Engine Controls - Tests W/codes - 3.1l Chevrolet Lumina I

INTRODUCTION

Most engine control problems are the result of mechanical breakdowns, poor electrical connections or damaged vacuum hoses. Before considering the computer system as a possible cause of problems, perform checks and inspections covered in appropriate BASIC TESTING article in this section. Failure to do so may result in lost diagnostic time.

If no faults were found while performing BASIC DIAGNOSTIC PROCEDURES, proceed with DIAGNOSTIC PROCEDURE under SELF-DIAGNOSTIC SYSTEM. If no fault codes are present and driveability problems exist, proceed to TESTS W/O CODES article in this section for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.). If only intermittent codes are present, see INTERMITTENTS in TESTS W/O CODES article in this section.

SELF-DIAGNOSTIC SYSTEM

Note. Powertrain Control Module (PCM) may also be referred to as Electronic Control Module (ECM) in some diagnostic charts and figures. Terms are used interchangeably.

Control module is equipped with a self-diagnostic system, which detects system failures or abnormalities. When a malfunction occurs, control module will store a numerical code and, in most cases, illuminate SERVICE ENGINE SOON light located on instrument panel. SERVICE ENGINE SOON light is also referred to as the Malfunction Indicator Light (MIL). Malfunctions are recorded as hard failures or as intermittent failures.

Note. Models equipped with OBD II diagnostic systems may have additional diagnostic information and procedures available; however, to fully utilize information and procedures requires the use of a Tech 1 scan tester. See tester owners manual for additional information.

HARD FAILURES

Most hard failures cause SERVICE ENGINE SOON light to glow and remain on until malfunction is repaired. If light comes on and remains on (light may flash) during vehicle operation, cause of malfunction must be determined using diagnostic (code) charts. If a sensor fails, control module will use a substitute value in its calculations to continue engine operation. In this condition, vehicle is functional, but it will most likely display degraded driveability.

INTERMITTENT FAILURES

Intermittent failures cause SERVICE ENGINE SOON light to flicker or glow and go out about 10 seconds after intermittent fault goes away. Corresponding trouble code, however, will be retained in control module memory. If related fault does not reoccur within 50 engine starts, trouble code will be erased from control module memory. Intermittent failures may be caused by sensor, connector or wiring related problems. See INTERMITTENTS in TESTS W/O CODES article in this section.

DIAGNOSTIC PROCEDURE

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

1. Ensure all engine systems not related to computer system are operating properly. DO NOT proceed with testing unless all other problems have been repaired. DIAGNOSTIC CIRCUIT CHECK must be performed before using trouble code charts. See «BASIC TESTING»(/chevrolet/lumina/i-1989-2001/remont/testing-diagnostics/#engine-controls-basic-testing) article in this section.
2. If trouble codes were displayed, determine whether codes are hard or intermittent trouble codes. Hard codes will cause SERVICE ENGINE SOON light to glow continuously while engine is running. See «HARD OR INTERMITTENT TROUBLE CODE DETERMINATION»(/chevrolet/lumina/i-1989-2001/remont/testing-diagnostics/#engine-controls-tests-wcodes-31l__hard-or-intermittent-trouble-code-determination) . For diagnosing hard codes, proceed to appropriate trouble code chart. For diagnosing intermittent codes, proceed to INTERMITTENTS in «TESTS W/O CODES»(/chevrolet/lumina/i-1989-2001/remont/testing-diagnostics/#engine-controls-tests-wo-codes) article in this section.
3. If no trouble codes are present and a driveability problem exists, refer to SYMPTOMS in «TESTS W/O CODES»(/chevrolet/lumina/i-1989-2001/remont/testing-diagnostics/#engine-controls-tests-wo-codes) article in this section. Doing so will help identify proper system or component to check in «SYSTEM/COMPONENT TESTS - 3.1L»(/chevrolet/lumina/i-1989-2001/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-31l) article in this section.
4. After necessary repairs are made, clear trouble codes, verify vehicle will enter "closed loop" operation and ensure code does not reset.

Note. For specific information on retrieving codes using a scan tester, refer to user and reference manuals supplied with tester.

Note. Field service mode check can only be performed on vehicles equipped with 12-pin DLC with a wire present in test terminal "B". See DATA LINK CONNECTOR (DLC) TEST TERMINALS table.

DATA LINK CONNECTOR (DLC) TEST TERMINALS

Scheme 3

READING TROUBLE CODES

Control module stores component failure information under a related trouble code which can be recalled for diagnosis and repair. Trouble codes may be read by counting flashes of SERVICE ENGINE SOON light (some models) or by reading digital display on a scan tester. Scan tester is faster to use, more accurate and capable of reading information which otherwise would necessitate testing individual control module and sensor/solenoid connector terminals using a digital voltmeter. See SCAN TESTER USAGE and SCAN DATA .

Note. When using most scan testers, a time delay exists between serial data updates. For instantaneous response, a digital voltmeter must be used.

If scan tester is not available, reading flashes of SERVICE ENGINE SOON light is possible by grounding diagnostic test terminal "B" (if a wire is present) of DLC with ignition on and engine off. (Scheme 3) For example, "FLASH, FLASH, pause, FLASH, longer pause" identifies Code 21. First series of flashes is first digit of trouble code. Second series of flashes is second digit of trouble code. Trouble codes are displayed starting with lowest numbered code. Each code is displayed 3 times. Codes will continue to repeat as long as DLC test terminal is grounded.

TROUBLE CODE DEFINITION

TROUBLE CODE DEFINITION

HARD OR INTERMITTENT TROUBLE CODE DETERMINATION

During any diagnostic procedure, determine if codes are hard failure codes or intermittent failure codes. Diagnostic charts will not usually help analyze intermittent codes. To determine hard codes and intermittent codes

1. Enter diagnostic mode. Read and record all stored trouble codes. Exit diagnostic mode, and clear trouble codes. See «CLEARING TROUBLE CODES»(/chevrolet/lumina/i-1989-2001/remont/testing-diagnostics/#engine-controls-tests-wcodes-31l__clearing-trouble-codes) .
2. Apply parking brake, and place transmission in Neutral or Park. Block drive wheels, and start engine. SERVICE ENGINE SOON light should go out. Run warm engine at specified curb idle for 2 minutes and note SERVICE ENGINE SOON light.
3. If SERVICE ENGINE SOON light comes on, enter diagnostic mode. Read and record trouble codes. This will reveal hard failure codes. Oxygen sensor related codes may require a road test to reset hard failure after trouble codes were cleared.
4. If SERVICE ENGINE SOON light does not come on, all stored trouble codes were intermittent failures, except as noted above.

