Vehicles predating 1996 use an internal computer called an Electronic Control Module. The ECM is the home for manufacturer-specific diagnostic self-testing, as well as trouble and fault coding. Collectively, these systems are now referred to as the first generation of On-Board Diagnostics, or OBD-I. No two manufacturer diagnostic systems are the same. Ford, for example, uses a different set of codes than Chrysler or General Motors. While testing procedures do differ, the process of reprogramming a vehicles ECM is fundamentally the same and takes roughly 10 minutes or less.
Park your vehicle and engage the emergency brake.
Open the hood over the engine compartment.
Disconnect the negative cable from the negative terminal on your vehicle's battery. Then, disconnect the positive cable from the battery's positive terminal. How to do this will differ by brands. Usually, battery cables are clamped onto the terminals, and removing them requires unscrewing a restraining bolt with a socket wrench.
Wait five to 10 minutes, for safety. While you have disconnected the main electrical power source, there will be residual power built up in your vehicle. By waiting, you are allowing all the power to completely drain out of the vehicle. This will wipe your ECM's memory.
Reconnect the positive cable to the positive battery terminal. Then, reconnect the negative cable to the negative terminal.
