2.0l Pfi Tests Pontiac Grand AM III

"SCAN" DIAGNOSTIC CIRCUIT CHECK

The following conditions must be met before testing

1. Engine at operating temperature.
2. Engine in closed loop operation.
3. Engine idling ("Engine Run" column).
4. Test terminal NOT grounded.
5. Scanner or ALDL tool NOT installed.

Note. In some of the diagnostic and trouble shooting charts the Assembly Line Data Link (ALDL) may also be referred to as the Assembly Line Communication Link (ALCL). This is the same test point for connection of aftermarket "Scan" testers.

The "Scan" Diagnostic Circuit Check is an organized approach for identifying fuel injection problems using an Assembly Line Data Link (ALDL). This communication link can provide diagnostic information for display on any "Scan" tester designed for this purpose.

If the "Scan" tester is not operating, check on another vehicle. If okay, the cigarette lighter socket should be checked for 12 volts and a good ground. Also check for an open diagnostic test terminal from ALDL terminal "B" and ECM. With ignition on, the diagnostic line should have about 5 volts present.

"Scan" Diagnostic Circuit Check. Scheme 527

CHART A-1, NO "SERVICE ENGINE SOON" LIGHT

"SERVICE ENGINE SOON" light should be on steady when ignition is on and engine is off. Battery voltage is supplied to the bulb. Bulb is grounded by ECM through circuit No. 419. Engine runs okay, check

1. Faulty light bulb.
2. Circuit No. 419 open.
3. Gauge fuse blown. This will result in no oil or generator lights, seat belt reminder, etc. Engine cranks but will not run.
4. Continuous battery fuse of fusible link open.
5. ECM ignition fuse open.
6. Battery circuit No. 240 to ECM open.
7. Ignition circuit No. 439 to ECM open.
8. Poor connection to ECM.

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

1. Probing circuit No. 419 to ground creates an alternate ground. If the "SERVICE ENGINE SOON" light comes on, this verifies the lamp is not at fault.
2. Relays and solenoids are operated by the ECM, using internal switches called "Drivers". Each driver is part of a group of 4 called "Quad Drivers". Failure of any driver can damage any other driver in the set. Use an ohmmeter to check resistance of solenoids listed in chart.

Chart A-1, No "Service Engine Soon" Light. Scheme 528

CHART A-2, NO CODE 12 "SERVICE ENGINE SOON" LIGHT ON

"SERVICE ENGINE SOON" light should be on steady when ignition is on and engine is off. Battery voltage is supplied to the bulb. Bulb is grounded by ECM through circuit No. 419. With the diagnostic terminal grounded, the light should flash a Code 12, followed by any trouble codes stored in memory. A steady light is possibly a short to ground in circuit No. 419, or an open in diagnostic circuit No. 451.

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

1. If there is a problem with the ECM that causes a "Scan" tester to not read serial data, then the ECM should not flash Code 12. If Code 12 does flash, be sure the "Scan" tester is functioning properly on another vehicle. If "Scan" tester is working properly, and circuit No. 461 is good, the MEM-CAL or ECM may be at fault.
2. This step checks for shorted circuit No. 419 to ground.
3. This step checks for an open circuit No. 451.
4. At this point, light wiring is okay. Problem is a faulty ECM or MEM-CAL. ECM is okay if Code 51 is stored when PROM is removed.

If Code 12 does not flash, the ECM should be replaced using the original MEM-CAL. Replace the MEM-CAL only after trying a different ECM, as a defective MEM-CAL is an unlikely cause of this problem.

Chart A-2, Won't Flash Code 12 ("Service Engine Soon" Light On). Scheme 529

CHART A-3-A, ENGINE CRANKS BUT WILL NOT RUN

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

1. A "SERVICE ENGINE SOON" light is a basic test to determine if there is a 12-volt battery supply and 12-volt ignition supply to the ECM. No ALDL function may be due to an ECM problem. The engine will not start without reference pulses. The "Scan" tester should read RPM during cranking.
2. If RPM was indicated during cranking, the ignition module is receiving a crank signal. "No Spark" at this test indicates the ignition module is not triggering the coil or there is a secondary ignition problem.

Chart A-3-A, Engine Cranks But Will Not Run. Scheme 530

CHART A-3-B, ENGINE CRANKS BUT WILL NOT RUN

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

1. This step checks for ignition voltage at the injector harness connector. Disconnect harness connector before probing terminal "B". Reconnect connector after test.
2. This test checks for open circuits at circuits No. 467 and 468 from connector to ECM. Be sure injector harness is connected.
3. Disconnect ECM connector "C-D" and check injector jumper circuits with a Digital Volt Ohm Meter (DVOM).

Chart A-3-B, Engine Cranks But Will Not Run. Scheme 531

CHART A-5, FUEL PUMP RELAY CIRCUIT

When the ignition switch is turned to the "ON" position, the ECM will turn on the electric fuel pump by energizing the fuel pump relay. The ECM will keep the pump on if the engine is running or cranking (ECM is receiving reference pulses from the ignition module). If there are no reference pulses, the ECM turns pump off within 2 seconds after key on. Normal pump pressure is approximately 35-38 psi (2.5-2.7 kg/cm 2 ) with the vacuum hose disconnected from the pressure regulator. Excess fuel is returned to the tank. When the engine is stopped, the pump can be turned on by applying battery voltage to the fuel pump test terminal, located in the engine compartment.