Note. Trouble codes will be recorded at various operating times. Some codes require operation of that sensor or switch for 5 seconds; others require operation for 5 minutes or longer at normal operating temperature, vehicle speed and load. Therefore, some codes may not set in a service bay operational mode and may require road testing vehicle in order to duplicate conditions under which code will set.

CLEARING TROUBLE CODES

To clear trouble codes from memory, either to determine if malfunction will occur again or after making necessary repairs, disconnect power supply to PCM for at least 30 seconds or clear codes using a scan tester with code clearing capability.

PCM LOCATION

On most models, engine control module is located behind right or left side of dash, behind right or left kick panel or on left or right side of engine compartment. For more precise location, see COMPONENT LOCATIONS in SYSTEM/COMPONENT TESTS - 3.1L article in this section.

Diagnostic Aids

Diagnostic aids (located in many trouble code charts) are additional tips used to help diagnose trouble codes when inspected circuit is okay. Diagnostic aids may help lead to a definitive solution to trouble code problem.

Field Service Mode Check (If Available)

If DLC test terminal "B" is grounded with engine running, SERVICE ENGINE SOON light will indicate operational mode of engine. This test confirms proper operation of fuel system and verifies "closed loop" operation. Clear codes and perform this test after any repair is completed. Most models require the use of a scan tester to enter field service mode. See FIELD SERVICE MODE CHECK in BASIC TESTING article in this section.

Note. Vehicles utilizing a 16-terminal DLC or 12-terminal DLC with terminal "B" not used, must use a scan tester for retrieving codes or enter Field Service Mode.

SPECIAL TOOLS (DIAGNOSTIC)

Note. A scan tester plugged into DLC may be used to read trouble codes and check voltages in system on serial data line. On most models scan tester is REQUIRED to retrieve vehicle information. For additional information, see SCAN TESTER USAGE and SCAN DATA .

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

SCAN TESTER USAGE

Note. Before connecting scan tester to vehicle, diagnostic system should be checked to determine if system is operating properly and if information received will be accurate. This is done by performing DIAGNOSTIC CIRCUIT CHECK or ON-BOARD DIAGNOSTIC SYSTEM CHECK located in BASIC TESTING article in this section. If vehicle does not pass diagnostic check, information received may be invalid.

Scan tester is a specialized tester which, when plugged into DLC, can be used to diagnose on-board computer control systems by providing instant access to circuit voltage information without need to crawl under dash or hood to backprobe sensors and connectors.

Scan tester cuts down diagnostic time dramatically by furnishing input data (voltage signals) which can be compared to specification parameters. See SCAN DATA . They may also furnish information on output device (solenoids and motors) status. However, status parameters only indicate output signals have been sent to devices by control module; they do not indicate whether devices have responded properly to signal. Verify proper response at output device using a voltmeter or test light.

A problem may exist even if trouble codes are not present. About 80 percent of driveability problems occur without trouble codes. Sensors that are out of calibration will not set a trouble code but will cause driveability problems.

Using 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 DATA .

Note. If erroneous voltage signals are suspected, verify tester information using a digital voltmeter and wiring schematic. If non-existent codes are displayed, DO NOT use scan tester for diagnosis. Contact tester manufacturer for additional information.

SCAN DATA

Note. Information contained in SCAN DATA is typical of readings taken on vehicle with engine idling, upper radiator hose hot, closed throttle, transmission in Park or Neutral, "closed loop" status achieved and all accessories off (except as noted in tables). Data parameters are updated a minimum of every 1 1/4 seconds, more often on most models. Not all devices and systems are used on all models; following lists only represent most commonly used parameters. For additional information, refer to owner manual furnished with tester.

SCAN DATA

ECM/PCM CODE CHARTS

Note. In following diagnostic tests, schematics and illustrations are courtesy of General Motors Corp.

CODE 13, OPEN OXYGEN SENSOR CIRCUIT

Code will set if Codes 21 and 22 are not set, engine is at normal operating temperature, oxygen sensor voltage is constant within a specified range (.35-.55 volt), throttle angle is greater than idle, a precalibrated amount of time has elapsed since start-up and all conditions have existed for a precalibrated amount of time. Most likely causes of code are: open sensor signal (high) circuit, faulty PCM connection, faulty PCM, faulty oxygen sensor connection or faulty oxygen sensor.

Note. Test numbers refer to numbers on diagnostic chart.

1. This tests if problem still exists. Vehicle cannot enter "closed loop" mode if oxygen sensor circuit is open. Code indicates an open in O2 sensor circuit (steady .35-.55 volt).
2. Determines if oxygen sensor, wiring or control module is at fault. If wiring is good, grounding oxygen sensor wire will cause .45 volt reference supplied by control module to pull low.
3. This tests oxygen sensor circuit wiring. Use only a high impedance (10-megohm minimum) digital voltmeter.

Control module will not go into closed loop if code is set. Code may set if vehicle runs out of fuel or stalls while vehicle is in motion. If oxygen sensor ground becomes loose, a false oxygen sensor reading will occur. This can result in a Code 13 being set. On models equipped with an oxygen sensor heating element, element resistance should be 3.5-14.0 ohms.

CODE 13 PCM TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 13 Schematic (3.1L) Open Oxygen Sensor Circuit. Scheme 4

Code 13 Diagnostic Flow Chart (3.1L) Open Oxygen Sensor Circuit. Scheme 5

CODE 14, COOLANT TEMP SENS SGNL VLTGE LOW (HI TEMP INDICATED)

Code will set if PCM sees an extremely high temperature signal (low voltage). Most likely causes of code are: sensor signal circuit shorted to ground, faulty coolant temperature sensor or faulty PCM.

Note. This chart assumes engine cooling system is functioning properly (not overheating). Test numbers refer to numbers on diagnostic chart.

1. This checks if conditions for code still exist.
2. This tests for grounded sensor signal line between control module and coolant sensor.

After engine is started, temperature should rise steadily to about 90°C and then stabilize when thermostat opens. At normal operating temperature, signal voltage at control module terminal should be 1.5-2.0 volts. Check sensor for shifted calibration by using sensor TEMPERATURE-TO-RESISTANCE VALUES table. When code is set, control module will turn on electric cooling fan(s) if equipped.

CODE 14 PCM TERMINAL & CIRCUIT WIRING IDENTIFICATION

TEMPERATURE-TO-RESISTANCE VALUES (Measure resistance across sensor terminals)

1. For shared sensor ground tie-offs, see appropriate wiring diagram in WIRING DIAGRAMS section.

Code 14 Schematic (3.1L) Coolant Temperature Sensor Signal Voltage Low (High Temp. Indicated). Scheme 6

Code 14 Diagnostic Flow Chart (3.1L) Coolant Temperature Sensor Signal Voltage Low (High Temp. Indicated). Scheme 7

CODE 15, COOLANT TEMP SENSOR SIGNAL VOLTAGE LOW

Code indicates control module has seen high resistance (high monitored voltage) in coolant sensor circuit. This could be due to high resistance (cold temperature) or high voltage at coolant sensor terminal at control module for a precalibrated period of time. Most likely causes of code are: open sensor signal circuit, open sensor ground circuit (not allowing path to ground for 5-volt reference), faulty sensor connection, faulty sensor, faulty PCM connection or faulty PCM.

