On-Board Diagnostic Codes

Many components of the emission control system are monitored for proper operation by the Electronic Control Unit or ECU. In the event improper operation is detected the ECU will store an error code or codes indicating which component has failed to operate correctly. These codes can be be extracted to assist in diagnosis and testing of emission control system failures. Be sure to extract diagnostic codes before disconnecting the vehicle battery or the codes will be lost. (The diagnostic codes will not be lost if power is restored to the ECU in less than ten seconds.)

Fig. 241 Normal (no failure) diagnostic code indication.:

UNDERSTANDING DIAGNOSTIC CODES

To read and interpret diagnostic codes you must use an analog voltmeter or logic probe. To understand what diagnostic code(s), if any, is being sent to the diagnostic connector, you must be able to interpret the pattern of ON/OFF voltages that Mitsubishi uses for this model, Fig. 241.

Fig. 242 Failure code timing diagram.:

All output codes start with a three second voltage ON signal which merely indicates the code extraction process has begun. What will then follow is a pattern of short or long 12V pulses to represent the failure code.
A voltage pulse that indicates an OFF signal is short (about 0.5 second) and a pulse that indicates an ON signal is longer (about 1.5 seconds), Fig. 242. The number and position of OFF to ON pulses represents the code stored in the ECU's memory. This ON/OFF pulse code can be compared to a list of diagnostic patterns as outlined in `DIAGNOSIS AND TESTING PROCEDURES.'


After the cause of a failure code has been detected and corrected the diagnostic code memory of the ECU should be cleared by disconnecting the vehicle battery for 15 seconds or more. NOTE! Do not disconnect the vehicle battery while the engine is running or while the ignition is on. Refer to `DIAGNOSIS AND TESTING PROCEDURES' for complete information on how to extract diagnostic codes.