Diagnostic trouble codes: Difference between revisions

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Fiero ECM trouble codes commonly documented for stock 1984-1988 Fieros include:
Fiero ECM trouble codes commonly documented for stock 1984-1988 Fieros include:


* [[Code 12]] - Diagnostic mode
{| class="wikitable"
* [[Code 13]] - Oxygen sensor circuit
! Code
* [[Code 14]] - Coolant temperature sensor circuit, high temperature indication
! Description
* [[Code 15]] - Coolant temperature sensor circuit, low temperature indication
! Related system
* [[Code 21]] - Throttle position sensor circuit, high voltage
! Notes
* [[Code 22]] - Throttle position sensor circuit, low voltage
|-
* [[Code 23]] - Manifold air temperature sensor circuit, low temperature indication
| [[Code 12]]
* [[Code 24]] - Vehicle speed sensor circuit
| Diagnostic mode
* [[Code 25]] - Manifold air temperature sensor circuit, high temperature indication
| ECM
* [[Code 32]] - EGR system
| Code 12 indicates that the ECM is in diagnostic mode. It should flash first when codes are read through the ALDL connector.
* [[Code 33]] - MAP sensor circuit, high voltage
|-
* [[Code 34]] - MAP sensor circuit, low voltage
| [[Code 13]]
* [[Code 35]] - Idle speed error
| Oxygen sensor circuit
* [[Code 42]] - Electronic spark timing circuit
| Fuel control / emissions
* [[Code 44]] - Lean exhaust indication
| May relate to the oxygen sensor, wiring, connector, exhaust condition, or ECM signal interpretation.
* [[Code 45]] - Rich exhaust indication
|-
* [[Code 51]] - PROM error
| [[Code 14]]
* [[Code 52]] - CALPAK error
| Coolant temperature sensor circuit, high temperature indication
* [[Code 53]] - System over-voltage
| Engine control
* [[Code 55]] - ECM error
| May indicate a coolant temperature sensor circuit reading that appears too hot to the ECM.
|-
| [[Code 15]]
| Coolant temperature sensor circuit, low temperature indication
| Engine control
| May indicate a coolant temperature sensor circuit reading that appears too cold to the ECM.
|-
| [[Code 21]]
| Throttle position sensor circuit, high voltage
| Engine control
| May relate to the throttle position sensor, adjustment, wiring, connector, or ECM input.
|-
| [[Code 22]]
| Throttle position sensor circuit, low voltage
| Engine control
| May relate to the throttle position sensor, wiring, connector, or ECM input.
|-
| [[Code 23]]
| Manifold air temperature sensor circuit, low temperature indication
| Engine control
| May relate to the MAT sensor, wiring, connector, or ECM input.
|-
| [[Code 24]]
| Vehicle speed sensor circuit
| Engine control / speed signal
| May relate to the vehicle speed sensor, wiring, speedometer circuit, or ECM speed input.
|-
| [[Code 25]]
| Manifold air temperature sensor circuit, high temperature indication
| Engine control
| May relate to the MAT sensor, wiring, connector, or ECM input.
|-
| [[Code 32]]
| EGR system
| Emissions / engine control
| Commonly associated with EGR system problems, especially on V6 cars.
|-
| [[Code 33]]
| MAP sensor circuit, high voltage
| Engine control
| May relate to the MAP sensor, vacuum line, wiring, connector, or ECM input.
|-
| [[Code 34]]
| MAP sensor circuit, low voltage
| Engine control
| May relate to the MAP sensor, vacuum line, wiring, connector, or ECM input.
|-
| [[Code 35]]
| Idle speed error
| Idle control
| May relate to idle air control, vacuum leaks, throttle body condition, idle setting, or ECM control.
|-
| [[Code 42]]
| Electronic spark timing circuit
| Ignition control
| May relate to the ignition module, ECM, wiring, connectors, or electronic spark timing circuit.
|-
| [[Code 44]]
| Lean exhaust indication
| Fuel control / emissions
| May relate to vacuum leaks, low fuel pressure, oxygen sensor signal, exhaust leaks, or fuel-control problems.
|-
| [[Code 45]]
| Rich exhaust indication
| Fuel control / emissions
| May relate to excessive fuel pressure, leaking injectors, sensor faults, oxygen sensor signal, or fuel-control problems.
|-
| [[Code 51]]
| PROM error
| ECM
| May relate to the ECM PROM, ECM internal fault, or PROM connection.
|-
| [[Code 52]]
| CALPAK error
| ECM
| May relate to the ECM CALPAK, ECM internal fault, or CALPAK connection.
|-
| [[Code 53]]
| System over-voltage
| Charging / electrical system
| May relate to charging voltage, alternator regulation, wiring, or ECM voltage input.
|-
| [[Code 55]]
| ECM error
| ECM
| May indicate an internal ECM fault or related ECM problem.
|}


== Diagnosis ==
== Diagnosis ==

Revision as of 18:35, 6 July 2026

Diagnostic trouble codes are fault codes stored by the ECM when the engine control system detects certain problems.