Note. Test numbers refer to numbers on diagnostic chart.

1. This checks if conditions for code still exist.
2. This test simulates a low voltage condition. If control module recognizes low voltage signal, scan tester will display greater than about 130°C. This indicates control module and wiring are not at fault.
3. This test determines if coolant sensor ground or signal circuit is open.

After engine is started, temperature should rise steadily to about 90°C and then stabilize when thermostat opens. At normal operating temperature, signal voltage at control module terminal should be 1.5-2.0 volts. Check sensor for shifted calibration by using sensor TEMPERATURE-TO-RESISTANCE VALUES table. When code is set, control module will turn on electric cooling fan(s) if equipped.

CODE 15 PCM TERMINAL & CIRCUIT WIRING IDENTIFICATION

TEMPERATURE-TO-RESISTANCE VALUES (Measure resistance across sensor terminals)

1. For shared sensor ground tie-offs, see appropriate wiring diagram in WIRING DIAGRAMS section.

Code 15 Diagnostic Flow Chart (3.1L) Coolant Temp Sensor Signal Voltage Low. Scheme 8

CODE 16, SYSTEM VOLTAGE LOW

Code 16 will set when ignition is on and engine speed is greater than 1000 RPM and the PCM is sensing ignition input voltage of less than about 9 volts for more than 2 seconds.

Note. Test numbers refer to numbers on diagnostic chart.

1. Test generator output to determine proper operation of voltage regulator. Run engine at greater than 1000 RPM and measure voltage across battery. If voltage reading is less than 9 volts, repair generator.

Check for intermittent by monitoring system voltage with a scan tester while wiggling related wiring. If an intermittent is induced, display will abruptly change. This may help to isolate location of problem.

Code 16 Schematic (3.1L) System Voltage Low. Scheme 9

Code 16 Diagnostic Flow Chart (3.1L) System Voltage Low. Scheme 10

CODE 17, CAMSHAFT SENSOR ERROR

Camshaft sensor is a Hall Effect switch located on the front of the engine. Sensor sends signals to the PCM when cylinder No. 1 is on the intake stroke. Signal is used by PCM to synchronize sequential fuel injection. If cam signal is not received by PCM, PCM will pulse fuel injectors sequentially, but the fuel injectors may not be in sync with each intake valve opening. A loss of this signal, or any extra signals (false signals), at engine speeds greater than 500 RPM will cause PCM to set a Code 17.

Note. Test numbers refer to numbers on diagnostic chart.

1. PCM performs a test for Code 17 when the engine is running or cranking. Code 17 will set when a cam pulse is missing on the first pass through the internal PCM self-test of the camshaft position sensor, or a cam pulse is missing for 3 seconds.
2. By repeatedly tapping the starter, camshaft timing mark and camshaft position sensor will align with each other. At this point, voltage from sensor to PCM should drop to near zero volts temporarily. This indicates the camshaft sensor is capable of sending a signal to the PCM.
3. Before replacing camshaft position sensor, inspect sensor for proper installation.

An intermittent cam reference signal can be caused by poor connection, cracked sensor or internal engine problems.

Code 17 Schematic (3.1L) Camshaft Sensor Error. Scheme 11

Code 17 Diagnostic Flow Chart (3.1L) Camshaft Sensor Error. Scheme 12

CODE 21, THROTTLE POSITION SENSOR SIGNAL VOLTAGE HIGH

Code will set if engine is idling (closed throttle), MAP value is low, a MAP code is not set and PCM sees high TP sensor voltage. Most likely causes for code are: TP sensor signal circuit shorted to voltage, open sensor ground circuit, faulty sensor connection, faulty TP sensor or faulty PCM.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks if code is result of a hard failure or an intermittent condition.
2. This test simulates a low-voltage condition. If control module recognizes change of state, control module and wiring are okay.
3. This step isolates a faulty sensor, control module or open sensor ground circuit. If sensor ground is shared by another sensor, an accompanying code related to that sensor may exist.

A scan tester displays throttle position in volts. Closed throttle voltage should be low. Voltage should increase gradually to about 4.5 volts at a steady rate as throttle angle is increased. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article in this section.

CODE 21 PCM TERMINAL & CIRCUIT WIRING IDENTIFICATION

1. For shared sensor ground tie-offs, see appropriate wiring diagram in WIRING DIAGRAMS section.

Code 21 Schematic (3.1L) Throttle Position Sensor Signal Voltage High. Scheme 13

Code 21 Diagnostic Flow Chart (3.1L) Throttle Position Sensor Signal Voltage High. Scheme 14

CODE 22, THROTTLE POSITION SENSOR SIGNAL VOLTAGE LOW

Code will set if engine is running and TP sensor voltage is lower than base idle voltage (auto zero voltage). Most likely causes of code are: TP sensor signal circuit open or shorted to ground, 5-volt reference open or shorted to ground (should also set other codes), faulty sensor connection, faulty sensor, faulty PCM connection or faulty PCM.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test checks if code is result of a hard failure or an intermittent condition.
2. This test simulates high voltage conditions. If control module recognizes change of state, control module and wiring are okay.
3. This simulates a high signal voltage to check for an open in TP sensor signal line to control module. Scan tester should recognize this signal and display high TP sensor voltage.

A scan tester displays throttle position in volts. Closed throttle voltage should be low. Voltage should increase gradually to about 4.5 volts at a steady rate as throttle angle is increased. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article in this section.

CODE 22 PCM TERMINAL & CIRCUIT WIRING IDENTIFICATION

1. For shared sensor ground tie-offs, see appropriate wiring diagram in WIRING DIAGRAMS section.

Code 22 Diagnostic Flow Chart (3.1L) Throttle Position Sensor Signal Voltage Low. Scheme 15

CODE 23, IAT SENSOR SIGNAL VOLTAGE HIGH (LOW TEMP INDICATED)

Code will set if engine has been running for a precalibrated period of time, has reached operating temperature and signal voltage indicates a low intake air temperature (high voltage). Most likely causes of code are: open sensor signal circuit, open sensor ground circuit (not allowing path to ground for 5-volt reference), faulty sensor connection, faulty sensor, faulty PCM connection or faulty PCM.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks if code is result of a hard failure or an intermittent condition.
2. This simulates low voltage (high temperature) conditions. If scan tester displays a high temperature, control module and wiring are not at fault.
3. This checks for continuity of sensor signal and ground circuits using process of elimination. If ground circuit is shared by other sensors and ground circuit is open, accompanying codes related to those sensors may be present.

