Mechanics

GM Trouble Codes OBD1

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Article describes diagnostic trouble code definitions for GM manufactured vehicles made before 1995

Most GM cars (non classic) made before 1995 are equipped with the ODB1 computer operating system. This system is unique to the GM manufactured cars which include: Buick, Cadillac, Chevy, GMC, Oldsmobile and Pontiac. To start diagnostic trouble code gathering locate the ALDL (assembly line data link) connector which is located on the driver’s side lower part of the dash on most cases. In this connector are two terminals that need to be connected with a wire jumper. Once the wire is inserted into the connector turn the ignition key to the "on" position without starting the engine. The check engine light will begin to flash, count the flashes, this will be the first digit of the two number code, example for code 14, one flash, pause, then four flashes.

There will be a pause in the check engine light once the first code flashing series has concluded, the second number flash is about to begin. Once the second set of flashing has stopped the end of trouble codes is finished. If there is more than one code stored in the computers memory the first digit of the second code will begin. If there are no additional trouble codes the light will continue to flash the code 12 which inducates there are no more codes stored. Code 12 is normal to sometimes flash first two or three times before the actual trouble code is displayed.

Diagnostic Connector

Use a jumper wire and connect terminals A and B on the ALDL.

GM Trouble Codes for OBD1 Computer Operating System

12 - System normal

13 - Oxygen sensor circuit open

14 - Coolant sensor high resistance or shorted

15 - Coolant sensor circuit low or open

16 - Direct ignition system (DIS) fault in circuit

17 - Camshaft position sensor fault

18 - Crankshaft or camshaft sensor error

19 - Crankshaft sensor circuit fault

21 - TPS sensor out of range

22 - TPS sensor signal voltage low

23 - Intake air temp sensor out of range, low

24 - Vehicle speed (vss) sensor circuit fault

25 - Air temp sensor sensor out of range, high

26 - Quad-driver module (computer) circuit #1 fault

27 - Quad-driver module (computer) 2nd gear circuit

28 - Quad-driver module (computer) circuit #2 fault

29 - Quad driver module (computer) 4th gear circuit

31 - Camshaft position sensor fault

32 - EGR circuit fault

33 - Map sensor signal out of range, high

34 - Map sensor signal out of range, low

35 - Idle air control sensor circuit fault

36 - Ignition system circuit error

38 - Brake input circuit fault

39 - Clutch input circuit fault

41 - Camshaft sensor circuit fault, ignition control circuit fault

42 - Electronic spark timing (EST) circuit grounded

43 - Knock sensor or electronic spark control circuit fault

44 - Oxygen sensor lean exhaust

45 - Oxygen sensor rich exhaust

46 - Pass-key II circuit or power steering pressure switch circuit fault

47 - PCM data circuit

48 - Misfire diagnosis

51 - Calibration error, mem-cal, ecm or eprom failure

52 - Engine oil temperature circuit, low temperature indicated

53 - Battery voltage error or egr or pass-key II circuit

54 - EGR system failure or fuel pump circuit low voltage

55 - Converter error or pcm not grounded or lean fuel or founded reference voltage

56 - Quad-driver module #2 circuit

57 - Boost control problem

58 - Vehicle anti-theft system fuel enable circuit

61 - AC system performance or degraded oxygen sensor signal

62 - Engine oil temperature high temperature indicated

63 - Oxygen sensor right side circuit open or map sensor out of range

64 - Oxygen sensor right side lean exhaust indicated

65 - Oxygen sensor right side rich exhaust indicated

66 - AC pressure sensor circuit low pressure

67 - AC pressure sensor circuit or a/c clutch circuit failure

68 - AC compressor relay circuit failure

69 - AC clutch circuit head pressure high

70 - AC refrigerant pressure circuit high

71 - AC evaporator temperature sensor circuit low

72 - Gear selector switch circuit

73 - AC evaporator temperature circuit high

75 - Digital egr #1 solenoid error

76 - Digital egr #2 solenoid error

77 - Digital egr #3 solenoid error

79 - Vehicle speed sensor (vss) circuit signal high

80 - Vehicle speed sensor (vss) circuit signal low

81 - Brake input circuit fault

82 - Ignition control (IC) 3X signal error

85 - Prom error

86 - Analog/digital pcm error

87 - Eprom error

99 - Power management

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AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2013-12-06)