Reading Trouble Codes
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.
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.
| Code No. | Circuit Affected | ||
|---|---|---|---|
| 12 (1) | No RPM reference pulse | ||
| 13 | Open oxygen sensor circuit open | ||
| 14 | Coolant Temp. Sensor (CTS) signal voltage low | ||
| 15 | CTS signal voltage high | ||
| 21 | Throttle Position Sensor (TPS) signal voltage high | ||
| 22 | TPS signal voltage low | ||
| 23 | Manifold Air Temp. (MAT) sensor signal voltage high | ||
| 24 | Vehicle Speed Sensor circuit | ||
| 25 | MAT sensor signal voltage low | ||
| 32 | EGR system error | ||
| 33 | Manifold Absolute Pressure (MAP) sensor signal voltage high | ||
| 34 | MAP sensor signal voltage low | ||
| 35 | Idle Air Control idle speed error (2.5L) | ||
| 42 | EST circuit open or grounded | ||
| 43 | ESC retard signal too low (Except 2.5L) | ||
| 44 | Lean exhaust indicated | ||
| 45 | Rich exhaust indicated | ||
| 51 | Faulty PROM, MEM-CAL or ECM | ||
| 52 | Faulty/missing CALPAC | ||
| 53 | System voltage high (2.5L) | ||
| 54 | Fuel pump voltage low | ||
| 55 | Faulty ECM (Except 2.5L) | ||
|
|||