P0301 — Cylinder 1 misfire detected

P0301 identifies misfire detected at cylinder 1, not a confirmed failure of that cylinder's spark plug or coil. Cylinder numbering varies between engines and must be verified before testing. Local ignition, injection, intake or compression faults can produce the same code. Diesel diagnosis does not use the spark-plug and ignition-coil checks applicable to petrol engines.

Affected system: Cylinder combustion

Symptoms

• Idle vibration or uneven engine running.
• Hesitation and reduced power, sometimes only cold or during acceleration.
• Engine light, potentially flashing during sufficiently severe misfire.

Possible causes

• Spark plug, coil, lead or ignition connector fault where fitted.
• Cylinder 1 injector or its control circuit.
• Local intake leakage or uneven air distribution.
• Low compression caused by a valve, cylinder or timing-related mechanical problem.

What to check

• Locate cylinder 1 from service information and check lamp behaviour; stop testing for severe rough running.
• Save freeze-frame data and compare cylinder 1 misfire counts with the other cylinders.
• On petrol engines inspect plug, coil, connectors and oil/water traces with the engine off.
• Component swapping is informative only when parts are compatible and the procedure permits it; fault migration is evidence, not the sole diagnostic criterion.
• Check injector operation, wiring and intake sealing using engine-specific tests; never open pressurised injection lines.
• If the fault stays on cylinder 1, request compression/leak-down and mechanical checks. Confirm the misfire is resolved before assessing the catalyst.

Step-by-step repair guide

• Misfires P0300–P0304: how to test ignition coils and spark plugs step by step

Also written as: P0301-00, P030100, PO301 (some scan tools add the fault-type byte, e.g. -00).