If engine is allowed to cool overnight, coolant and IAT sensor values should be close to each other when measured by scan tester. Code will result if signal and ground circuits become open. Check sensor for shifted calibration by using sensor TEMPERATURE-TO-RESISTANCE VALUES table.

CODE 23 PCM TERMINAL & CIRCUIT WIRING IDENTIFICATION

TEMPERATURE-TO-RESISTANCE VALUES (Measure resistance across sensor terminals)

1. For shared sensor ground tie-offs, see appropriate wiring diagram in WIRING DIAGRAMS section.

Code 23 Schematic (3.1L) IAT Sensor Signal Voltage High (Low Temp Indicated). Scheme 16

Code 23 Diagnostic Flow Chart (3.1L) IAT Sensor Signal Voltage High (Low Temp Indicated). Scheme 17

CODE 24, VEHICLE SPEED SENSOR

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 about 2 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.

Note. Prior to testing Corvette for Code 24, disable ASR system.

1. PM generator only produces a voltage signal if drive wheels are turning greater than about 2 MPH.

A faulty or misadjusted park/neutral switch may set a false Code 24. Use scan tester to check for proper signal in Drive while wiggling shifter. Code 24 may set if vehicle is power braked (brakes applied and throttle depressed) for more than 10 seconds.

Code 24 Schematic (3.1L) Vehicle Speed Sensor. Scheme 18

Code 24 Diagnostic Flow Chart (3.1L) Vehicle Speed Sensor. Scheme 19

CODE 25, IAT SENSOR SIGNAL VOLTAGE LOW (HIGH TEMP INDICATED)

Code will set if intake air temperature is extremely high (sensor signal voltage low) for a precalibrated period of time. Most likely causes of code are: sensor signal circuit shorted to ground, faulty coolant temperature sensor or faulty PCM.

Note. Test numbers refer to numbers on diagnostic chart.

1. This checks if code is hard failure or intermittent condition.
2. This simulates low temperature (high resistance) conditions. If control module recognizes open circuit and scan tester displays temperature of less than about -30°C, control module and wiring are okay.

If engine is allowed to cool overnight, coolant temperature sensor and IAT sensor values should be close to each other when measured by a scan tester. A Code 25 will result if sensor signal circuit is shorted to ground. Check sensor for shifted calibration by using sensor TEMPERATURE-TO-RESISTANCE VALUES table.

CODE 25 PCM TERMINAL & CIRCUIT WIRING IDENTIFICATION

TEMPERATURE-TO-RESISTANCE VALUES (Measure resistance across sensor terminals)

1. For shared sensor ground tie-offs, see appropriate wiring diagram in WIRING DIAGRAMS section.

Code 25 Diagnostic Flow Chart (3.1L) IAT Sensor Signal Voltage Low (High Temp Indicated). Scheme 20

CODE 28, TRANS. RANGE PRESSURE SWITCH

Transaxle range switch is a multiple signal switch that sends gear selector position information to the PCM. PCM determines gear selection from an internal table. This is important to maintain idle quality when shifting from Park to Drive as well as maintaining correct TCC engagement points. When Code 28 sets, PCM will default to 3rd gear until a correct signal is received by PCM.

Note. Test numbers refer to numbers on diagnostic chart.

1. & 2) These steps check for proper operation of transmission range switch.
2. This test checks for an open circuit, grounded circuit or short to voltage.

An intermittent may be caused by a poor connection, rubbed-through wire insulation or wire broken inside insulation. Inspect PCM harness connectors for backed-out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal-to-wire connection and damaged harness. Monitor Tech 1 while moving related connectors and wiring harness. If failure is induced, reading will change. This may assist in isolating the location of the malfunction.

Code 28 Schematic (3.1L) Transmission Range Pressure Switch. Scheme 21

Code 28 Diagnostic Flow Chart (3.1L) Transmission Range Pressure Switch. Scheme 22

CODE 33, MAP SENSOR SIGNAL VOLTAGE HIGH

Code 33 will set when MAP signal voltage reading is high for greater than a precalibrated period of time, TPS voltage indicates throttle is closed and neither Code 21 nor 22 is present. Most likely causes for code are: open sensor ground circuit, sensor signal circuit shorted to voltage, faulty MAP sensor, MAP sensor vacuum leak or faulty PCM.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test confirms Code 33 and determines if it is result of a hard failure or an intermittent condition.
2. This step simulates conditions for a Code 34. If control module recognizes and sets Code 34, low MAP signal, control module and 5-volt reference and MAP signal circuits are not at fault. If ground circuit is shared with other sensors and ground circuit becomes open, additional codes related to these sensors may be set.

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

CODE 33 PCM TERMINAL & CIRCUIT WIRING IDENTIFICATION

1. For shared sensor reference and shared sensor ground tie-offs, see appropriate wiring diagram in WIRING DIAGRAMS section.

Code 33 Schematic (3.1L) MAP Sensor Signal Voltage High. Scheme 23

Code 33 Diagnostic Flow Chart (3.1L) MAP Sensor Signal Voltage High. Scheme 24

CODE 34, MAP SENSOR SIGNAL VOLTAGE LOW

Code 34 will set when ignition is on, throttle is off idle, no TP sensor code exists and MAP signal voltage is lower than expected for off idle engine operation. Most likely causes for code are: MAP sensor signal circuit open or shorted to ground, 5-volt reference circuit open or shorted to ground, faulty sensor connection, faulty sensor or faulty PCM.

Note. Test numbers refer to numbers on diagnostic chart.

1. This confirms conditions for Code 34 and determines if code was caused by a hard failure or an intermittent fault.
2. Jumpering MAP signal to 5-volt reference at MAP harness connector will determine if sensor is at fault or if a problem exists with control module or wiring.
3. Scan tester may not display 12 volts. Control module recognizes voltage as greater than 4 volts (high MAP voltage signal), indicating control module and MAP signal circuit are not at fault.

With ignition switch in ON position and engine stopped, manifold pressure is equal to atmospheric pressure and signal voltage will be high. Comparing BARO readings with a known good vehicle using same sensor is a good way to check accuracy of suspect sensor.

Readings should be same within .4 volt. A Code 34 will also result if 5-volt reference and MAP signal circuits are open or shorted to ground.

CODE 34 PCM TERMINAL & CIRCUIT WIRING IDENTIFICATION

1. For shared sensor reference and shared sensor ground tie-offs, see appropriate wiring diagram in WIRING DIAGRAMS section.

Code 34 Diagnostic Flow Chart (3.1L) MAP Sensor Signal Voltage Low. Scheme 25

CODE 35, IAC IDLE SPEED ERROR

Code 35 will set when closed throttle engine speed is 150-200 RPM greater or less than desired idle speed, IAC is attempting to adjust idle speed (high or low IAC counts) and conditions have been present for a precalibrated period of time.

