Draft guidance: awaiting Claude spot-check, qualified-mechanic review and named-editor approval.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

P0301: Cylinder 1 misfire detected

The engine control unit has detected that cylinder 1 is not contributing consistently.

What P0301 means

The engine control unit has detected that cylinder 1 is not contributing consistently. P0301 identifies the affected cylinder, but it does not prove that its spark plug, coil or injector has failed. Cylinder numbering varies between engines and must be confirmed for the exact vehicle.

Can I still drive?

Treat an active cylinder 1 misfire as urgent because unburnt fuel may overheat the catalytic converter. Stop, switch off and arrange recovery for a flashing or red warning light, heavy shaking, major power loss, repeated stalling, overheating, smoke, a strong raw-fuel smell or a new mechanical knock or tick. With only a steady amber light, no noticeable roughness and no other warning signs, restrict driving to a short, low-load trip to a nearby repairer. Avoid hard acceleration and high engine speed. A smooth-running engine does not make the code safe to ignore; the misfire may occur only when cold, hot or under load.

Symptoms

Engine-management light on or flashing
Uneven idle or vibration
Hesitation and reduced acceleration
Judder under load
Hard starting or occasional stalling
Increased fuel use
Fuel smell from the exhaust
Little or no obvious symptom when intermittent

Common causes

Worn, fouled or damaged cylinder 1 spark plug on a petrol engine
Weak cylinder 1 ignition coil, damaged coil boot or faulty HT lead
Restricted, leaking or electrically faulty cylinder 1 injector
Damaged coil or injector wiring, connector or earth
Local intake-manifold, injector-seal or vacuum leak
Low compression caused by a burnt or sticking valve, worn rings, head-gasket leakage or a valvetrain fault
Oil, coolant or heavy carbon contamination in that cylinder
A wider fuel-pressure, timing, air-metering or control fault registering most strongly on cylinder 1

Safe first checks

Save all codes and freeze-frame information; do not focus only on P0301 if other cylinders or fuel-trim codes are present.
Confirm cylinder 1's location using reliable information for the exact engine.
With the engine off and cold, inspect accessible coil and injector connectors, wiring, intake hoses and the plug-well area for looseness, damage or moisture.
Check oil and coolant safely, noting unexplained coolant loss, oil contamination or white exhaust smoke.
Review when the spark plugs were replaced and whether the correct specification was used.
Do not handle ignition components with the engine running or open a pressurised fuel system. Component substitution and compression or leak-down testing require the proper procedure.

Typical UK fix cost

Budget approximately £100–£450 for diagnosis and a common cylinder-specific repair such as plugs, one accessible coil, local wiring attention or one injector. Low compression, valve, head-gasket, timing or catalytic-converter faults can take the bill to £600–£2,500+. These are broad editorial estimates for a 2026 UK independent garage and must be checked against a vehicle-specific written quotation covering diagnosis, parts, labour and VAT.

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.