How the On-Board Diagnostic System Operates

The Engine Control Unit (ECU) monitors input and output circuits within the fuel injection system. Some circuits are checked continually, others are only checked under certain conditions (starting, etc.). If the OBD system determines that one of the circuits is malfunctioning, a fault code will be stored in memory. Each circuit has its own fault number and message. It is important to note that fault codes will stay in memory as long as there is battery power to the ECU, terminal 103. Therefore, it is impossible to tell when a fault code has been set.

HOW TO ERASE FAULT CODES
Remove negative battery cable for 10 seconds or remove "room" fuse from fuse panel for 10 seconds. On a limited number of MMC vehicles, fault codes can be erased using the DRBII. Use the adjustments screen on the DRBII to do this.

THE CHECK ENGINE LAMP
If a critical fault code is set, the electronic control unit will turn on the "check engine" warning lamp and substitute a value for the failed sensor or component. All fault codes, except the speed sensor, are considered critical.

THE "MAINTENANCE REQUIRED" WARNING LIGHT
This light is not part of the EFI system. When this light is lit, please refer to the maintenance section for proper maintenance procedures and instructions on how to turn off this light.