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

1. This test confirms the ECM is operating the fuel pump. Since the engine is not running and the ECM is not receiving reference pulses, the fuel pump relay will be turned off after 2 seconds.
2. This test turns pump on if circuit No. 120 is okay.
3. Checks relay ground circuit No. 450.
4. If fuse is blown, this test will confirm a short to ground in circuit No. 120. To prevent mis-diagnosis, be sure fuel pump is disconnected before test.
5. Checks for battery voltage at pump relay.
6. This test checks for ECM control of relay through circuit No. 465.
7. The fuel pump control circuit includes an oil pressure switch, parallel to the relay. Should the relay fail, the switch will provide voltage to the pump as long as oil pressure is more than 4 psi. If the relay fails, engine crank time may be extended because pump relay will not activate fuel pump until engine builds up oil pressure. If engine cranks slowly or switch is defective, engine may not start.
8. This test checks if oil pressure switch provides voltage to pump.
9. This test checks that oil pressure switch is open when engine is not running. Switch sticking closed will allow pump to run continuously and discharge battery.

Diagnostic Aids

Improper fuel system pressure may contribute to one or all of the following symptoms

1. Cranks but will not run.
2. Code 44 or 45.
3. Cuts out (may feel like ignition problems).
4. Hesitation, loss of power or poor fuel economy.

Chart A-5, Fuel Pump Relay Circuit (1 of 2). Scheme 532

Chart A-5, Fuel Pump Relay Circuit (2 of 2). Scheme 533

CHART A-7, FUEL SYSTEM DIAGNOSIS

The ECM will turn on the electric fuel pump when the ignition is turned on. The fuel pump is capable of producing fuel pressure as high as 65 psi (4.6 kg/cm 2 ). Normal pump pressure, with the vacuum hose disconnected from the pressure regulator, is 35-38 psi (2.5-2.7 kg/cm 2 ). Excess fuel passes through the pressure regulator and is returned to the tank via a fuel return line.

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

1. This test checks for adequate fuel delivery pressure. Pressure is controlled by spring pressure within the regulator assembly. Pressure should not leak down after the fuel pump is shut off.
2. Manifold vacuum is applied to fuel pressure regulator to control fuel pressure. When engine is idling, manifold pressure is low (high vacuum) and this will lower fuel line pressure of approximately 25-30 psi (1.8-2.1 kg/cm 2 ). This pressure drop at idle indicates proper regulator control.
3. The application of vacuum to the pressure regulator should result in a fuel pressure drop.
4. Pressure that continues to fall is caused by one of the following: In-tank fuel pump check valve not holding. Pump coupling hose leaking. Fuel pressure regulator valve leaking. Injector sticking open.
5. Pressure less than 35 psi (2.5 kg/cm 2 ) can be of 2 types: Regulated pressure is low when the amount of fuel to injector is okay, but pressure is too low. System may run lean and set Code 44. In addition, engine may be hard starting and have poor overall performance. Restricted fuel flow can cause a pressure drop. Normally, a vehicle with a system pressure of less than 9 psi (.6 kg/cm 2 ) at idle will not be driveable. If the pressure drop occurs only while driving, the engine will begin to surge and then stop as pressure drops rapidly.
6. Restricting fuel return line allows pump to develop maximum pressure. Pressure should exceed 65 psi (4.6 kg/cm 2 ).
7. This test determines if high pressure is due to a restricted fuel return line or a pressure regulator problem.

Improper fuel system pressure may contribute to one or all of the following symptoms

1. Cranks but wont run.
2. Code 44 or 45.
3. Cuts out (may feel like ignition problems).
4. Hesitation, loss of power or poor fuel economy.

Chart A-7, Fuel System Diagnosis (1 of 2). Scheme 534

Chart A-7, Fuel System Diagnosis (2 of 2). Scheme 535

CODE 13, OPEN OXYGEN SENSOR CIRCUIT

Code 13 will set 2 minutes after engine start if the ECM receives a steady O2 signal voltage between .35-.55 volts for more than one minute with TPS signal at more than 6.5 percent. Engine must also be at normal operating temperature.

Possible causes of a Code 13 are a faulty O2 sensor, faulty wiring or terminals, or a faulty ECM.

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

1. This test allows the ECM to confirm either open or closed loop operation. If the condition for Code 13 exists, the system will not go into closed loop.
2. This test will determine if the sensor is at fault or the wiring to the ECM is the cause of Code 13.
3. Use only a high impedance digital volt ohm meter when doing this test. This test checks the continuity of circuits No. 412 and 413. If circuit No. 413 is open the ECM voltage on circuit No. 412 will be over .6 volts (600 mv).

Normal "Scan" tester voltage varies between .1-1.0 volts (100-999 mv) while in closed loop. Code 13 will set in 3 seconds if all criteria have been met and the system will go into open loop.

