Decoding 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.

To select another year or component, simply reselect your
options above and click Select again.

2 Responses to “Decoding ECM Trouble Codes”

  1. necesito alguien que reprograme chips para mi TBI

  2. Hi Sanney – glad to see another post!

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