See Figures 1 and 2
The engine computer is designed to test its own input and output circuits. The engine computer monitors the input and output signals coming from and going to the various sensors. The computer then compares the in-coming and out-going signals to specific parameters, which are written into the computer's memory at the factory. If a sensor's signal is showing operation of the sensor to be out of the given range, the computer stores a fault code. The fault code (a 2 digit number) is stored in the engine computer for eventual display to the person performing the readout procedure. The fault code does not indicate which component is faulty, rather which circuit was perceived as functioning out of the given parameters. Therefore, once a fault code is known, the entire circuit must be checked for problems, rather than simply replacing the main sensor of the circuit.
The fault codes can be displayed by means of the instrument panel CHECK ENGINE light (except for 1988-90 and early 1991 Premier and Monaco vehicles) or by connecting a Diagnostic Readout Box (DRB or DRBII) and reading a numbered display code, which indicates the circuit in question. Some inputs and outputs are checked continuously and others are checked under certain conditions.
If the problem is repaired or no longer exists, the engine computer (PCM, ECU or SBEC) cancels the fault code after 50-100 key ON/OFF cycles.
When a fault code is detected, it appears as either a flash of the "CHECK ENGINE'' on the instrument panel or by watching the Diagnostic Readout Box II (DRB II). This indicates that an abnormal signal in the system has been recognized by the engine computer.
Fault codes DO indicate that a circuit has performing outside of pre-established parameters, but DO NOT identify the faulty components directly.