1985 - 1990 ECM Trouble Codes

Diagnosing a problem within the Computer Command Control System (CCCS) is not a difficult task as the CCCS has a built-in diagnostic system, which indicates a problem by flashing the "Check Engine" light on the instrument panel. All we need to do now is extract the codes. By using the chart below, we will be able to narrow the problem area down considerably.  The following trouble codes are for 1985 to 1990 Camaro IROC-Z vehicles.

Code Problem or Description Possible Causes
12

No reference pulses to ECM.
(All)

  • This code should flash whenever the "test" terminal is grounded with the ignition on and the engine not running.

  • If the engine is running and the code appears, this indicates that the ECM is not receiving any references from the distributor.
  • Faulty or loose EST connector at the distributor.
13

Oxygen sensor circuit
(All)

  • Sticking or misadjusted TPS

  • Faulty wiring and/or connectors from the oxygen sensor.  

  • Faulty oxygen sensor.
14

Coolant sensor circuit
(All)

  • If the engine is experiencing overheating problems, rectify before diagnosing further.

  • Faulty or loose wiring and/or connectors from the coolant sensor.

  • Faulty coolant sensor.
15

Coolant sensor circuit
(All) 

  • See note for code 14.

  • Faulty or loose wiring and/or connectors from the coolant sensor.  

  • Faulty coolant sensor.  

  • Faulty or loose wiring and/or connections at the ECM.
21

Throttle position sensor (TPS) circuit
(All)

  • Sticking or misadjusted TPS plunger.

  • Faulty or loose wiring/and or connectors at TPS and/or at the ECM.  

  • Faulty TPS.
22

Throttle position sensor (TPS) circuit
(All)

  • TPS misadjusted.  

  • Faulty or loose wiring and/or connections at the ECM.  

  • Faulty TPS.
23

Manifold Air Temperature (MAT) sensor

  • Faulty MAT sensor.  

  • Faulty or loose wiring and/or connections to the MAT sensor.
24

Vehicle speed sensor (VSS) circuit
(All)

  • A code 24 should only be set while the vehicle is in motion.  Disregard code 24 if set when drive wheels are not turning.

  • Faulty or loose wiring and/or connections at the ECM.  

  • TPS misadjusted.  

  • Faulty VSS.

25

Manifold Air Temperature (MAT) sensor

  • Incorrect voltage level of signal from the MAT sensor to the ECM.  Should be above 4 volts
32

Baro sensor circuit

  • Short between sensor terminals B and C or faulty wiring therein.  

  • Faulty or loose wiring and/or connections at the ECM (Terminals 1, 21 and 22).  

  • Faulty Baro sensor
32

EGR system

  • Faulty EGR valve.  

  • Faulty or loose wiring and/or connections at the EGR solenoid.

  • Faulty, loose and/or leaking vacuum hoses to EGR valve.
33

Manifold Absolute Pressure (MAP) sensor
(All 1988 and later TBI V8, 1990 and later TPI V8)

  • Low vacuum sensed

       

 

  • Faulty or disconnected vacuum hoses.  

  • Faulty or loose wiring and/or connections at the ECM.  

  • Faulty MAP sensor.
33

Mass Air Flow (MAF) sensor
(1985 to 1989 vehicles, TPI V8)

  • Excessive airflow indicated.

  •  Incorrect voltage level at
    terminal C on the MAF sensor.  Should be 0.5 volts at idle, 4.7 volts at wide open throttle (WOT)

  •  Faulty or loose wiring and/or connections at the MAF sensor.

  • Faulty MAF sensor.
34

Manifold Absolute Pressure (MAP) sensor
(1988 and later TBI V8, 1990 and later TPI V8)

High vacuum sensed

  • Faulty or loose wiring and/or connections at the ECM.

  • Faulty MAP sensor
34

Mass Air Flow (MAF) sensor
(1985 to 1989 vehicles, MPFI V6 and TPI V8)

Low airflow indicated.

  •  Incorrect voltage level at terminal C on the MAF sensor.  Should be 0.5 volts at idle, 4.7 volts at wide open throttle (WOT).

  •  Faulty or loose wiring and/or connections at the MAF sensor.

  • Faulty MAF sensor.
35

Idle Air Control (IAC) circuit
(1987 to 1989 vehicles, TPI V8)

  • Closed throttle engine speed is 125 RPM above or below desired (commanded) idle speed for 45 seconds.

  • See a dealer service department for trouble diagnosis.
41

Cylinder select error
(Fuel injected vehicles)

  • Terminal D3 of ECM not properly grounded to engine.

  • Faulty or loose wiring and/or connections to the ECM.
41 No distributor reference pulses to ECM with engine running
(Carbureted vehicles)

  • Poor electrical connection.

  • Open or short in circuit

  • Defective distributor pick up coil.

  • Fault in the MAP or differential pressure sensor circuit.
42

Bypass or EST problem
(All)

  • If vehicle will not start, check wire leading to ECM terminal 12.