Code 13, Open Oxygen Sensor Circuit. Scheme 536

CODE 14, COOLANT SENSOR SIGNAL VOLTAGE LOW

The Coolant Temperature Sensor uses a thermistor to control the signal voltage to the ECM. The ECM applies a voltage on circuit No. 410 to the sensor. When the engine is cold, sensor resistance is high and the ECM sees a high signal voltage. As the engine warms, the sensor resistance becomes less and the voltage drops. At operating temperature, voltage will measure about 1.5-2.0 volts at ECM terminal "E-16".

Code 14 will set if signal voltage indicates a coolant temperature more than 275°F (135°C) for more than 3 seconds. Code 14 will also set if circuit No. 410 is shorted to ground.

Coolant temperature is used to control: Fuel delivery, engine timing (EST), knock control (ESC), idle (IAC), and Torque Converter Clutch (TCC).

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

1. Checks to see if code was set as a result of a hard failure or an intermittent condition.
2. This test 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.

After the engine is started, the temperature should rise steadily to about 90°C (32°F), then stabilize when thermostat opens. If the engine is allowed to cool overnight, the coolant and MAT sensors, when measured with a "Scan" tester, should read close to each other.

When Code 14 is set, the ECM will turn on the cooling fan.

Code 14, Coolant Sensor Signal Voltage Low. Scheme 537

CODE 15, COOLANT SENSOR SIGNAL VOLTAGE HIGH

The Coolant Temperature Sensor (CTS) uses a thermistor to control the signal voltage to the ECM. The ECM applies a voltage on circuit No. 410 to the sensor. When the engine is cold, sensor resistance is high and the ECM sees a high signal voltage. As the engine warms, the sensor resistance becomes less and the voltage drops. At operating temperature, voltage will measure about 1.5-2 volts at ECM terminal "E16". Code 15 will set if signal voltage indicates a coolant temperature less than -22°F (-30°C) for 4 seconds. Coolant temperature is used to control: fuel delivery, engine timing (EST), knock control (ESC), idle (IAC) and Torque Converter Clutch (TCC).

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

1. This test checks to see if code was set as a result of a hard failure or intermittent condition.
2. This test simulates conditions for a Code 14. If the ECM recognizes the open circuit by displaying a low temperature, the ECM and wiring are not at fault.
3. This test will determine if there is a wiring problem or a faulty ECM. If circuit No. 452 is open, there may also be a Code 21 stored.

After the engine is started, the temperature should rise steadily to about 90°C (32°F), then stabilize when thermostat opens. If the engine is allowed to cool overnight, the coolant and MAT sensors should read close to each other when measured with a "Scan" tester. When Code 14 is set, the ECM will turn on the cooling fan.

Code 15, Coolant Sensor Signal Voltage High. Scheme 538

CODE 21, TPS SIGNAL VOLTAGE HIGH

The Throttle Position Sensor (TPS) provides a voltage signal that changes with the position of the throttle valve. Signal voltage will vary from one volt at idle to 4.6 volts at wide open throttle. Code 21 will set if

1. Engine is running and TPS voltage is greater than 2.5 volts for 5 seconds with engine speed less than 1300 RPM (auto. trans.) or 1400 (man. trans.).
2. MAP sensor detects high manifold vacuum (no load).

A Code 21 may also set if circuit No. 155 is open or circuit No. 41 is shorted to voltage.

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

1. This step determines if Code 21 is a hard fault or an intermittent condition.
2. This step simulates Code 22. If the ECM recognizes the low voltage signal and sets Code 22, the ECM and circuits No. 416 and 417 are not at fault.
3. This step isolates a faulty sensor, ECM, or an open circuit No. 155.

A "Scan" tester displays throttle position in volts. Closed throttle voltage should be less than one volt. Voltage should increase gradually to about 4.6 volts, at a steady rate, as throttle angle is increased.

Code 21, TPS Signal Voltage High. Scheme 539

CODE 22, TPS SIGNAL VOLTAGE LOW

The Throttle Position Sensor (TPS) provides a voltage signal that changes relative to the position of the throttle valve. Signal voltage will vary from one volt at idle to 4.6 volts at wide open throttle.

Code 22 will set if engine is running and TPS voltage is less than .2 volts for 5 seconds. A Code 22 may also set if circuits No. 416 or 417 are open or shorted to ground.

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

1. This step determines if Code 22 is a hard fault or an intermittent condition.
2. This step simulates Code 21. If the ECM recognizes the high voltage signal (over 4 volts) and sets Code 21, the ECM and wiring are not at fault.
3. This step checks for voltage from the TPS. The important thing is that the ECM recognizes the voltage as over 4 volts, indicating that circuit No. 417 and the ECM are not at fault.

A "Scan" tester displays throttle position in volts. Closed throttle voltage should be less than one volt. Voltage should increase gradually to about 4.6 volts, at a steady rate, as throttle angle is increased.

