DISPLAYING THE TROUBLE CODES:
**You can but a GM code reader that plugs right in for around $20. Run a wire/hairpin/Paper Clip from Pin 6 to Pin 12 with the ignition off (although on later cars it should be from Pin 5 to Pin 6), then turning the ignition to the on position without starting the engine will flash the "Service Engine Soon" light in the following sequence: flash, pause, flash-flash, long pause flash, pause, flash-flash, long pause flash, pause,long pause Following this "12" code repeating three times will come any trouble codes stored, flashing each of them three times. If you do not see the "12" flash three times, your diagnostic circuit is defective.

CLEARING THE TROUBLE CODES:
Turn the keyswitch to the off position. To clear any trouble codes, disconnect the power feed/battery for 30 seconds. If this is done at the battery, and your car stereo is equipped and programmed with a four digit pin code, you may have to re-enter that as well to use your stereo again. A better place to remove power is at the fuse.

TROUBLE CODES:

12. No distributor reference pulses to Electronic Control Module (ECM). 
(This code will flash whenever the diagnostic terminal is grounded with the 
ignition on and the engine not running.  If additional trouble codes are 
stored they will flash/appear after this code has flashed 3 times.

13. Oxygen sensor Circuit 
(Sticking TPS, poor electircal connection, open or short circuit, 
defective oxygen sensor, defective ECM)

14. Coolant Temperature Sensor Circuit 
(Engine overheating; poor electrical connection, open or short in circuit; 
defective coolant temperature sensor; defective ECM)

15. Coolant Temperature Sensor Circuit
(Poor electrical connection, open or short in circuit; defective coolant 
temperature sensor; defective ECM.)

16. High battery voltage or direct inition system open or shorted to ground 

17. RPM signal problem 

21. Throttle Position Sensor 
(Sticking or misadjusted TPS; poor electrical connection, open 
or short in circuit; defective TPS; defective ECM.)

22. Throttle Position Sensor 
(Sticking or misadjusted TPS; poor electrical connection, open 
or short in circuit; defective TPS; defective ECM.)

23. Carbureted Models - Mixture Control Solenoid
(Open circuit or short to ground in the mixture solenoid circuit; defective ECM-if the 
mixture control solenoid is shorted to ground, the ECM may have been damaged.)

23. Fuel Injected Models - Circuit fault in vehicle speed sensor, defective MAT sensor.

24. Vehicle Speed Sensor 

25. High temperature at manifold air temperature sensor or Vacuum switching 
valve circuit open or shorted to ground or  High voltage at ATS sensor

26. Fault in quad driver module

27. Fault in 2nd gear switch

28. Fault in 3rd gear switch

29. Fault in 4th gear switch

31. Low voltage at manifold absolute pressure sensor or Fuel injector or
Park or neutral switch or CAM diagnostic OR Governor malfunction or  Wastegate 
overboost or Wastegate eletrical signal open or shorted to ground
32. Fault in barometric pressure sensor circuit orFault in exhaust gas 
recirculation valve diagnostic switch or Fault in electronic vacuum regulator
valve

33. MAP Sensor - High voltage (low vacuum) at mass air flow sensor (or MAP sensor)
(Leaking or restricted vacuum hoses; poor electrical connection,
open or short in circuit; defective MAF sensor; defective ECM.) 

34. MAF Sensor - Low voltage (high vacuum) at mass air flow sensor (or MAP sensor)
(Poor electrical connection, open or short in circuit; defective MAF sensor; defective ECM.)

35. Idle speed Control 
(TPS sticking or misadjusted; poor electrical connection, open or short in circuit; 
defective IAC valve; defective ECM.)

36. Burn off at mass air flow sensor, problem in transmission shift, Fault in direct 
ignition system or missing pulses in electronic spark timing signal
38. Fault in torque converter clutch brake switch

39. Fault in torque converter clutch circuit

41. Fault at cam senso,  Cylinder select error, tach input error

42. Fault at electronic spark timing circuit, fault at direct ignition system, 
fault at fuel cutoff relay circuit

43. Low voltage at electronic spark timing circuit 
(Poor electrical connection, open or short in circuit; defective ignition module; defective ECM.)

44. Oxygen sensor lean 
(Poor electrical connection, open or short in circuit; defective ESC module; 
defective knock sensor; defective ECM.)

45. Rich Exhaust

46. Fault at vehicle anti-theft sytem or Fault at power steering switch

47. Problem at Electronic Control Module (ECM) 

48. Misfire

49. Vacuum leak

51. PROM - ECM Chip 

52. Fuel Calpak 
(Missing fuel calpac missing or Analog to digital converter error or Fault at 
quad driver module or Low voltage at oil temperature sensor.) 

53. System Over Voltage 
(High voltage at battery or high voltage at exhaust gas recirculation valve or
Voltage reference error or Problem at vehicle anti-theft system.) 

54. Mixture Control Solenoid
(Low voltage at fuel pump or low voltage at Fuel pump relay or output 
failure at quad driver module.) 

55. Electronic Control Module (ECM) 
(ECM failure or Serial bus error or Fuel lean malfunction.)

56. Low coolant or corrosivity or fault in port throttle system vacuum sensor

58. Problem at vehicle anti-theft system

61. Oxygen sensor
 
(degraded or Port throttle system error or Cruise control problems.) 

62. Gear switch input diagnostics or High voltage at oil temperature sensor or
Fault in cruise control- vacuum solenoid circuit

63. High voltage at manifold absolute pressure sensor or Fault in exhaust 
gas recirculation valve or Fault at right oxygen sensor. 

64. Low voltage at manifold absolute pressure sensor or Fault in exhaust 
gas recirculation valve or Right oxygen sensor lean. 

65. Failure at exhaust gas recirculation valve or Faulure at injector 
peak/hold diagnostic or Right oxygen sensor rich or Fault at cruise control 
position sensor.  

66. Internal reset of Electronic Control Module (ECM) 

67. Fault at cruise control switch 

68. Fault at cruise control switch  

69. Fault at air conditioner pressure switch  

88. Internal reset of Electronic Control Module (ECM)