Misfire Diagnosis: Coils, Plugs, Injectors, or Compression?
A misfire means one cylinder is not producing expected combustion. The cause may be ignition, fuel, air, timing, or mechanical compression. Guessing parts can get expensive quickly.
Start with the pattern
The first question is whether the misfire follows one cylinder, one bank, cold start, hot idle, boost load, or random operation. The pattern usually narrows the system.
Common starting checks include:
- Spark plug condition
- Coil swap test
- Injector correction or contribution
- Fuel pressure
- Intake leaks
- Compression or leak-down
- Cam timing data
Why simple swaps help
On many petrol engines, moving a coil from cylinder 2 to cylinder 4 and watching whether the misfire follows can save time. If the misfire stays on the same cylinder, the fault may be plug, injector, wiring, compression, or valve timing rather than the coil.
Buyer takeaway
A flashing engine light under load is not a small detail. Misfires can damage catalytic converters, wash cylinder walls with fuel, and hide deeper mechanical problems. Diagnose before buying, not after the invoice arrives.