Computers and Control Systems: Description and Operation

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.)

A normally operating ECI system will be indicated by a constant 12 vdc signal at the diagnostic connector. If a failure occurs it is indicated by the voltage at the diagnostic connector alternating between 0 and 12 vdc. The number of pulses from 0 to 12 vdc can be counted to determine what the error code is.

Fig. 241 Sample diagnostic error code indication.:

An error code begins with two seconds of 0 vdc followed by repeated 0.4 second duration transitions to 12 vdc, Fig. 241. At the end of the code there will again be two seconds of 0 vdc followed by the next higher failure code if there is one. Error codes can be read using a voltmeter or logic probe connected to the diagnostic connector. Count the number of transitions to 12 vdc and compare that to the Diagnostic Codes Chart to determine the failed component.


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 negative terminal 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.