Code 22, TPS Signal Voltage Low. Scheme 540

CODE 23, MAT SENSOR SIGNAL VOLTAGE HIGH

The Manifold Air Temperature (MAT) sensor uses a thermistor to control signal voltage to the ECM. The ECM applies a voltage on circuit No. 472 to the sensor. When manifold air is cold, the sensor resistance is high and the ECM will set a high signal voltage. As the incoming air warms, the sensor resistance becomes less, and the voltage drops.

Code 23 will set if signal voltage indicates a manifold air temperature less than -40°F (-40°C) for more than 5 seconds during a boost condition. Due to conditions necessary to set a Code 23, the "SERVICE ENGINE SOON" light will only stay on while the fault is present.

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

1. A "Scan" tester may not be used to diagnose this fault due to the ECM transmitting "default values" when a fault is present. This test will determine if the ECM, sensor and wiring are at fault.
2. If the resistance is greater than 25,000 ohms, replace the sensor.

Code 23, MAT Sensor Signal Voltage High. Scheme 541

CODE 24, VEHICLE SPEED SENSOR (VSS) (PM GENERATOR)

The speed sensor, which is a Permanent Magnet (PM) generator, provides the ECM with vehicle speed information. The PM generator, mounted in the transmission, produces a pulsing voltage signal whenever the vehicle speed is over 3 MPH. The voltage level and the number of pulses increase with vehicle speed. The ECM converts the pulsing voltage to MPH, which is used by the ECM in calculations to determine vehicle adjustments.

A Code 24 will set when MPH reads zero, transmission is not in Park or Neutral, engine speed is between 1400-4400 RPM, TPS is at 2 percent (closed throttle), and a low airflow condition is sensed. All of these conditions must be met for at least 3 seconds.

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

1. This test monitors the ECM voltage on circuit No. 389. With wheels turning, the pulsing action created in circuits No. 400 and 401 will result in a voltage signal to the ECM and be displayed as MPH. The PM generator only produces a voltage signal if drive wheels are turning greater than 3 MPH.
2. Before replacing the ECM, PROM should be checked for correct application.

A faulty or misadjusted Park/Neutral switch may set a false Code 24. Use "Scan" tester and check for proper signal in Drive, while wiggling shifter.

Code 24, Vehicle Speed Sensor (VSS) (PM Generator). Scheme 542

CODE 25, MAT SENSOR SIGNAL VOLTAGE LOW

The Manifold Air Temperature (MAT) sensor uses a thermistor to control signal voltage to the ECM. The ECM applies and monitors a voltage on circuit No. 472 to the sensor. When manifold air is cold, the sensor resistance is high and the ECM will see a high signal voltage. As the incoming air warms, the sensor resistance becomes less and the voltage drops.

Code 25 will set if signal voltage indicates a manifold air temperature greater than 275°F (135°C) for at least 30 seconds. Due to the conditions necessary to set Code 25, the "SERVICE ENGINE SOON" light will only stay on while the fault is present.

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

1. A "Scan" tester may not be used to diagnose this fault due to the "default values" substituted by the ECM. If voltage is above 4 volts, the ECM and wiring are not at fault.
2. If the resistance is less than 100 ohms, replace the sensor.

Code 25, MAT Sensor Signal Voltage Low. Scheme 543

CODE 31, WASTEGATE OVERBOOST

Code 31 will set when the manifold pressure exceeds about 27 psi (1.9 kg/cm 2 ) of boost for 2 seconds and Code 33 has not been previously set. Code 31 will set and the "SERVICE ENGINE SOON" light will stay on only while overboost exists. The light will stay on for 10 seconds after the condition exists and then go off. An overboost condition could be caused by circuit No. 435 shorted to ground, a sticking wastegate actuator or wastegate, a control valve stuck in the closed position, a cut or pinched hose, or a faulty ECM. An underboost condition could be caused by a wastegate actuator or wastegate sticking open, no ignition feed to control valve or an open wire to ECM, or a faulty ECM.

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

1. With ignition off, the control valve solenoid is open and should allow pressure to be applied to wastegate actuator.
2. After pressure is applied to valve and the pressure source is removed, the actuator should slowly move back and close wastegate. If pressure does not bleed off, the vent in the control valve solenoid could be plugged.
3. With key on and diagnostic terminal grounded, control valve solenoid should be energized. This closes off the manifold to the wastegate actuator.
4. This test checks electrical control portion of the system. With ignition on and engine not running, solenoid should not be energized.

Code 31, Wastegate Overboost. Scheme 544

CODE 32, EGR CIRCUIT

The EGR vacuum control has an ECM controlled solenoid that pulses manifold vacuum to the EGR valve. The solenoid is always de-energized in Park, Neutral and at idle.

Code 32 will set if the EGR diagnostic vacuum switch does not sense vacuum with moderate to heavy engine load, but less than wide open throttle.

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

1. Be sure to check for presence of vacuum at throttle body side of EGR solenoid before continuing test.
2. Grounding the test terminal of the ALDL causes the ECM to energize the solenoid by grounding circuit No. 471. Test light should be on.
3. This step checks 12-volt source from ECM. A test light cannot be used.

