Draft guidance: awaiting Claude spot-check, qualified-mechanic review and named-editor approval. Do not use a fault code alone to replace parts.

P0303: Cylinder 3 misfire detected

P0303 means the engine control unit has detected a misfire associated with cylinder 3.

What P0303 means

P0303 means the engine control unit has detected a misfire associated with cylinder 3. It is a location clue rather than a diagnosis: ignition, fuel delivery, air leakage, wiring or the cylinder's mechanical condition could be responsible. Manufacturer information is needed to identify cylinder 3 correctly.

Can I still drive?

Treat an active P0303 as urgent. Misfiring can worsen and may overheat the catalytic converter. Stop safely and switch off if the warning light flashes, the engine runs very roughly, acceleration becomes unsafe, the car stalls, or there is overheating, smoke, a strong fuel smell or a serious new noise. Arrange recovery. A short, gentle drive to a nearby repairer may be reasonable only when the light is steady amber, the engine remains smooth and there are no other warning signs. Do not undertake a normal journey simply to see whether the code returns.

Symptoms

  • Steady or flashing engine-management light
  • Lumpy idle or noticeable vibration
  • Hesitation and weak acceleration
  • Judder, especially uphill or under load
  • Poor starting or stalling
  • Higher fuel consumption
  • Unusual exhaust smell
  • No clear symptom if the misfire was brief or condition-specific

Common causes

  • Worn, fouled, cracked or incorrect cylinder 3 spark plug on a petrol engine
  • Weak cylinder 3 coil, damaged coil boot, HT lead or shared coil-pack output
  • Blocked, leaking or electrically faulty cylinder 3 injector
  • Connector, loom, earthing or control-unit driver problem
  • Intake-manifold or injector-seal leak affecting that cylinder
  • Low compression due to a valve, piston ring, head gasket, camshaft or valvetrain fault
  • Oil, coolant or carbon deposits interfering with combustion
  • A broader mixture, fuel-pressure or timing fault when other cylinders or related codes are affected

Safe first checks

  • Read and record all codes and freeze-frame data. P0300, another P030x or a mixture code may show that the fault is not confined to cylinder
  • 2. Verify cylinder numbering using trusted service information for the exact engine.
  • With the ignition off and engine cold, inspect accessible cylinder 3 wiring and connectors, intake pipework and the plug-well area.
  • Check oil and coolant levels, noting coolant loss, contaminated oil, white smoke or a fuel smell.
  • Check whether plugs are overdue and whether the fault began after servicing, engine-bay washing or repair work.
  • Do not probe ignition outputs while running or loosen fuel pipes. Misfire data, controlled component substitution, injector tests and compression/leak-down measurements should replace guesswork.

Typical UK fix cost

Allow around £100–£450 for diagnosis and a common cylinder-specific repair. An internal-engine, cylinder-head, timing or catalytic-converter repair may cost £600–£2,500+. This range is indicative only and must be checked against current UK parts prices, labour, VAT and the vehicle-specific diagnosis before publication.

Related models

This is a generic OBD-II code used across many makes and models. The exact test thresholds, component layout and repair route vary by engine and manufacturer, so check vehicle-specific technical information before ordering parts.

Related fault codes

Sources

A diagnostic trouble code identifies a system or circuit to investigate. It does not prove that a named part has failed. Preserve freeze-frame data and seek a vehicle-specific diagnosis before clearing the code.