System Diagnosis: Notes

Engine Control Unit (ECU) monitors several different engine control system circuits. If an abnormal input signal occurs, a fault code is stored in ECU memory and given a fault code number. Each circuit has its own fault number and message. A specific fault code indicates a particular system failure, but it DOES NOT indicate that cause of failure is necessarily within system. A fault code DOES NOT condemn any specific component; it simply points out a probable malfunctioning area. If a critical fault code is set, ECU will turn on CHECK ENGINE light. All fault codes except speed sensor are considered critical.

Fault codes can be confirmed by using a voltmeter. See ENTERING ON-BOARD DIAGNOSTICS (USING VOLTMETER)  in this article.

System malfunctions encountered are identified as either hard failures or intermittent failures as determined by ECU.