Service Code Output Format

Self-Test Code Output Format. Key ON/Engine OFF:

The Electronic Engine Control (EEC-IV) system communicates service through the self test codes. These codes are two or three digit numbers representing the results of the self test.
The service codes are transmitted on the Self Test Output (STO) wire located on the vehicle self test connector. The codes are in the form of timed pulses, and are read by the technician on a scan tool or the CHECK ENGINE light.
NOTE: Two digit codes are shown in the images. Three digit codes are similar.