Note. Test numbers refer to numbers on diagnostic chart.

1. Tech 1 RPM control mode is used to extend and retract IAC valve. Movement is verified by an engine speed change. If no change in speed occurs, valve can be retested when removed from throttle body. If IAC valve is retracted beyond control range (about 1500 RPM), many flashes in extend position may occur before engine speed begins to drop. This is normal on certain engines. Fully extending IAC may cause engine to stall. This may be normal.
2. This test uses Tech 1 to command IAC-controlled idle speed. Control module issues commands to obtain requested idle speed. Each light on node light should flash Red and Green while IAC valve is cycled. While sequence of color is not important, check circuits for faults beginning with poor terminal contacts if either light is off or does not flash Red and Green.

A slow, unstable idle may be caused by a system problem which cannot be overcome by IAC. Scan counts will be greater than 60 if idle is too low and zero counts if idle is too high. If idle is too high, stop engine. Fully extend IAC with driver. Start engine. If idle speed is greater than 800 RPM, look for possible vacuum leaks.

System Too Lean

If air/fuel ratio is too lean, idle speed may be either too high (check for vacuum leaks) or too low. Engine speed may vary up and down; disconnecting IAC may not help. Scan and/or digital voltmeter (10-megohm) will read an oxygen (O2) sensor output less than 300 mV (.3 volt). Check for low fuel pressure or water in fuel. A contaminated O2 sensor (caused by silicone) will produce lean air/fuel mixtures with an O2 sensor output fixed greater than 800 mV (.8 volt). This may also set Code 45.

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. System may be obviously rich, with Black smoke from exhaust pipe. Scan tester and/or voltmeter will read an O2 sensor voltage signal fixed greater than 800 mV (.8 volt). Look for high fuel pressure and injectors leaking or sticking. Remove IAC, and inspect bore for foreign material and evidence of IAC valve dragging bore.

Throttle Body

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

IAC Valve Connections

Inspect carefully for loose or corroded connections.

PCV Valve

An incorrect PCV valve may cause incorrect idle speed.

Code 35 Schematic (3.1L) IAC Idle Speed Error. Scheme 26

Code 35 Diagnostic Flow Chart (3.1L) IAC Idle Speed Error. Scheme 27

CODE 36, 24X SIGNAL ERROR

The 24X crankshaft position sensor is used to improve spark during low engine speeds. Code 36 will set if the engine is running and the PCM detects 6 or more 24X crankshaft sensor reference pulses have not occurred. Code 36 diagnosis occurs once per ignition cycle.

Note. Test numbers refer to numbers on diagnostic chart.

1. The 24X signal RPM should be the same as engine speed up to 2000 RPM, at which time it stops. This test determines if the PCM is putting out a 24X signal.
2. Determines if signal voltage is present at the PCM.
3. Determines is battery voltage is present at sensor input.
4. Applying battery voltage to the 24X pigtail circuit determines pigtail and sensor integrity.
5. Determines ground path continuity.

An intermittent 24X signal and Code 36 can be caused by poor connections. Visually and physically check connections.

Code 36 Diagnostic Flow Chart (3.1L) 24X Signal Error. Scheme 28

CODE 37, TCC BRAKE SWITCH ERROR

PCM monitors activity of the normally-closed brake switch. Switch opens when brake pedal is applied.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks for voltage at brake switch.
2. Simulates brake switch closed or brakes off.
3. Checks TCC brake switch circuit between PCM and switch.
4. Checks for open TCC brake switch input circuit and simulates brakes being applied.

Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal-to-wire connections or damaged harness. Perform intermittent check by monitoring circuit and code status while wiggling related wiring and connectors. If failure is induced, monitored circuit status will change rapidly and/or related code will set.

Code 37 Schematic (3.1L) TCC Brake Switch Error. Scheme 29

Code 37 Diagnostic Flow Chart (3.1L) TCC Brake Switch Error. Scheme 30

CODE 41, CYLINDER SELECT ERROR

Ignition timing is controlled by the PCM. Inputs from various sensors allow PCM to calculate optimum spark timing.

Note. Test number refers to number on diagnostic chart.

1. Confirms Code 41 and fault causing code is present.
2. Checks for normal IC ground path through ignition module

Scan tester does not have the ability to help diagnose Code 41. If PCM is faulty and must be replaced, the new PCM must be reprogrammed. This requires special equipment.

Code 41 Schematic (3.1L) Cylinder Select Error. Scheme 31

Code 41 Diagnostic Flow Chart (3.1L) Cylinder Select Error. Scheme 32

CODE 42, IC CIRCUIT OPEN/GROUNDED

Code 42 indicates PCM has seen an open or short to ground in IC or by-pass circuit.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test confirms Code 42 and determines if fault is a hard failure or intermittent condition.
2. This test checks for a normal IC ground path through ignition module. If EST circuit is shorted to ground, reading will be less than 500 ohms.
3. As test light voltage touches by-pass circuit, module should switch. This will cause ohmmeter to switch from hundreds of ohms to thousands of ohms. This test assures module "switched".
4. If module did not switch, this tests for a short in IC circuit, an open in by-pass circuit and a faulty ignition module connection or module.
5. This step confirms Code 42 is a faulty PCM and not an intermittent problem in IC or by-pass circuits.

Scan tester does not have ability to help diagnose a Code 42 problem. See INTERMITTENTS in TESTS W/O CODES article in this section.

Code 42 Diagnostic Flow Chart (3.1L) IC Circuit Open/Grounded. Scheme 33

CODE 43, KNOCK SENSOR ERROR

Note. Test numbers refer to numbers on diagnostic chart.

1. If conditions for code exist, scan tester will always indicate "yes" when knock signal is monitored. If an audible knock is heard from engine, repair internal engine problem. No engine knock should be detected at idle.
2. PCM applies 5-volts on sensor signal line.

Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal-to-wire connections or damaged harness. Perform intermittent check by monitoring circuit and code status while wiggling related wiring and connectors. If failure is induced, monitored circuit status will change rapidly and/or related code will set.

Code 43 Schematic (3.1L) Knock Sensor Error. Scheme 34

Code 43 Diagnostic Flow Chart (3.1L) Knock Sensor Error. Scheme 35

CODE 44, LEAN EXHAUST INDICATION

Note. Some models are equipped with 2 oxygen sensors. On these models, Code 44 will set if the left sensor circuit is lean. Code 64 will set if the right sensor circuit is lean. Use this chart for Code 64 also and perform tests for applicable sensor.

O2 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. On models with 2 oxygen sensors, Code 44 will reflect a lean left O2 sensor; Code 64 will indicate a lean right O2 sensor. Perform test procedures for right or left sensor as necessary.

