ASE A1 Engine RepairGeneral Engine DiagnosisEasy
A technician is diagnosing an engine with a persistent misfire under load. A scan tool indicates a P0301 code (Cylinder 1 Misfire Detected). After swapping the spark plug and ignition coil from Cylinder 1 to Cylinder 2, the misfire now occurs in Cylinder 2, and the scan tool shows a P0302 code. What does this diagnostic process indicate?
- AThe engine control unit (ECU) is faulty.
- BThe fuel injector for Cylinder 1 is faulty.
- CThere is an internal engine mechanical issue in Cylinder 1.
- DThe ignition coil or spark plug that was originally in Cylinder 1 is faulty.
Show answer & explanationAnswer & explanation
Correct answer: D. The ignition coil or spark plug that was originally in Cylinder 1 is faulty.
By swapping components and observing the misfire 'move' to the new cylinder, the technician successfully isolated the faulty component. Since the misfire moved with the spark plug and ignition coil, it indicates that one of these components (or both) that were originally in Cylinder 1 is defective.
Why the other options are wrong
- A. A faulty ECU would likely cause multiple or erratic issues, not a misfire that follows a specific swapped component.
- B. If the fuel injector was faulty, the misfire would likely remain in Cylinder 1 even after swapping ignition components.
- C. An internal mechanical issue (e.g., low compression) would cause the misfire to remain in Cylinder 1 regardless of swapped ignition components.
Misfire Diagnosis (Swap Test)
A diagnostic technique involving swapping suspected faulty components (like coils or plugs) between cylinders to see if the problem moves with the component.
- Effective for isolating issues with spark plugs, ignition coils, and sometimes fuel injectors.
- If the misfire follows the component, that component is likely faulty.
- If the misfire stays in the original cylinder, the issue is likely mechanical or wiring-related to that cylinder.
Memory trick: Misfire's dance, swap parts for a chance.