Description of On-Board Diagnostics

The self-diagnostic system used on the 1990 300ZX differs from some other Nissan vehicles in that it uses only two modes and one LED for performing self-diagnosis. The "CHECK ENGINE" lamp will blink simultaneously with the LED.
Mode 1:
a. When the key is on and the engine is stopped this the "CHECK ENGINE" lamp self-check. If the lamp does not illuminate in this mode refer to CHASSIS ELECTRICAL DIAGRAMS and check the lamp and circuit.
b. When the engine is running this is the normal operating mode. The "CHECK ENGINE" will illuminate when a trouble is found in certain circuits (CA. only) or when the ECU's C.P U. is malfunctioning (Non-CA. models). Refer to READING AND CLEARING TROUBLE CODES for identification of trouble codes.
Mode 2:
a. When the key is on and the engine is stopped this is the self-diagnostic mode. The trouble codes will be extracted in this mode.
b. When the engine is running it is used to monitor the air/fuel mixture and O2 sensor operation.