Code 32, EGR Circuit. Scheme 545

CODE 33, MAP SENSOR SIGNAL VOLTAGE HIGH

The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). The ECM receives this information as signal voltage that will vary from one volt at idle to 4-4.5 volts at wide open throttle. If the MAP sensor fails, the ECM will substitute a fixed value and use the TPS sensor to control fuel delivery.

Code 33 will set when signal voltage is too high, TPS is less than 2 percent and these conditions have been met for more than 5 seconds. A Code 33 will also set if circuit No. 452 is open, or circuit 432 is shorted to voltage or to circuit No. 474.

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

1. This test confirms Code 33, and determines if the fault is the result of a hard failure or and intermittent condition.
2. This step simulates conditions for a Code 34. If the ECM recognizes and sets Code 34, low MAP signal, the ECM and circuits No. 474 and 432 are not at fault.

Possible causes of a Code 33 are a faulty MAP sensor, faulty wiring or terminals, or a faulty ECM. Disconnect MAP sensor and system will go to back-up mode. If misfire or idle condition remains, MAP sensor is okay.

With the ignition switch in the "ON" position and the engine stopped, manifold pressure is equal to atmospheric pressure and the signal voltage will be high. Comparison of the BARO readings from a known good vehicle using the same sensor is a good way to check the accuracy of the suspected sensor. Readings should be the same F .4 volts.

Code 33, MAP Sensor Signal Voltage High. Scheme 546

CODE 34, MAP SENSOR SIGNAL VOLTAGE LOW

The Manifold Absolute Pressure sensor (MAP) responds to changes in manifold pressure (vacuum). The ECM receives this information as a signal voltage that will vary from about one volt at idle to 4-4.5 volts at wide open throttle. If MAP sensor fails the ECM will substitute a fixed MAP value and use the TPS to control fuel delivery.

Code 34 will set when engine speed is less than 1200 RPM, TPS is greater than 20 percent and MAP signal voltage is low. A Code 34 will also result if circuits No. 474 and 432 are open or shorted to ground.

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

1. This test confirms Code 34, and determines if code is the result of a hard failure or an intermittent condition.
2. Jumpering harness terminal "B" to "C" will determine if sensor is at fault or if there is a problem with the ECM or wiring.
3. "Scan" tester may not display 12 volts. The important thing is that the ECM recognizes voltage as more than 4 volts and sets Code 33, high MAP signal, indicating the ECM and circuit No. 432 are not at fault.

With the ignition switch in the "ON" position and the engine stopped, manifold pressure is equal to atmospheric pressure and the signal voltage will be high. Comparison of the BARO readings from a known good vehicle using the same sensor is a good way to check the accuracy of the suspected sensor. Readings should be the same F .4 volts.

Code 34, MAP Sensor Signal Voltage Low. Scheme 547

CODE 35, IDLE SPEED ERROR

Code 35 will set when the closed throttle engine speed is 150 RPM above or below the correct idle speed for 20 seconds.

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

1. Continue with test even if engine will not idle. If idle is too low, "Scan" tester will display 80 or more counts. If idle is high it will display zero counts. Occasionally an erratic or unstable idle may occur. Engine speed may vary 200 RPM or more up and down. Disconnect IAC. If the condition is unchanged, the IAC is not at fault. There is a system problem.
2. When the engine was stopped, the IAC valve retracted to a fixed position for increased airflow during the next engine start. A "Scan" tester will display 80 or more counts.
3. Be sure to disconnect the IAC valve prior to this test. The test light will confirm the ECM signals by a steady or flashing light on all circuits.
4. There is a remote possibility that one of the circuits is shorted to voltage which would have been indicated by a steady light. Disconnect ECM and turn the ignition on. Probe terminals to check for this condition.

A slow, unstable idle may be caused by a system problem that cannot be overcome by IAC. "Scan" counts will be above 80 if if too low, and zero counts, if too high.

1. If idle is too high, stop engine. With ignition on, ground diagnostic terminal. Wait 45 seconds for IAC to seat, then disconnect IAC. Start engine. If idle speed is above minimum rate, look for possible vacuum leaks. If no leaks are detected, adjust minimum idle speed.
2. If air/fuel ratio is too lean the idle speed may be either too high or too low. Engine speed may vary up and down and disconnecting the IAC may not help. "Scan" and/or voltmeter will read an oxygen sensor output less than 300 mv (.3 volts). Check for low fuel pressure or water in the fuel. A contaminated O2 sensor (usually silicone) will cause a lean air/fuel mixture with an oxygen sensor output fixed above 800 mv (.8 volts). This may also set Code 45.
3. If air/fuel ratio is too rich, idle speed will be too low and "Scan" tester counts will usually be above 80. The system may be obviously rich, with Black smoke from the tailpipe. "Scan" tester and/or voltmeter will read an oxygen sensor voltage signal fixed above 800 mv (.8 volts). Look for high fuel pressure or injectors leaking or sticking. Remove IAC and inspect bore for foreign material or evidence of IAC valve dragging the bore.
4. Inspect area beneath throttle body for "coking". If coking exists, the throttle body must be cleaned before setting minimum air rate or replacing the IAC.

