How to Read OBD1 Codes on GM or Ford Vehicles (Pre-1996)?

Last Updated on April 10, 2025

Knowing how to read trouble codes is extremely useful—especially if you live in a remote area or prefer DIY repairs. Whether your car was manufactured before or after 1996, being able to read diagnostic codes can help you identify problems early and avoid unnecessary mechanic visits. Fortunately, reading codes from older vehicles using the OBD1 system is simpler than you might think. If you’re looking for a reliable tool to read older vehicle codes, don’t miss our expert picks in this guide to the best OBD1 scanners available today.

Related: What is OBDii?

OBD1 Connector Location

First, locate the OBD1 port in your vehicle. In most GM models such as Cadillac, Chevy, Buick, Pontiac, Oldsmobile, and GMC, the OBD1 connector is typically found under the dashboard, below the steering column. The connector usually has a black plastic cover and contains 12 pins, though not all are active on every vehicle.

Related article: Follow these steps to pass OBD ii emissions test

OBD1 Connector Wiring Pinout

The OBD1 connector is standardized to some extent, with key pins used for diagnostics, power, and ground. Here’s a typical pinout you might see:

• A – Ground
• B – Diagnosis Terminal
• C – A.I.R.
• D – “Service Engine Soon” light
• E – Serial Data
• F – T.C.C.
• G – Fuel Pump
• H – Brake Speed Input
• J/K/L – Not Used (varies by model)
• M – Additional Serial Data

⚠️ Tip: Always handle the connector carefully. Miswiring or shorting the wrong pins can damage sensitive control units.

How to Read OBD1 Codes Without a Scanner

Reading OBD codes is surprisingly simple, even for beginners. One of the best things about OBD1 systems—particularly in GM vehicles—is that you don’t need a scanner to read the codes. You can use a jumper wire or paperclip to connect Pin A (Ground) and Pin B (Diagnostic Terminal). Follow these steps:

Step-by-Step Guide

1. Connect a battery charger to your car during the procedure to maintain consistent voltage. Low voltage may cause inaccurate readings or additional fault codes.
2. Insert a jumper wire between pins A and B on the OBD1 connector.
3. Turn the ignition key to the “ON” position (do not start the engine).
4. Watch the check engine light (CEL)—it will start flashing.
   • Count the flashes carefully. For example:
     Code 16 = 1 long flash, pause, then 6 quick flashes.
   • A longer pause separates different codes; a short pause separates digits within a code.
5. Code 12 (1 long flash followed by 2 short flashes) always appears first and repeats 3 times. It confirms that the OBD1 system is functioning correctly.
6. After that, the system will flash any stored trouble codes, each 3 times.
7. Write down each code, and then reference a code chart to interpret what each code means for your vehicle.
8. When done, remove the jumper wire and disconnect the battery charger.

OBD1 Codes List

Below is a comprehensive OBD1 codes list along with their meanings. It’s important to note that OBD1 vs OBD2 codes differ in structure and compatibility. While OBD1 code definitions can vary slightly depending on the vehicle manufacturer and model, most follow a common standard—especially among GM, Ford, and Chrysler vehicles.

