Fault Tree: Cylinder-specific misfire (P0301–P0308)

Decision flow

1. Confirm it is truly cylinder-specific
   Check misfire counters if available. If it’s random across cylinders, use P0300 random misfire →
2. Basic ignition sanity (fast wins)
   • Inspect plug: gap, oil fouling, cracked porcelain.
   • Swap the coil (and plug if easy) to another cylinder and re-test.
   If the misfire follows the coil/plug, you’re done.
3. If ignition swap does not move it: injector check
   • Listen for injector clicking.
   • If supported: injector balance / contribution test.
   Related: Fuel pressure testing basics →
4. Air leak / runner-specific issues
   • Check for intake gasket leak near that runner (smoke test).
   • Check PCV / breather routing faults that favour a cylinder.
   Use: Smoke testing done right →
5. Mechanical check if it persists
   • Compression test: compare cylinders.
   • If low: consider leakdown (valve, rings, head gasket) and timing.
   If you suspect timing: Crank/Cam checks (no scope) →
6. Wiring / ECU driver possibility (rare but real)
   • Check injector and coil power/ground + connector pin fit.
   • Do a voltage drop test on grounds under load.
   Use: Voltage drop testing →
7. After the fix: verify
   • Clear codes, reproduce the same condition, confirm misfire counts stay low.