Ignition

P0301 — Cylinder 1 Misfire Detected

Severity: medium Drive with care, to a shop

What P0301 means

The engine control unit watches crankshaft speed through the crankshaft position sensor. When cylinder 1 fails to burn its air-fuel mixture properly, the crank speed dips and the ECU stores P0301. The cause can be in ignition, fuel delivery, cylinder sealing or an air leak near that cylinder.

Symptoms

  • Check engine light on or flashing
  • Rough, shaky idle
  • Engine vibration
  • Loss of power or hesitation when accelerating
  • Higher fuel consumption
  • Fuel smell from the exhaust
  • Possible stalling at idle

Possible causes

  1. Worn or faulty spark plug in cylinder 1
  2. Failed ignition coil or coil connector on cylinder 1
  3. Clogged or leaking fuel injector on cylinder 1
  4. Vacuum leak near the cylinder 1 intake port
  5. Low compression: valve, piston rings or head gasket
  6. Damaged wiring or poor connector contact
  7. Crankshaft or camshaft position sensor fault
  8. MAF sensor fault (less likely)

How to diagnose

  1. Read all codes and freeze frame data to see the RPM and load when the misfire occurred
  2. Check per-cylinder misfire counters to confirm cylinder 1 is the problem
  3. Inspect the cylinder 1 plug, coil and connector for cracks, oil, carbon tracking or wear
  4. Swap the coil (and the plug if needed) with another cylinder and see whether the misfire follows
  5. Test the injector's operation, resistance and driver signal
  6. Check the intake system and hoses for vacuum leaks, for example with a smoke machine
  7. Run a compression test or leak-down test on cylinder 1
  8. Inspect the wiring and the crank/cam sensors

Common fixes

  • Replace the spark plug
  • Replace the cylinder 1 ignition coil
  • Clean or replace the fuel injector
  • Repair the vacuum leak
  • Repair the engine if compression is low (valves, rings, head gasket)
  • Repair the wiring or connector

Questions and answers

What does P0301 mean?

The ECU detected a misfire in cylinder 1. It is a generic OBD-II code, so the cause could be ignition, fuel delivery or an engine mechanical problem.

Can I drive with P0301?

Briefly and under light load, yes, but fix it soon. If the check engine light flashes, the catalytic converter is at risk, so stop driving and have the car towed.

How can I tell if it's the coil or the plug?

Swap the coil with one from a neighboring cylinder and clear the codes. If the code moves to P0302 or similar, the coil is bad; if not, check the plug, injector and compression.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.