Code is set when O2 sensor voltage signal at control module is low (less than 0.3 volt) for a precalibrated period and fuel system is operating in "closed loop" and attempting to richen mixture. Most likely causes for setting of code are: an actual lean condition, a short to ground on sensor signal circuit or a faulty PCM.

Note. Test number refers to number on diagnostic chart.

1. Checks to see if O2 sensor is registering a lean condition.

Using scan tester, observe Block Learn Memory (BLM) value at different RPMs. If conditions for a lean exhaust code exist, block learn value will be about 150.

O2 Sensor Wire

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

Fuel Contamination

Water, even small amounts, near in-tank fuel pump inlet can be delivered to injector. Water may cause a lean exhaust, setting code.

Fuel Pressure

System will be lean if fuel pressure is low. If necessary, 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.

Misfire Or Stall

If engine misfires or stalls (including running out of fuel) while vehicle is moving, code may set. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article in this section.

CODE 44 PCM TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 44 Diagnostic Flow Chart (3.1L) Lean Exhaust Indication. Scheme 36

CODE 45, RICH EXHAUST INDICATION

Note. Some models are equipped with 2 oxygen sensors. On these models, Code 45 will set if the left sensor circuit is rich. Code 65 will set if the right sensor circuit is lean. Use this chart for Code 65 also and perform tests for applicable sensor.

O2 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. Diagnosis should begin with fuel pressure, leaking injector, ignition shielding (ground), vapor canister fuel saturation, coolant sensor, MAP sensor, O2 sensor contamination and TPS intermittent output.

Code is set when fuel system is operating in "closed loop", throttle angle is greater than idle, O2 sensor signal at control module is greater than .7 volt for a precalibrated period, fuel system is attempting to lean mixture and time since engine start is one minute or more. Most likely causes of code are: an actual rich condition, a short to voltage on sensor signal circuit or a faulty PCM.

Note. Test number refers to number on diagnostic chart.

1. Test checks to see if O2 sensor is registering a rich condition.

If other codes of lower number are set with code, use those charts first. Malfunction in MAP or TPS sensor circuits can cause a rich exhaust code to set. If other codes are not set, rich exhaust is most likely caused by

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. Control module can compensate for slight increases, but a code will be set if air/fuel ratio becomes too rich.

Ignition Ground

If an open occurs on HEI ground circuit, HEI induced electrical "noise" may result, causing simulated reference pulses to be picked up by control module on reference line of EST harness. Additional pulses result in a higher than actual engine speed signal. Control module will increase injector pulse width ("on" time) to match increased RPM signal. Scan tester will show higher than actual RPM, which can help in diagnosing this problem.

Evaporative Fuel Canister

Fuel saturation of charcoal canister will cause a rich air/fuel ratio. If canister is full of fuel, check canister control valves and hoses.

MAP Sensor

An output causing control module to sense a higher than normal manifold pressure (low vacuum) can cause system to go rich. Disconnecting MAP sensor will allow control module to substitute a fixed value for MAP sensor. If condition disappears, substitute a different 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 exterior of O2 sensor. False high signal voltage (low oxygen content sensed) produced is interpreted by control module as a rich mixture, causing control module to set code.

EGR Problem

EGR valve sticking open at idle is usually accompanied by a rough idle and/or stalling. Also check for shorted/leaking injector and fuel-contaminated oil. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article in this section.

CODE 45 PCM TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 45 Diagnostic Flow Chart (3.1L) Rich Exhaust Indication. Scheme 37

CODE 46, PASS-KEY CIRCUIT

The PASS-Key system is designated to disable vehicle operation if the incorrect key or starting procedure is used. PASS-Key decoder module sends a signal to the PCM if the correct key is being used. If the proper signal does not reach the PCM, the PCM will not pulse the injectors, thus not allowing the vehicle to start. Code 46 will set if proper signal is not being received on fuel enable signal circuit to PCM when ignition is turned on.

Note. Test numbers refer to numbers on diagnostic chart.

1. If engine cranks but does not start, it indicates the portion of the module which generates the signal to the PCM is not operating or fuel enable signal circuit is open or shorted to ground. If decoder module is found to be okay, PCM may be at faulty, but this not a likely condition.
2. If Code 46 is stored and engine will not crank, it indicates that there is a Pass-Key problem or incorrect key or starting procedure is being used.

Code 46 Schematic (3.1L) Pass-Key Circuit. Scheme 38

Code 46 Diagnostic Flow Chart (3.1L) Pass-Key Circuit. Scheme 39

CODE 51, FAULTY PROM/MEM-CAL

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

CODE 52, FAULTY CAL-PAK

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

CODE 53, SYSTEM VOLTAGE HIGH

This code indicates a basic charging system problem. Code 53 will set when voltage at control module terminal is greater than 16.5 volts. If voltage at PCM battery voltage terminal is not within specification, check and repair charging system.

Note. Test numbers refer to numbers on diagnostic chart.

1. Normal battery output is 8.0-16.5 volts.
2. Checks if high voltage is due to generator or PCM. With engine running, voltage at battery should be greater than 12.1 volts.
3. Checks to see if generator is faulty under load condition. If voltage is greater than 16.5 volts, service generator.

Code 53 Diagnostic Flow Chart (3.1L) System Voltage High. Scheme 40

CODE 54, FUEL PUMP VOLTAGE LOW

Fuel pump circuit No. 120 is monitored and used by PCM for fuel system compensations based on system voltage. Signal is also used to store a trouble code if fuel pump relay is defective or fuel pump voltage is lost while engine is running. About 12 volts should exist on fuel pump signal circuit for 2 seconds after ignition is turned on or anytime reference pulses are being received by PCM.

Code 54 will set if voltage on circuit is less than 7 volts for 3 seconds. The oil pressure switch will provide a current path for fuel pump when oil pressure becomes great enough to close the contacts. Malfunction Indicator Light (MIL) will illuminate.

Code 54 Schematic (3.1L) Fuel Pump Voltage Low. Scheme 41

Code 54 Diagnostic Flow Chart (3.1L) Fuel Pump Voltage Low. Scheme 42

CODE 55, PCM ERROR

Ensure PCM grounds are okay. Ensure correct EPROM is being used and that it is properly installed. If okay, replace PCM. Clear codes, confirm closed loop operation and check operation of Malfunction Indicator Light (MIL).

CODE 58, TRANS TEMPERATURE SENSOR HIGH TEMPERATURE

Temperature sensor is a 2 wire thermistor located in transaxle. This sensor operates in the same manner as engine coolant temperature sensor and intake air temperature sensor. PCM applies 5 volts to sensor on signal circuit. Voltage sensed at PCM signal circuit terminal will be high when transmission fluid is cold and as fluid warms voltage signal will drop. A very low signal voltage indicates a high fluid temperature or a failure in sensor circuit. Normal fluid temperature range is 75-100°C.

