OBD-I was to encourage auto manufacturers to design reliable emission control systems.
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The DTC Codes can be found without an expensive scan tool. Each manufacturer used their own diagnostic link connector to read the DTCs from the vehicle. DTC are read through the blinking patterns of the ‘Check Engine Light’. By connecting certain pins of the diagnostic connector, the ‘Check Engine’ light will blink out a two-digit number.
If you own a 1982 to 1995 GM car or truck, you just might be able to read your trouble codes for free!
- 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.
- 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.