Draft guidance: awaiting Claude spot-check, qualified-mechanic review and named-editor approval.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

P0304: Cylinder 4 misfire detected

The engine control unit has detected that cylinder 4 failed to contribute normally during some combustion events.

What P0304 means

The engine control unit has detected that cylinder 4 failed to contribute normally during some combustion events. P0304 does not distinguish between an ignition, fuelling, air-leak, electrical or mechanical fault. Cylinder numbering is engine-specific, so do not assume which physical cylinder is number 4.

Can I still drive?

Treat an active P0304 as urgent. The risk depends on the severity of the misfire, not the cylinder number. Stop and arrange recovery if the engine-management light flashes or turns red, the engine shakes strongly, power loss makes driving unsafe, or there is stalling, smoke, overheating, a raw-fuel smell or an abnormal mechanical sound. If the light is steady amber and the engine is smooth, with normal fluid levels and no other warning signs, limit driving to a short, gentle trip to a nearby garage. Avoid heavy throttle, high revs, steep climbs, towing and motorway driving until assessed.

Symptoms

Engine-management light illuminated or flashing
Rough idle or cabin vibration
Hesitation, juddering or reduced power
Misfire that appears only during cold starting or hard acceleration
Difficult starting or stalling
Increased fuel consumption
Fuel-like exhaust smell or unusual smoke
No obvious symptom when the stored fault is intermittent

Common causes

Worn, contaminated, cracked or incorrectly specified cylinder 4 spark plug on a petrol engine
Weak ignition coil, damaged boot, HT lead or coil-pack output for cylinder 4
Restricted, leaking or electrically failed cylinder 4 injector
Damaged wiring, loose connector, poor earth or control-circuit fault
Intake or injector-seal leak close to cylinder 4
Valve leakage, sticking valve, broken valve spring, cam or lifter wear, worn rings or head-gasket leakage
Oil, coolant or carbon fouling
Shared low fuel pressure, incorrect mixture, cam-timing or sensor problem if other codes or cylinders are involved

Safe first checks

Preserve all stored and pending codes plus freeze-frame information before clearing the warning light.
Confirm cylinder 4's location from the correct workshop information; inline, flat and V engines differ.
With the engine switched off and cold, inspect visible connectors, wiring, intake hoses and the plug-well area for looseness, rubbing, moisture or contamination.
Check oil and coolant safely and investigate unexplained losses rather than repeatedly topping them up.
Review service and repair history, particularly plug replacement, timing work or recent disturbance of intake and ignition components.
Do not touch coils or HT leads on a running engine and do not open fuel pipes. Testing should establish whether the fault follows a component or remains with cylinder 4 before parts are authorised.

Typical UK fix cost

Budget approximately £100–£450 for diagnosis and a common repair involving plugs, an accessible coil, local wiring or one injector. Difficult access, diesel or direct-injection parts and manufacturer-specific procedures can increase the price. Mechanical, timing, cylinder-head or catalytic-converter work can reach £600–£2,500+. Treat these figures as editorial estimates requiring a current human price check and a written vehicle-specific quotation.

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.