Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Reading Trouble Codes
The ECM stores component failure information for the CCC system under a related trouble code which can be recalled for diagnosis and repair. Trouble codes may be read by counting flashes of the "SERVICE ENGINE SOON" light, or by reading the output of a diagnostic "Scan" tester connected to the ALDL connector. The tester is faster, more accurate, and capable of reading information which otherwise would necessitate testing individual ECM and sensor/solenoid connector terminals with a digital voltmeter. See SCAN TESTER - TEST DATA PARAMETERS table and SCAN TESTER USAGE in this article.
If "Scan" tester is not available, it is possible to read flashes of the "SERVICE ENGINE SOON" light by grounding the diagnostic terminal of the ALDL with ignition on and engine off. For example, "FLASH", "FLASH", pause, "FLASH", longer pause, identifies 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 numbered code. Each code is displayed 3 times. Codes will continue to repeat as long as ALDL "test" terminal is grounded.
| Code No. | Circuit Affected | ||
|---|---|---|---|
| 12 (1) | No RPM reference pulse | ||
| 13 | Open oxygen sensor circuit | ||
| 14 | CTS signal voltage low | ||
| 15 | CTS signal voltage high | ||
| 16 | System voltage high (3.3L & 3.8L) | ||
| 21 | TPS signal voltage high | ||
| 22 | TPS signal voltage low | ||
| 23 | MAT sensor signal voltage high M/C solenoid voltage low | ||
| 24 | VSS circuit | ||
| 25 | MAT sensor signal voltage low | ||
| 26 | Quad-Driver error | ||
| 27, 28 & 29 | Gear switch problem (3.8L) | ||
| 31 | Wastegate error (turbo) Purge solenoid voltage high (carb.) Park/Neutral switch (3.8L) | ||
| 32 | EGR vacuum control signal | ||
| 33 | MAP sensor signal voltage high MAF sensor signal voltage high (PFI) | ||
| 34 | MAP sensor signal voltage low Pressure sensor circuit MAF sensor signal voltage low (PFI) | ||
| 35 | IAC Idle speed error | ||
| 36 | MAF sensor burnoff (5.0L & 5.7L) Closed throttle airflow high (2.3L) | ||
| 38 | Brake switch (3.3L & 3.8L) | ||
| 39 | TCC (3.3L & 3.8L) | ||
| 41 | No distributor reference (carb.) C(3)I ignition - cam sensor loss Cylinder select error (MEM-CAL) | ||
| 42 | EST circuit open or grounded | ||
| 43 | ESC retard signal too low | ||
| 44 | Lean exhaust indicated | ||
| 45 | Rich exhaust indicated | ||
| 46 | Anti-theft fault (5.7L) Power steering pressure switch (3.3L) | ||
| 48 | Misfire diagnosis (3.8L) | ||
| 51 | Faulty PROM, MEM-CAL or ECM | ||
| 52 | Faulty/missing CALPAC or MEM-CAL | ||
| 53 | Faulty alternator, voltage high EGR system malfunction Anti-theft circuit fault | ||
| 54 | Fuel pump voltage low MC solenoid voltage high (carb.) | ||
| 55 | Faulty ECM | ||
| 61 | Degraded O2 sensor (2.8L & 3.1L) | ||
| 62 | Gear switch error (2.3L) | ||
| 63 | EGR flow error (3.8L) | ||
| 64 | EGR flow error (3.8L) | ||
| 65 | Fuel injector current low (2.3L) EGR flow error (3.8L) | ||
| 66 | A/C press. sensor voltage out of spec. (2.3L & 3.3L) | ||
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NO RELATED
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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.