Reading Service Codes
The engine control system communicates malfunctions to the technician through service codes. These service codes are 2 digit numbers representing a particular fault. The service codes are transmitted through the Self-Test Output (STO) line found in the vehicle self-test connector.
Codes are displayed in the form of timed pulses and read by the technician on a voltmeter or STAR tester. See Fig 1 . When using analog volt/ohmmeter, codes are displayed in the form of needle sweeps on the volt/ohmmeter.