How to read OBD1 Codes?

How to read OBD1 Codes? Step-By-Step Guide

Knowing how to read trouble codes is very crucial, especially if you live in a remote area. It doesn’t matter whether your car was made after 1996 or before. Actually, using an OBD code reader is easy if you know the steps to follow.

In this post, we’re going to help you understand how to read codes on your old 1995 or older GM or Ford vehicles. This will help you save the cost of visiting a mechanic to troubleshoot your car.

OBD1 connector location

First and foremost, you should know where the OBD port is located. Most GM vehicles, such as Cadillac, Buick, Chevy, Pontiac, GMC, and Oldsmobile, have the OBD1 connector situated below the steering wheel, under the dash. The connector usually has a black plastic cover, which you have to remove before accessing it. Besides, the connector is also typically black and comes with 12 pins on it.

OBD1 connector wirings pinout

The OBD1 connector typically comes with 12 pins. However, not all pins are used in most vehicle models. What’s more, the standard, which includes power, ground, and a diagnostic pin, is the same in most car models.

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

When connecting your wires, ensure to be very careful as a slight mistake can hurt your car’s electrical system and control units, which are pretty costly. If you’re wondering how a pinout of the OBD1 connector looks like in most cars, here is an image.

Below is what the different letters in the image mean: 

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 – None 

K – None

L – None

M – Serial data 

How to read OBD1 codes?

Reading OBD codes is super easy. Actually, you don’t need a diagnostic code reader. All you need is a wire that makes short the circuit of pin A and B. The vehicle will automatically enter the diagnostic mode and flash the check engine light multiple times. To know what the diagnostic codes mean, you will be required to count the flashes.

Here is the procedure:

Step 1: Ensure a car battery charger is connected to your car before carrying out this procedure. This is important as low voltage can bring many issues with the troubleshooting and many faults you wouldn’t want.

Step 2: As stated earlier, place a jumper wire between pin A and B.

Step 3: Leave the jumper wire connected and turn the ignition on. However, you should not start the engine.

Step 4: After some time, the check engine light will begin to flash. It would help if you counted the flashes. You will notice a longer pause between the codes if you have multiple trouble codes and a shorter pause between the fault codes’ digits.

For instance, you will see something like code 16 = 1 flash * Pause * 6 flashes.

Step 5: When all trouble codes have been exhausted, it will flash the code 12 to 1 flash * Pause * 2 flashes. All codes should begin with code 12, which is one long flash followed by 2 short flashes. Code 12 means that the OBD1 diagnostic system is okay and will repeat itself continuously if there are no trouble codes. On the contrary, if there is a problem, the code will flash 3 times before flashing the fault codes.

Step 6: Note down all the trouble code numbers you got and confirm them on the table below.

Step 7: When you’re done, remove the jumper wire and car battery charger.

OBD1 Codes List

Below is a list of OBD1 codes list and what they mean. There are differences between OBD1 and OBD2 codes. Note that OBDI codes’ meanings can vary depending on your car model, but most of them are the same among different vehicle manufacturers.  

  1. System fine (Diagnosis mode active)
  2. Oxygen O2 Sensor – Open circuit
  3. Coolant temperature sensor – Extreme resistance or shorted circuit
  4. Coolant temperature sensor – Low or open circuit
  5. Direct ignition system (DIS) – Shorted circuit or bad circuit
  6. Camshaft position sensor fault – Circuit or timing error 
  7. Camshaft or Crankshaft – Sensor or circuit or timing error
  8. Crankshaft sensor – Sensor or circuit or timing error
  9. Throttle position sensor (TPS) – Out of range/Performance
  10. Throttle position sensor (TPS) – Signal voltage low
  11. Intake Air temp sensor (IATS) – Out of range or low resistance
  12. Vehicle speed sensor (VSS) – Circuit fault
  13. Intake Air temp sensor (IATS) – Out of range, high resistance
  14. Quad-driver module (QDM A) – Circuit #1 fault
  15. Quad-driver module (QDM A) – 2nd gear circuit
  16. Quad-driver module (QDM A) – Circuit #2 fault
  17. Quad driver module (QDM A) – 4th gear circuit
  18. Wastegate Solenoid – Circuit fault 
  19. Exhaust Gas Recirculation (EGR) – Circuit fault
  20. Manifold Air Pressure Sensor (MAP) – Signal out of range, high resistance
  21. Manifold Air Pressure Sensor (MAP) – Signal out of range, low resistance
  22. Idle air control valve – Circuit error or sensor 
  23. Ignition system – Circuit error or fault
  24. Brake input (Brake switch sensor) – A circuit fault
  25. Clutch input (Clutch sensor) – A circuit fault or error 
  26. Camshaft sensor – Circuit fault/ignition control – circuit error (depending on the vehicle model)
  27. Electronic spark timing (EST) – Circuit grounded or shorted
  28. Knock sensor or Electronic spark control – Circuit error (depending on the vehicle model)
  29. Oxygen O2 sensor – Lean Mixture
  30. Oxygen O 2sensor – Rich Mixture
  31. Pass-key II – Circuit/Power steering pressure switch – circuit error or fault (depending on the vehicle model)
  32. PCM data – Circuit error
  33. Misfire (diagnosis)
  34. Calibration error – ECM, EPROM, or Mem-cal misfiring 
  35. Engine oil temperature circuit – Low temperature indicated or circuit error
  36. Battery voltage – Low/high/fault, or Egr valve solenoid 1 – circuit fault (depending on the car model)
  37. Fuel Pump – Circuit error/Egr valve solenoid 2 – Circuit fault (depending on the car model)
  38. ECM PCM – Circuit error/EGR valve solenoid 3 – Circuit fault (depending on the car model)
  39. Quad-driver module (QDM B) – Circuit error
  40. Boost control – Faulty
  41. Vehicle anti-theft system (VATS) – Circuit error
  42. Air conditioning (A/C) system performance or degraded oxygen sensor signal (depending on the car model)
  43. Engine oil temperature sensor – High temperature shown
  44. Oxygen O2 sensor right side – Circuit open/manifold air pressure sensor (MAP) – out of range (depending on the vehicle model)
  45. Oxygen O2 sensor test right side – Lean mixture indicated
  46. Oxygen O2 sensor right side – Rich mixture indicated
  47. Air conditioning (A/C) pressure sensor – Circuit low pressure
  48. Air conditioning (A/C) pressure sensor – Circuit or A/C clutch – circuit failure (depending on the vehicle model)
  49. Air conditioning (A/C) compressor relay – Circuit failure error
  50. Air conditioning (A/C) clutch – Circuit or pressure high
  51. Air conditioning (A/C) refrigerant pressure – Circuit high
  52. Air conditioning (A/C) evaporator temperature sensor – circuit low
  53. Gear selector switch – Circuit fault
  54. Air conditioning (A/C) Evaporator temperature – Circuit high
  55. Digital EGR #1 – Solenoid faulty or error
  56. Digital EGR #2 – Solenoid faulty or error
  57. Digital EGR #3 – Solenoid faulty or error
  58. Vehicle speed sensor (SS) – Circuit signal high resistance
  59. Vehicle speed sensor (VSS) – Circuit signal low resistance
  60. Brake input data – Bad circuit
  61. Ignition control (IC) 3X – Signal error/faulty circuit
  62. Prom – Faulty/error circuit
  63. Analog/digital – Pcm faulty/error circuit
  64. Eprom – Faulty/error circuit
  65. Power management – Faulty/error circuit


Reading an old car make and model made before 1996 is easy and first with the above steps. You don’t need to visit a professional mechanic. However, if you have an OBD2 scan tool, your work will even be much easier.

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