The ECM stores component failure information under a related trouble code which can be recalled for diagnosis and repair. Read trouble codes by counting SERVICE ENGINE SOON light flashes or with diagnostic "Scan" tester connected to the ALDL connector. The tester is faster, more accurate and capable of reading information which would require testing individual ECM and sensor/solenoid connector terminals with a digital voltmeter. See SCAN DATA TABLES and SCAN TESTER USAGE in this article.

NOTE: When using a "Scan" tester, there is a time delay between serial data updates. For instantaneous response, a digital voltmeter must be used.

If "Scan" tester is not available, SERVICE ENGINE SOON light flashes can be read by grounding ALDL diagnostic terminal with ignition on and engine off. For example, FLASH, FLASH, pause, FLASH, longer pause, indicates Code 21. The first series of flashes are the first digit of trouble code. The second series of flashes are the second digit of trouble code. Trouble codes are displayed starting with the lowest code. Each code is displayed 3 times and will continue as long as ALDL is grounded.

NOTE: Trouble codes will be recorded at various operating times. Some codes require sensor or switch operation for 5 seconds and others may require longer under certain conditions. Some codes may not set in a service bay operational mode.

ECM TROUBLE CODE DEFINITION

Code No.

Circuit Affected

12 ⁽¹⁾

No RPM reference pulse

Open oxygen sensor circuit open

Coolant Temp. Sensor (CTS) signal voltage low

CTS signal voltage high

Throttle Position Sensor (TPS) signal voltage high

TPS signal voltage low

Manifold Air Temp. (MAT) sensor signal voltage high

Vehicle Speed Sensor circuit

MAT sensor signal voltage low

EGR system error

Manifold Absolute Pressure (MAP) sensor signal voltage high

MAP sensor signal voltage low

Idle Air Control idle speed error (2.5L)

EST circuit open or grounded

ESC retard signal too low (Except 2.5L)

Lean exhaust indicated

Rich exhaust indicated

Faulty PROM, MEM-CAL or ECM

Faulty/missing CALPAC

System voltage high (2.5L)

Fuel pump voltage low

Faulty ECM (Except 2.5L)

⁽¹⁾	Display of a Code 12 is normal when no reference pulses are received by ECM (engine not running).

NOTE: Trouble code charts should only be used if SERVICE ENGINE SOON light is illuminated (indicating a current problem exists). Exceptions are Codes 13, 15, 24, 44 and 45 charts, which may be used to help diagnose intermittent codes. Anytime Codes 51, 52, 54 or 55 are displayed with another code, start with 50-series code first and proceed to low profile numbered codes.

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.

• Brakes feel soft, pull hard to one side, or make grinding noises.

• The engine overheats, stalls repeatedly, or misfires under load.

• You are missing required tools, torque specs, or safe lifting equipment.

• You are not confident in the next step or safety outcome.

Safety Warning

Reading Trouble Codes

Use the Manual With the Right Hardware

When to See a Mechanic

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