Code 35, Idle Speed Error. Scheme 548

CODE 42, ELECTRONIC SPARK TIMING (EST)

Code 42 indicates that the ECM has seen an open or short to ground in the EST or by-pass circuits.

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

1. This test confirms Code 42, and that fault is present.
2. This test checks for a normal EST ground path through the ignition module. An EST circuit No. 423 shorted to ground will read less than 500 ohms.
3. As the test light voltage touches terminal "C7", the module should switch causing the ohmmeter to "overrange" if the meter is in the 1000-2000 ohm position. Selecting a higher ohms position will indicate above 5000 ohms. It is only important that the module "switched".
4. If the module did not switch, this step will check for EST circuit No. 423 shorted to ground, an open by-pass circuit No. 424, or a faulty ignition module connection or module.
5. This test confirms that Code 42 is a faulty ECM and not an intermittent in circuits No. 423 or 424.

Code 42, Electronic Spark Timing (EST). Scheme 549

CODE 43, ELECTRONIC SPARK CONTROL

Code 43 indicates the ECM has seen high or low voltage at circuit No. 496 for longer than 5 seconds with the engine running.

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

1. If conditions for test as described above are being met, Code 43 will currently set and the "SES" light will be illuminated.
2. The ECM has a 5-volt pull-up resistor, which should be present at the knock sensor terminal.
3. This test determines if internal resistance of knock sensor is within an acceptable range.

Check circuit No. 496 for a potential short or open to ground. Also check for proper installation of MEM-CAL. Mechanical engine knock can cause a knock sensor signal. Abnormal engine noise must be corrected before using this chart.

Note. Code 43 circuit diagram and flow chart has been updated by Technical Service Bulletin No. 91-6-6, dated 8/24/90 for Pontiac models and Technical Service Bulletin No. 91-6E-01, dated 8/24/90 for Buick models.

Code 43 Circuit Diagram, Electronic Spark Control. Scheme 550

Code 43 Flow Chart, Electronic Spark Control. Scheme 551

CODE 44, LEAN EXHAUST INDICATION

Code 44 indicates that the ECM has seen O2 sensor voltage (at ECM terminal "E14") lower than .2 volts for 50 seconds or more, one minute after engine start. The ECM supplies a voltage of about .45 volt between terminals "E14" and "E15". The O2 sensor varies the voltage from one volt (rich indication) to .10 volt (lean indication). An open sensor circuit or a bad sensor causes open loop operation.

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

1. This test allows ECM to confirm conditions for Code 44.

Observe the Block Learn Memory (BLM) value at different RPMs using the "Scan" tester. If conditions for a Code 44 exist, the block learn value will be around 150.

1. O2 sensor wire may be mispositioned and laying against the exhaust manifold. Check for ground between sensor and wire connector.
2. Water, even small amounts, near the in-tank fuel pump inlet can be delivered to the injector. The water may cause a lean exhaust and set Code 44.
3. 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.

Code 44, Lean Exhaust Indication. Scheme 552

CODE 45, RICH EXHAUST INDICATION

The ECM supplies a voltage of about .45 volts between circuits No. 412 and 413. The O2 sensor varies the voltage from one volt (rich exhaust) to .10 volt (lean exhaust). The sensor acts like an open sensor circuit and produces no voltage when exhaust temperature is low. An open sensor circuit or cold sensor causes open loop operation.

Code 45 is set when O2 sensor signal at ECM is above .75 volts for 30 seconds and time since engine start is one minute or more. Code 45 indicates a rich exhaust and diagnosis should begin by checking for proper fuel pressure, leaking injectors, HEI shielding, canister purge saturation, coolant sensor, MAP sensor, O2 sensor contamination and TPS intermittent output.

Note. "Scan" tester measures and displays temperatures in degrees centigrade only.

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

1. Test determines if O2 sensor is registering a rich condition.

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

1. If fuel pressure is too high, air/fuel ratios will be rich. The ECM can compensate for some increase, however, if air/fuel ratio becomes too rich, a Code 45 will be set. See fuel system diagnosis «CHART A-7»(/pontiac/grand-am/iii-1984-1991/remont/testing-diagnostics/#20l-pfi-tests__chart-a-7-fuel-system-diagnosis) .
2. An open ignition ground may result at circuit No. 453 in EMI, or HEI induced electrical "noise" may occur, causing simulated reference pulses to be picked up by the ECM. The 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. Engine tachometer will show higher than actual RPM, which can help in diagnosing this problem.
3. Fuel saturation of the charcoal canister will cause a rich air/fuel ratio. If full of fuel, check canister control and hoses.
4. An output that causes the ECM to sense a higher than normal manifold pressure (low vacuum) can cause the system to go rich. Disconnecting the MAP sensor will allow the ECM to substitute a fixed value for the MAP sensor. If the condition disappears, substitute a different MAP sensor and continue testing.
5. O2 sensor contamination, caused by silicone in improper RTV sealants, may cause a White powdery coating to cover the exterior of the O2 sensor. The false high signal voltage (low oxygen content) produced is interpreted by the ECM as a rich mixture, causing the ECM to set Code 45.
6. Check pressure regulator for a ruptured vacuum diaphragm by checking the manifold vacuum hose for fuel.

