Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairEasy
A technician is diagnosing a misfire on cylinder #3 of an engine equipped with a Coil-On-Plug (COP) ignition system. Using a scan tool, the technician observes a consistent misfire count on cylinder #3. Which of the following is the MOST appropriate next step in diagnosing the issue?
- ACheck the fuel injector resistance for cylinder #3.
- BPerform a compression test on all cylinders.
- CSwap the cylinder #3 COP coil with a known good coil from another cylinder.
- DInspect the wiring harness for the crankshaft position sensor.
Show answer & explanationAnswer & explanation
Correct answer: C. Swap the cylinder #3 COP coil with a known good coil from another cylinder.
Swapping the COP coil is an efficient diagnostic step to quickly determine if the coil itself is faulty. If the misfire moves with the coil, the coil is defective.
Why the other options are wrong
- A. While a fuel issue could cause a misfire, a COP system misfire count points more directly to ignition, making the coil swap a better first step.
- B. A compression test is relevant for mechanical issues, but swapping the coil is a more direct test for an ignition component in this scenario.
- D. A faulty crankshaft position sensor would likely affect multiple cylinders or cause a no-start condition, not typically a single-cylinder misfire.
COP Coil Swap
A diagnostic technique where a Coil-On-Plug (COP) ignition coil from a misfiring cylinder is exchanged with a coil from a properly functioning cylinder to determine if the coil is faulty.
- Identifies a defective ignition coil quickly.
- If misfire moves, coil is bad.
- If misfire stays, look elsewhere.
Memory trick: Coil swap is the quick way to solve a COP misfire.