12. System fine (Diagnosis mode active)
13. Oxygen O2 Sensor – Open circuit
14. Coolant temperature sensor – Extreme resistance or shorted circuit
15. Coolant temperature sensor – Low or open circuit
16. Direct ignition system (DIS) – Shorted circuit or bad circuit
17. Camshaft position sensor fault – Circuit or timing error 
18. Camshaft or Crankshaft – Sensor or circuit or timing error
19. Crankshaft sensor – Sensor or circuit or timing error
20. Throttle position sensor (TPS) – Out of range/Performance
21. Throttle position sensor (TPS) – Signal voltage low
22. Intake Air temp sensor (IATS) – Out of range or low resistance
23. Vehicle speed sensor (VSS) – Circuit fault
24. Intake Air temp sensor (IATS) – Out of range, high resistance
25. Quad-driver module (QDM A) – Circuit #1 fault
26. Quad-driver module (QDM A) – 2nd gear circuit
27. Quad-driver module (QDM A) – Circuit #2 fault
28. Quad driver module (QDM A) – 4th gear circuit
29. Wastegate Solenoid – Circuit fault 
30. Exhaust Gas Recirculation (EGR) – Circuit fault
31. Manifold Air Pressure Sensor (MAP) – Signal out of range, high resistance
32. Manifold Air Pressure Sensor (MAP) – Signal out of range, low resistance
33. Idle air control valve – Circuit error or sensor 
34. Ignition system – Circuit error or fault
35. Brake input (Brake switch sensor) – A circuit fault
36. Clutch input (Clutch sensor) – A circuit fault or error 
37. Camshaft sensor – Circuit fault/ignition control – circuit error (depending on the vehicle model)
38. Electronic spark timing (EST) – Circuit grounded or shorted
39. Knock sensor or Electronic spark control – Circuit error (depending on the vehicle model)
40. Oxygen O2 sensor – Lean Mixture
41. Oxygen O 2sensor – Rich Mixture
42. Pass-key II – Circuit/Power steering pressure switch – circuit error or fault (depending on the vehicle model)
43. PCM data – Circuit error
44. Misfire (diagnosis)
45. Calibration error – ECM, EPROM, or Mem-cal misfiring 
46. Engine oil temperature circuit – Low temperature indicated or circuit error
47. Battery voltage – Low/high/fault, or Egr valve solenoid 1 – circuit fault (depending on the car model)
48. Fuel Pump – Circuit error/Egr valve solenoid 2 – Circuit fault (depending on the car model)
49. ECM PCM – Circuit error/EGR valve solenoid 3 – Circuit fault (depending on the car model)





50. Quad-driver module (QDM B) – Circuit error
51. Boost control – Faulty
52. Vehicle anti-theft system (VATS) – Circuit error
53. Air conditioning (A/C) system performance or degraded oxygen sensor signal (depending on the car model)
54. Engine oil temperature sensor – High temperature shown
55. Oxygen O2 sensor right side – Circuit open/manifold air pressure sensor (MAP) – out of range (depending on the vehicle model)
56. Oxygen O2 sensor test right side – Lean mixture indicated
57. Oxygen O2 sensor right side – Rich mixture indicated
58. Air conditioning (A/C) pressure sensor – Circuit low pressure
59. Air conditioning (A/C) pressure sensor – Circuit or A/C clutch – circuit failure (depending on the vehicle model)
60. Air conditioning (A/C) compressor relay – Circuit failure error
61. Air conditioning (A/C) clutch – Circuit or pressure high
62. Air conditioning (A/C) refrigerant pressure – Circuit high
63. Air conditioning (A/C) evaporator temperature sensor – circuit low
64. Gear selector switch – Circuit fault
65. Air conditioning (A/C) Evaporator temperature – Circuit high
66. Digital EGR #1 – Solenoid faulty or error
67. Digital EGR #2 – Solenoid faulty or error
68. Digital EGR #3 – Solenoid faulty or error
69. Vehicle speed sensor (SS) – Circuit signal high resistance
70. Vehicle speed sensor (VSS) – Circuit signal low resistance
71. Brake input data – Bad circuit
72. Ignition control (IC) 3X – Signal error/faulty circuit
73. Prom – Faulty/error circuit
74. Analog/digital – Pcm faulty/error circuit
75. Eprom – Faulty/error circuit
76. Power management – Faulty/error circuit`

Conclusion

Reading OBD1 codes on older vehicles manufactured before 1996 is quick and simple if you follow the correct steps. You don’t necessarily need to visit a professional mechanic. However, using an OBD2 scanner—if your car is compatible or upgraded—can make the diagnostic process even easier and more efficient.