Code 45, Rich Exhaust Indication. Scheme 553

CODE 51, FAULTY MEM-CAL

Check that pins are fully inserted in the socket and that MEM-CAL is properly latched in position. If no problem is visible, replace MEM-CAL, clear codes and recheck. If Code 51 reappears, replace ECM.

CHART C1 - ECM REPLACEMENT CHECK CHART

In order to reduce incidents of repeat ECM failure, a revised ECM diagnostic procedure is available. Beginning in 1982, most ECMs are equipped with Integrated Circuits (IC) in place of separate transistors to operate various controlled components.

These ICs, called Quad-Drivers (QDR), have 4 separate outputs, meaning that each QDR can operate up to 4 different components. An inoperative QDR can result in ECM output becoming open or shorted to ground. Often, all 4 outputs of a QDR will fail, even if just one QDR circuit is faulty.

Refer to the following tables to determine which ECMs contain QDRs. Since this procedure is not applicable to ECMs which do not contain QDRs, those ECMs are not listed.

Performing the diagnostic flow chart will identify an inoperative QDR. Once the circuit is identified, it must be repaired to eliminate repeat ECM failure. This diagnostic procedure must be used when "Replace ECM" is the conclusion of any procedure.

ECM QDR IDENTIFICATION

Scheme 554

Diagnosis

The Park/Neutral switch contacts are a part of the Neutral Start switch and are closed to ground in Park or Neutral and open in Drive ranges. The ECM supplies ignition voltage through a current limiting resistor to circuit No. 434 and senses a closed switch when the voltage on circuit No. 434 drops to less than one volt. The ECM uses the P/N signal as one of the inputs to control Idle Air Control (IAC) and VSS diagnostics.

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

1. This test checks for a closed switch to ground in Park position. If using an ohmmeter, instead of a test light or "Scan" tester, the resistance will be low, indicating continuity to ground.
2. This test checks for an open switch in Drive range.
3. Be sure "Scan" tester indicates Drive while wiggling shifter to test for a faulty or misadjusted switch.

Chart C-1A, Park Neutral Switch Diagnosis. Scheme 555

C-1D, MAP SENSOR OUTPUT CHECK

The MAP sensor measures manifold pressure (vacuum) and sends that signal to ECM. The ECM uses this information for fuel and spark control.

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

1. Check MAP sensor output voltage to ECM. This voltage, without engine running, represents a barometer reading to ECM.
2. Applying vacuum to MAP sensor should cause voltage to be 1.2 volts less than voltage at step 1). Upon applying vacuum to sensor, change in voltage should be instantaneous. A slow voltage change indicates a faulty sensor.
3. Check vacuum hose to sensor for leaks or restriction. Be sure no other vacuum devices are connected to MAP sensor signal hose.

C-1D Map Sensor Output. Scheme 556

CHART C-1E, POWER STEERING PRESSURE SWITCH

Power steering pressure switch is normally open to ground and circuit No. 495 will be near battery voltage. Turning the steering wheel increases power steering oil pressure and its load on an idling engine. The pressure switch will close before the load can cause an idle problem. Closing the switch causes circuit No. 495 to read less than one volt. The ECM will increase the idle air rate and retard the timing. Pressure switch that will not close, or an open circuit No. 450 or 495 may cause the engine to stop when power steering loads are high. A switch that will not open, or a shorted to ground circuit No. 450 or 495, will cause timing to retard at idle, and may affect idle quality.

1. This test checks for ECM signal voltage on circuit No. 495, and confirms that ground circuit No. 450 is okay.
2. Checks to determine if circuit No. 495 is shorted to ground.
3. This should simulate a closed switch.

Chart C-1E, Power Steering Pressure Switch. Scheme 557

CHART C-4C, IGNITION SYSTEM CHECK

1. This test checks for proper output from the ignition system. The spark tester requires a minimum of 25,000 volts to fire. Tester should be used in case of an ignition miss because the system may provide enough voltage to run the engine but not enough to fire a spark plug under heavy load. 1A) If spark occurs with EST connector disconnected, pick-up coil is too low for EST operation.
2. A spark indicates that the problem is a faulty distributor cap or rotor.
3. Normally, there should be battery voltage at the "+" terminal. Low voltage would indicate an open or high resistance circuit from the distributor to coil or ignition switch.
4. This test checks for a shorted module or grounded circuit from the ignition coil to the module. The distributor module should be turned off so normal voltage should be about 12 volts. If the module is turned on, the voltage would be low, but above one volt. This could cause the ignition coil to fail from excessive heat. With an open ignition coil primary winding, a small amount of voltage will leak through the module from the "BAT" terminal to the tach terminal.
5. This test checks for an open module or circuit to it. Applying 12 volts to the terminal "P" will turn the module on and the voltage should drop to about 7-9 volts.
6. This should turn off the module and cause a spark. If no spark occurs, the fault is most likely in the ignition coil because most module problems would have been found before this point in the procedure. A module tester could determine which is at fault.