Code 58 will set when sensor indicates a high fluid temperature for a precalibrated period of time. Malfunction Indicator Light (MIL) will illuminate and PCM will use a warm default sensor value.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks for a short to ground or an out-of-calibration sensor.
2. Checks for an internal fault within the transmission by creating an open.

Check harness routing for a potential short to ground in signal circuit. Temperature should rise steadily to about 90°C then stabilize. A skewed sensor could result in delayed shifts or TCC enabled complaints.

Use temperature-to-resistance table to check sensor at various levels to determine if sensor is out-of-calibration, which could result in firm shifts or TCC enable.

TEMPERATURE-TO-RESISTANCE VALUES (Measure resistance across sensor terminals)

Code 58 Schematic (3.1L) Transmission Temperature Sensor High Temperature. Scheme 43

Code 58 Diagnostic Flow Chart (3.1L) Transmission Temperature Sensor High Temperature. Scheme 44

CODE 59, TRANS. TEMPERATURE SENSOR LOW TEMPERATURE

Temperature sensor is a 2 wire thermistor located in transaxle. This sensor operates in the same manner as engine coolant temperature sensor and intake air temperature sensor. PCM applies 5 volts to sensor on signal circuit. Voltage sensed at PCM signal circuit terminal will be high when transmission fluid is cold and as fluid warms voltage signal will drop. A very high signal voltage indicates a low fluid temperature or a failure in sensor circuit. Normal fluid temperature range is 75-100°C.

Code 59 will set when sensor indicates a low temperature for a precalibrated period of time. Malfunction Indicator Light (MIL) will not illuminate and PCM will use a warm default sensor value.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test determines if a current condition exists.
2. Simulates a Code 58. If PCM recognizes low signal voltage, and scan displays about 146°C or higher, PCM and wiring are okay.
3. Checks if signal circuit is open. Five volts should be present at sensor connector.

Temperature should rise steadily to about 90°C, then stabilize. A faulty connection or an open in ground or signal circuit can result in Code 59. A skewed sensor could result in firm shifts or TCC enabled complaints. Use temperature-to-resistance table to check sensor at various levels to determine if sensor is out-of-calibration, which could result in firm shifts or TCC enable.

TEMPERATURE-TO-RESISTANCE VALUES (Measure resistance across sensor terminals)

Code 59 Diagnostic Flow Chart (3.1L) Transmission Temperature Sensor Low Temperature. Scheme 45

CODE 66, LOW A/C CHARGE

The A/C refrigerant pressure sensor responds to changes in A/C refrigerant system high side pressure. Input indicates how much load A/C compressor is putting on the engine and is one of the factors used by the PCM to determine IAC valve position for idle speed control, and engine cooling fan operation. The circuit consists of a 5-volt reference and a ground, both provided by the PCM, and a signal line to the PCM. The signal is a voltage which is proportional to the pressure. Code 66 sets if the voltage is less than a calibrated level for 5 seconds or more. The A/C compressor is disabled by the PCM if Code 66 is present or if pressure is greater than or less than calibrated values.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step checks the voltage signal being received by the PCM from the A/C refrigerant pressure sensor.
2. Checks to see if low voltage signal is from the sensor or the circuit. Jumpering the sensor signal circuit to 5 volts checks the circuit, connections and PCM.
3. This step checks to see if the low voltage signal was due to an open in the sensor signal circuit or the 5-volt reference circuit, since the proper step eliminated the pressure sensor.

Code 66 sets when signal voltage falls outside the normal possible range of the sensor and is not due to a refrigerant system problem. If problem is intermittent, check for opens or shorts in harness or poor connections.

Code 66 Schematic (3.1L) Low A/C Charge. Scheme 46

Code 66 Diagnostic Flow Chart (3.1L) Low A/C Charge. Scheme 47

CODE 70, HIGH A/C PRESSURE

A/C pressure sensor responds to changes in A/C refrigerant system high side pressure. PCM uses A/C compressor load input to determine engine idle speed. Sensor uses a 5-volt reference signal from PCM and returns an input signal to PCM on a separate line. Low pressure (zero psi) will return a signal of about .1 volt. High pressure will return a signal of about 4.9 volts. PCM will disable A/C if Code 66 is present.

Note. Test numbers refer to numbers on diagnostic chart.

1. This step checks the voltage signal being received by the PCM from the A/C refrigerant pressure sensor.
2. Checks to see if the high voltage signal is from a shorted sensor or a short to voltage in the circuit. Normally, disconnecting the sensor would make a normal circuit go to near zero volts.

Code 70 sets when signal voltage falls outside of the normal possible range of the sensor and is not due to a refrigerant system problem. If problem is intermittent, check for opens or short in the harness or poor connections.

Code 70 Diagnostic Flow Chart (3.1L) High A/C Pressure. Scheme 48

CODE 72, VEHICLE SPEED SENSOR CIRCUIT

A vehicle speed signal is provided to the PCM by the Vehicle Speed Sensor (VSS). This sensor produces a pulsing waveform voltage whenever vehicle speed is greater than 2 MPH. Voltage level or amplitude and frequency of pulses increases with vehicle speed. PCM conditions this signal and calculates vehicle speed. The PCM will then supply the signal to the instrument panel (4000 pulses per mile).

Note. Test numbers refer to numbers on diagnostic chart.

1. Sensor will produce a signal and vehicle speed only if the wheels are turning faster than 3 MPH.
2. If circuits 389, 400 and 401 are okay, and if speedometer works correctly, DTC 24 is being caused by a faulty PCM or an incorrect program.

Tech 1(R) scan tool should indicate a vehicle speed whenever the drive wheels are turning at greater than 3 MPH.

A problem in circuit 389 will not affect the VSS input or readings on scan tool. Check circuits 400 and 401 for proper connections. Ensure connections are clean and tight, and harness is routed correctly.

DTC 72 is set when PCM detects a loss of the VSS signal as vehicle is moving at road speeds. This would typically indicate a hardware or circuit failure rather than a software or calibration error.

Code 72 Schematic (3.1L) Vehicle Speed Sensor Circuit. Scheme 49

Code 72 Diagnostic Flow Chart (3.1L) Vehicle Speed Sensor Circuit. Scheme 50

CODE 75, 76 OR 77, EGR SOLENOID ERROR

PCM performs an EGR diagnostic check to monitor flow of exhaust gases through 3 EGR solenoids. When vehicle is under deceleration, EGR valves are normally closed. PCM then opens each valve in succession (while keeping previous valves open) and manifold vacuum is monitored for a calculated MAP increase associated with each valve's application. Should expected response of manifold vacuum not be seen by PCM, a Code 75 (solenoid No.1), 76 (solenoid No. 2) or 77 (solenoid No. 3) will set.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test determines if there is power to EGR valve.
2. This test will determine if there is an open circuit in EGR wiring or if the EGR valve is at fault.
3. This test will determine if there is a short to ground in solenoid drive circuits or if PCM is at fault.

