Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGauges, Warning Devices, and Driver Information Systems Diagnosis and RepairEasy
A customer complains that the 'Check Engine' light on their vehicle illuminates intermittently, but the engine performance seems normal. A technician connects a scan tool and finds no stored Diagnostic Trouble Codes (DTCs). Which of the following is the MOST likely cause of this intermittent warning light illumination?
- AAn intermittent electrical connection to a sensor or the ECM
- BA constant short to ground in the warning light circuit
- CA completely failed Engine Control Module (ECM)
- DA permanently open circuit in the 'Check Engine' light bulb
Show answer & explanationAnswer & explanation
Correct answer: A. An intermittent electrical connection to a sensor or the ECM
Intermittent issues often point to loose or corroded electrical connections rather than complete component failures or constant circuit faults. If a sensor or ECM connection is intermittently losing contact, it could trigger the light without storing a hard code if the fault isn't present during the diagnostic cycle.
Why the other options are wrong
- B. A constant short to ground would either keep the light continuously on or blow a fuse, neither of which matches an intermittent illumination without DTCs.
- C. A completely failed ECM would likely result in significant engine performance issues and a continuously illuminated 'Check Engine' light, often with multiple DTCs.
- D. A permanently open circuit in the bulb would prevent the light from ever illuminating, contradicting the customer's complaint.
Intermittent Electrical Faults
Temporary disruptions in electrical continuity or signal integrity, often caused by loose connections, corroded terminals, or damaged wiring that flexes.
- Often difficult to diagnose as they may not be present during testing.
- Can cause intermittent warning lights, performance issues, or erratic readings.
- Require careful inspection of wiring, connectors, and sensor terminals.
Memory trick: Wobbly wires warn intermittently, rarely leaving a trace.