On the Pontiac Fiero, trouble codes can be read through the ALDL diagnostic connector by placing the ECM into diagnostic mode and watching the check engine light flash the stored codes.

Overview

Fiero diagnostic trouble codes are not the same as modern OBD-II codes. They are earlier GM ECM trouble codes and are displayed as two-digit flash codes.

When the ECM is placed into diagnostic mode, the check engine light should flash Code 12 first. Code 12 indicates that the ECM is operating in diagnostic mode. Stored trouble codes, if present, are then flashed after Code 12.

Reading codes

Trouble codes are commonly read by jumpering the appropriate terminals on the ALDL connector, turning the ignition on without starting the engine, and counting the flashes of the check engine light.

Each code is flashed three times before moving to the next code.

Related topics:

Fiero trouble codes

Fiero ECM trouble codes commonly documented for stock 1984-1988 Fieros include:

Code Description Related system Notes
Code 12 Diagnostic mode ECM Code 12 indicates that the ECM is in diagnostic mode. It should flash first when codes are read through the ALDL connector.
Code 13 Oxygen sensor circuit Fuel control / emissions May relate to the oxygen sensor, wiring, connector, exhaust condition, or ECM signal interpretation.
Code 14 Coolant temperature sensor circuit, high temperature indication Engine control May indicate a coolant temperature sensor circuit reading that appears too hot to the ECM.
Code 15 Coolant temperature sensor circuit, low temperature indication Engine control May indicate a coolant temperature sensor circuit reading that appears too cold to the ECM.
Code 21 Throttle position sensor circuit, high voltage Engine control May relate to the throttle position sensor, adjustment, wiring, connector, or ECM input.
Code 22 Throttle position sensor circuit, low voltage Engine control May relate to the throttle position sensor, wiring, connector, or ECM input.
Code 23 Manifold air temperature sensor circuit, low temperature indication Engine control May relate to the MAT sensor, wiring, connector, or ECM input.
Code 24 Vehicle speed sensor circuit Engine control / speed signal May relate to the vehicle speed sensor, wiring, speedometer circuit, or ECM speed input.
Code 25 Manifold air temperature sensor circuit, high temperature indication Engine control May relate to the MAT sensor, wiring, connector, or ECM input.
Code 32 EGR system Emissions / engine control Commonly associated with EGR system problems, especially on V6 cars.
Code 33 MAP sensor circuit, high voltage Engine control May relate to the MAP sensor, vacuum line, wiring, connector, or ECM input.
Code 34 MAP sensor circuit, low voltage Engine control May relate to the MAP sensor, vacuum line, wiring, connector, or ECM input.
Code 35 Idle speed error Idle control May relate to idle air control, vacuum leaks, throttle body condition, idle setting, or ECM control.
Code 42 Electronic spark timing circuit Ignition control May relate to the ignition module, ECM, wiring, connectors, or electronic spark timing circuit.
Code 44 Lean exhaust indication Fuel control / emissions May relate to vacuum leaks, low fuel pressure, oxygen sensor signal, exhaust leaks, or fuel-control problems.
Code 45 Rich exhaust indication Fuel control / emissions May relate to excessive fuel pressure, leaking injectors, sensor faults, oxygen sensor signal, or fuel-control problems.
Code 51 PROM error ECM May relate to the ECM PROM, ECM internal fault, or PROM connection.
Code 52 CALPAK error ECM May relate to the ECM CALPAK, ECM internal fault, or CALPAK connection.
Code 53 System over-voltage Charging / electrical system May relate to charging voltage, alternator regulation, wiring, or ECM voltage input.
Code 55 ECM error ECM May indicate an internal ECM fault or related ECM problem.

Diagnosis

A trouble code does not always mean that the named sensor or component is bad.

A code may be caused by wiring problems, connector corrosion, poor grounds, vacuum leaks, fuel pressure problems, mechanical problems, exhaust leaks, or other faults that cause the ECM to see an unexpected signal.

Before replacing parts, diagnosis should include inspection, basic testing, wiring checks, connector checks, and comparison with service information.

Clearing codes

Stored trouble codes can be cleared after diagnosis and repair. The usual method is to disconnect ECM power long enough to erase stored memory.

After clearing codes, the vehicle may need to be driven again to see whether the code returns.

See also

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