An intermittent may be caused by a poor connection, chafed wire insulation, or a wire broken inside insulation. Intake plenum should be checked for possible plugged passages.

Code 75, 76 OR 77 Schematic (3.1L) EGR Solenoid Error. Scheme 51

Code 75, 76 OR 77 Diagnostic Flow Chart (3.1L) EGR Solenoid Error. Scheme 52

CODE 79, TRANSMISSION FLUID OVERTEMP

PCM supplies and monitors a 5-volt reference to sensor. As transmission temperature changes, sensor resistance changes, affecting monitored voltage. When fluid is cold, resistance is high, resulting in a high monitored voltage. When fluid is hot, resistance is low, resulting in a low monitored voltage. Code will set if transmission temperature is 120-130°C for 5 seconds and Codes 58 and 59 are not set.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks for out-of-calibration sensor or shorted circuit. 2 Simulates Code 59.

Check harness for potential short to ground. See Code 59 chart to check transmission temperature sensor temperature-to-resistance values. Also, check transmission fluid level. Check for conditions which could cause transmission to overheat i.e. trailer towing, steep grades, etc.

TEMPERATURE-TO-RESISTANCE VALUES (Measure resistance across sensor terminals)

Code 79 Diagnostic Flow Chart (3.1L) Transmission Fluid Overtemp. Scheme 53

CODE 80/90, TRANSAXLE COMPONENT SLIPPING

PCM monitors difference between engine speed and transaxle input shaft speed. With selector in D3 or D4 (indicated gear 3 or 4), and TCC is locked, scan should display engine speed closely matching input speed.

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks continuity of internal transaxle harness and apply solenoid.
2. Checks for open in power supply to transaxle.
3. Checks PWM return circuitry to PCM.

Check all connections to transaxle pass-through connector. An open in the ignition feed circuit will cause multiple codes to set. If scan tester indicates TCC is working, road test vehicle. If engine RPM does not decrease when TCC engagement is indicated on TCC, diagnose transaxle for possible mechanical problem.

Code 80/90 Diagnostic Flow Chart (3.1L) Transaxle Component Slipping. Scheme 54

CODE 82, 3X SIGNAL ERROR

Magnetic crank sensor is used to determine engine crankshaft position much the same way as the pick-up coil did in distributor type systems. Sensor is mounted in block near a 7-slot wheel on crankshaft. Rotation of wheel creates a magnetic field in sensor which induces a voltage signal. Electronic ignition module then processes this signal and creates reference pulses needed by PCM to trigger correct coil at correct time.

Note. Test numbers refer to numbers on diagnostic chart.

1. This test will determine if 12-volt supply and a good ground are available at ignition module.
2. Checks for resistance of crank sensor and connections.
3. Voltage will vary in this test depending on cranking speed of engine.

An open or shorted 3X signal will cause an "engine cranks but won't start" condition. An intermittent problem may set a Code 82.

Code 82 Diagnostic Flow Chart (3.1L) 3X Signal Error. Scheme 55

CODE 85, PROM ERROR & CODE 87, EEPROM ERROR

Ensure all PCM connectors are fully inserted in socket. If okay, have PCM reprogrammed using appropriate equipment. If equipment is not available, have PCM serviced through dealership.

CODE 86, ANALOG/DIGITAL ERROR

If any other codes are present, use those charts first. The Analog/Digital (A/D) multiplexer chip is an internal part of the PCM. Some PCM sensor inputs lead to the A/D multiplexer. If any one of them shorts to battery voltage for more than 15 seconds, the A/D error diagnostic code will set, resulting in a Code 86.

If Code 86 is intermittent, all circuits leading to the PCM that are connected to the A/D multiplexer should be checked for an intermittent short to battery voltage. Monitor each input circuit on a scan tester and wiggle related circuits (wiring and connectors). Any circuit that intermittently shorts to battery voltage will show a change in data on the scan tester.

Code 86 Schematic (3.1L) Analog/Digital Error. Scheme 56

CODE 96, TRANSMISSION VOLTAGE LOW

Note. Test numbers refer to numbers on diagnostic chart.

1. Checks for normal battery voltage of 9-15 volts.
2. Checks if low voltage is due to generator voltage supply circuit, or PCM. If voltage is less than 8.6 volts (3.1L) or 9 volts (3.4L), PCM is okay.

If code sets when an accessory is operated, check for poor connections or excessive current draw. Also, check for poor connections at starter solenoid or fusible link.

Code 96 Schematic (3.1L) Transmission Voltage Low. Scheme 57

CODES 98 & 99, INVALID PCM PROGRAM

Attempt to reprogram PCM. If Code resets, replace PCM. Reprogramming of PCM requires special equipment.

SUMMARY

If no hard fault codes are present, driveability symptoms exist or intermittent codes exist, proceed to appropriate TESTS W/O CODES article in this section for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.) or intermittent diagnostic procedures.

3.1L W/3-Speed A/T (VIN M) PCM Wiring Diagram (Grand Prix - 1 Of 2). Scheme 58

3.1L W/3-Speed A/T (VIN M) PCM Wiring Diagram (Grand Prix - 2 Of 2). Scheme 59

3.1L W/4-Speed A/T (VIN M) PCM Wiring Diagram (All Models - 1 Of 2). Scheme 60

3.1L W/4-Speed A/T (VIN M) PCM Wiring Diagram (All Models - 2 Of 2). Scheme 61

See also:
• BASIC TESTING
• TESTS W/O CODES
• SYSTEM/COMPONENT TESTS - 3.1L
• HARD OR INTERMITTENT TROUBLE CODE DETERMINATION
• SCAN TESTER USAGE
• SCAN DATA
• CODE 13
• CODE 14
• CODE 15
• CODE 16
• CODE 17
• CODE 21
• CODE 22
• CODE 23
• CODE 24
• CODE 25
• CODE 28
• CODE 33
• CODE 34
• CODE 35
• CODE 36
• CODE 37
• CODE 41
• CODE 42
• CODE 43
• CODE 44
• CODE 45
• CODE 46
• CODE 51
• CODE 52
• CODE 53
• CODE 54
• CODE 55
• CODE 58
• CODE 59
• CODE 66
• CODE 70
• CODE 72
• CODE 75, 76 OR 77
• CODE 80/90
• CODE 82
• CODE 85
• CODE 86
• CODE 96
• CODE 98 & 99
• CLEARING TROUBLE CODES
• SCAN DATA