  • An improper HEI module can cause this code.
43

Electronic Spark Control (ESC) system
(All)

  • Faulty or loose wiring and/or connections to ECM terminal L.

  • Faulty or loose wiring and/or connections from the ESC controller to the ECM.

  • Faulty or loose wiring and/or connections from the knock sensor to ESC controller.

  • Voltage at ECM A-B connector terminal B7 should be over 6 volts unless the system is sensing detonation.

  • Faulty ESC sensor and/or module.
44

Lean exhaust
(All)

On carburetor-equipped vehicles:

  •  Faulty or sticking mixture control (M/C) solenoid.

  •   Faulty or loose wiring and/or connections at the ECM, terminals 9 and 14.

  •  Vacuum leakage at carburetor base gasket.

  • Faulty or loose vacuum hoses.

  • Faulty or leaking intake manifold gasket.

  • Air leakage at air management system-to-exhaust ports and at decel valve.

  • Faulty oxygen sensor.

On fuel injection vehicles:

  • Faulty or loose wiring and/or connections at the ECM.

  • Incorrect fuel pressure.

  • Faulty or leaking throttle body gasket.

  • Faulty or leaking intake manifold gasket.

  •  Faulty or loose vacuum hoses.

  • Water in fuel.

  • Faulty oxygen sensor.
45

Rich exhaust
(All)

On carburetor-equipped vehicles:

  • Faulty or sticking mixture control (M/C) solenoid and/or wiring.

  • Fuel in evaporative charcoal canister and its components indicate rich condition exists.

  •  Faulty oxygen sensor.

On fuel injection vehicles:

  •  Faulty or loose wiring and/or connections at the ECM.

  •  Incorrect fuel pressure.

  • Leaking fuel injectors.

  • Intermittent bursts of fuel from the injectors at idle indicate a faulty TPS.

  • Faulty oxygen sensor.
51

PROM problem
(All)

  • The PROM is located inside the ECM and is very delicate and easily broken.  An authorized mechanic should do all diagnostic procedures.

  • PROM not properly installed in the ECM.

  • Faulty PROM.

  • Faulty ECM.
52

Fuel CALPAK

  •  CALPAK PROM not properly installed.

  • Faulty PROM.
53

System over-voltage
(except 5.0L carbureted)

  • Voltage at ECM terminal B2 is greater than 17.1 volts for two or more seconds.

  • Faulty charging system.
53

EGR control error
(5.0L carbureted)

  • Faulty or loose wiring and/or connections to EGR solenoid.

  • Faulty or loose vacuum hoses to EGR valve.
54

Fuel pump circuit

  • Voltage at terminal B2 is less than 2 volts for 1.5 seconds since last reference pulse was received.

  • Faulty fuel pump relay, circuit and connections.

  • Faulty oil pressure switch.
55

ECM

  • Faulty ground connections to ECM.

  • Faulty ECM.

To extract this information from the ECM, we must use a jumper wire to ground the "Test" terminal on the ALDL connector.  This terminal is part of a wiring connector located just underneath the dashboard, next to the steering column. A small plate is used to cover the connector and must be removed to gain access to the terminals. With the connector exposed, push one end of the jumper wire into the "Test" terminal (B) and the other end into the "Ground" terminal (A). See figure 1.  Keep in mind that the ignition must be OFF so the risk of damage to the ECM is prevented. Once the terminals are shorted, turn the ignition to the ON position without starting the vehicle. The "Check Engine" light will begin to flash a series of codes, the first one, code 12, will consist of one flash, followed by a short pause, and then two flashes in quick succession.  After a longer pause, the code will repeat itself two more times, then proceed with any additional stored codes in the same manner, displaying each code three times before continuing on to the next. Once all of the codes have been displayed, the ECM will start over the sequence with code 12. If no codes are stored, code 12 will repeat until the ignition is turned OFF and the jumper wire removed. Once you see code 12 display for a forth time in a row, you can safely bet that no additional codes have been stored. 

Another method is to obtain a GM Code Scanner from most automotive stores (and others like Sears) for around $20.  Follow the instructions that come with the unit to extract the codes.  Although they are more expensive than a paper clip, they are much easier to deal with and extract codes much faster.  Just ensure you obtain a scanner that includes your year vehicle.

Once the codes have been extracted from the ECM, use the following chart to further diagnose the problem area. A good rule of thumb is to ensure that the code isn’t falsely triggered. This can be done by resetting the ECM (by disconnecting the positive (+) battery cable for 30 seconds, then reconnecting) and checking to see if the code re-appears during normal driving conditions. In some cases, codes may be stored without displaying a “Check Engine” light. The most common is code 42, which is the EST or bypass circuit. If the ECM detects loss of signal with the EST (when performing base timing adjustment for example) then regains connectivity, the “Check Engine” light will extinguish, however, a code 42 will be stored until erased from the ECM’s memory.  

Once the problem area has been determined, locate where the problem area resides. Once a repair action has been taken, reset the ECM and see if the code re-appears during normal driving conditions.

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