Chart C-4C, Ignition System Check. Scheme 558

CHART C-5, ELECTRONIC SPARK CONTROL SYSTEM CHECK

Electronic Spark Control (ESC) is accomplished through a knock sensor that modifies a voltage sent by the ECM under detonation conditions. As the knock sensor detects engine knock, the voltage in circuit No. 496 drops from 5 volts to approximately 2.5 volts. This is sensed by the ECM and spark timing is modified.

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

1. With the engine idling, there should be no knock signal present.
2. Tapping the lift hood bracket should simulate a knock signal to determine if the sensor is capable of detecting detonation. If no knock is detected, try tapping on the engine block closer to the sensor, before replacing sensor.
3. If the engine has internal problems which are creating the knock, the sensor may be responding to internal failure.
4. This test determines if the knock sensor is faulty or if the ESC portion of MEM-CAL is faulty. If the MEM-CAL is suspected, be sure that it is properly installed and latched in place.

While observing knock signal on the "Scan" tester, there should be an indication that knock is present, when detonation can be heard. Detonation is most likely to occur during high engine load conditions.

Chart C-5, Electronic Spark Control System Check. Scheme 559

CHART C-7, EGR CHECK

The EGR valve is opened by vacuum to let exhaust gasses into the combustion chamber through the intake manifold. These inert gasses help to lower peak combustion chamber temperatures, which will eliminate detonation (pinging) and reduce the formation of oxides of nitrogen (NOx). If the volume of these gasses is too great or gasses are allowed to flow at the improper time, combustion air/fuel mixture may become "diluted" and a misfire occur. The amount of EGR valve flow is controlled by variations in vacuum and the EGR vacuum control solenoid.

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

1. Checks for sticking EGR valve. If sticking, remove and examine valve to determine whether it can be cleaned, or must be replaced. A sticking EGR valve will cause a rough idle condition.
2. Checks for plugged EGR passages. If passages are plugged, the engine may have severe detonation on acceleration.

Chart C-7, EGR Check. Scheme 560

CHART C-8, TRANSMISSION/TRANSAXLE TORQUE CONVERTER CLUTCH

Fused battery ignition is supplied to the TCC solenoid through the brake switch and transmission/transaxle third gear apply switch. The ECM will engage TCC by grounding circuit No. 422 to energize the solenoid. TCC will engage when engine is warmed up, vehicle speed is above a calibrated value, throttle position sensor output is not changing (indicating a steady road speed), transaxle third gear apply switch is closed and brake switch is closed.

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

1. Light off confirms transmission/transaxle third gear supply switch is open.
2. At 30 MPH the transmission/transaxle third gear switch should close. Test light will come on and confirm battery supply and closed brake switch.
3. Grounding the diagnostic terminal with engine off should energize the TCC solenoid. This test checks the capability of the ECM to control the solenoid.
4. Solenoids and relays are turned on and off by the ECM internal electronic switches called "drivers". Each driver is part of a group of 4 called Quad-Drivers. Failure of one can damage any other driver with in the set.

Check TCC solenoid resistance. Disconnect the TCC at transmission. Connect ohmmeter between transmission connector, opposite harness connector terminal "A" and "D". Raise drive wheels and run vehicle in Drive to about 30 MPH to close third gear apply switch. Replace TCC solenoid, and ECM, if resistance measures less than 20 ohms when switch is closed.

An engine coolant thermostat that is stuck open, or opens at too low a temperature, may result in an inoperative TCC.

Chart C-8, Transmission/Transaxle Torque Converter Clutch (TCC). Scheme 561

CHART C-10A, A/C CLUTCH CONTROL

With A/C on, ignition voltage is supplied to the A/C low pressure switch located in the low pressure line. If system refrigerant level is okay, the low pressure switch will be closed, completing the circuit to the closed high pressure cut-off switch and to circuits No. 901 and 459. The voltage in circuit No. 459 is shown on a "Scan" tester as A/C request on (voltage present) or off (no voltage). When a request for A/C is seen by the ECM, the ECM will respond by grounding circuit No. 458. The A/C clutch relay will then close and current will flow from circuit No. 459 to circuit No. 59 and engage the A/C compressor clutch. When A/C request voltage is seen by the ECM on circuit No. 459, the cooling fan will be turned on.

Chart C-10A, A/C Clutch Control. Scheme 562

CHART C-12, ENGINE COOLING FAN, TURBO N BODY

Battery voltage is supplied to fan relay terminal No. 1 and from ignition to terminal No. 5. Grounding relay terminal No. 2 will close relay and supply battery voltage to fan motor. Above 30 MPH, the ECM will remove ground from circuit No. 335. If coolant temperature and A/C pressure switches are open, fan will stop. Cooling fan will also run continuously if a Code 14 or 15 (CTS failure) is set.

Chart C-12, Engine Cooling Fan, Turbo N Body. Scheme 563

Scheme 564

Scheme 565

Scheme 566

See also:
• CHART A-7