System Diagnosis: Notes

System diagnosis can only be accomplished using a voltmeter or appropriate scan tester. See ENTERING ON-BOARD DIAGNOSTICS. 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 fault code is set, ECU will turn on CHECK ENGINE light. Fault codes can be confirmed by using a voltmeter. System malfunctions encountered are identified as either hard failures or intermittent failures as determined by ECU.