich Exhaust Indication. Scheme 135

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 (EXCEPT 2.5L)

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 (2.5L)

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 54, FUEL PUMP CIRCUIT

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

Code 54 Schematic (Astro, Safari, S & T Series) Fuel Pump Circuit. Scheme 136

Code 54 Schematic (C, K, R, V, G, & P Series W/O 4L80-E Trans) Fuel Pump Circuit. Scheme 137

Code 54 Schematic (All Models W/4L80-E Trans. Except G Series) Fuel Pump Circuit. Scheme 138

Code 54 Schematic (G Series W/4L80-E Trans.) Fuel Pump Circuit. Scheme 139

Code 54 Schematic (3.1L) Fuel Pump Circuit. Scheme 140

Code 54 Flow Chart Fuel Pump Circuit. Scheme 141

CODE 55, ECM ERROR (EXCEPT 2.5L)

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