If you own a 1982 to 1995 GM car or truck, you just might be able to read your trouble codes for free! If you have a connector like this under your dash, read on... (note: this connector may have a black plastic cover over it, so also look for a cover shaped like this, sorry for the messy wires here) This is referred to as the Assembly Line Communication Link (ALDL) connector.

ALDL Diagnostics Connector (Located Underdash)

How to read the codes

1. Ground terminals A and B (see diagram below) with the ignition "ON" and engine not running. The system will enter the diagnostic mode. In this mode, the computer will display a Code 12, three consecutive times, by flashing the "CHECK ENGINE", "SERVICE ENGINE SOON" or "MALFUNCTION INDICATOR LAMP" or light. A Code 12 consists of one flash, a short pause, then two flashes in rapid succession.

2. After Code 12 is displayed, any stored trouble codes will be displayed by flashing the "CHECK ENGINE", "SERVICE ENGINE SOON" or "MALFUNCTION INDICATOR LAMP" light. All codes are displayed 3 times each then a code 12 will repeat. In the diagnostic mode, the computer will also energize all computer controlled relays and solenoids.

GM 2 Digit Trouble Codes

Tech -- GM 2 Digit codes for 82 and up (non OBD-II) code - code definition

12- diagnostic mode

13- 02 sensor or circuit

14- coolant sensor or circuit/ high temp indicated

15- coolant sensor or circuit/ low temp indicated

16- system voltage out of range

19- crankshaft position sensor or circuit

21- throttle position sensor or circuit- voltage high

22- throttle position sensor or circuit- voltage low

23- mixture control (m/c) solenoid or circuit (carbureted models)

23- manifold air temperature (mat) sensor or circuit (1990 and earlier models)

23- intake air temperature (iat) sensor circuit (fuel-injected models)

24- vehicle speed sensor (vss) or circuit

25- manifold air temperature (mat) sensor or circuit - high temperature indicated (1990 and earlier models)

25- intake air temperature (iat) sensor or circuit - high temperature indicated (1991 and later models)

26- quad driver module circuit

27- quad driver module circuit

28- quad driver module circuit

29- quad driver module circuit

31- park/neutral position (pnp) switch circuit

32- baro sensor or circuit (carbureted models)

32- egr circuit (fuel-injected models)

33- manifold absolute pressure (map sensor signal voltage high

33- mass air flow (maf) sensor or circuit - excessive airflow indicated

34- manifold absolute pressure (map) sensor signal voltage low

34- mass air flow (maf) sensor signal - low airflow indicated

35- idle speed control (isc) switch or circuit (shorted) (carbureted models)

35- idle air control (iac) valve/ circuit

38- brake switch circuit

39- torque converter clutch (tcc) circuit

41- no distributor signals to ecm, or faulty ignition module ( carbed models)

41- cylinders select error- mem-cal or ecm problem ( fuel-injected models

41- cam sensor circuit (3.8 engine)

42- bypass or electronic spark timing (est) circuit

43- low voltage at ecm terminal l (carbureted models)

43- knock sensor circuit

44- oxygen sensor or circuit - lean exhaust detected

45- oxygen sensor or circuit - rich exhaust detected

46- power steering pressure switch circuit

48- misfire diagnosis

51- prom, mem-cal or ecm problem

52- calpak or ecm problem

53- egr fault (carbureted models only)

53- system over-voltage (ecm over 17.7 volts)

54- mixture control (m/c) solenoid or circuit (carbureted models)

54- fuel pump circuit (1986 and later models)

55- oxygen sensor circuit or ecm

55- fuel lean monitor (2.2l engine)

61- oxygen sensor signal faulty (possible contaminated sensor)

62- transaxle gear switch signal circuits

63- manifold absolute pressure (map) sensor voltage high (low vacuum detected)

64- manifold absolute pressure (map) sensor voltage low (high vacuum detected)

66- pressure sensor or circuit air conditioning