Mini P0304

Cylinder 4 misfire on Mini — N14/N18 Prince engine coil/plug failure plus timing chain stretch causing cyl 4 misfire on high-mileage Cooper S.

Mini P0304 — Cylinder 4 misfire on Mini — N14/N18 Prince engine coil/plug failure plus timing chain stretch causing cyl 4 misfire on high-mileage Cooper S. Typical UK independent garage cost: £140–£1800.

What actually causes P0304 on a Mini

Mini N14/N18 (BMW/PSA shared Prince engine) coil packs fail by 60k miles — cyl 4 first. But on Cooper S with cyl 4 misfire and rattling on cold start, the cause is usually a stretched timing chain — Mini's notorious 'death rattle' is the early warning. Cyl 4 misfires because the cam timing has drifted out of phase.

Mini engines that log P0304

• N14 1.6 turbo (Cooper S R56, JCW R56)
• N18 1.6 turbo (Cooper S R56 LCI, F56)
• N12/N16 1.6 (Cooper R56, Cooper F56)

How we diagnose P0304 on a Mini

1. INPA / ISTA Mini — read per-cylinder misfire counters
2. Listen for cold-start chain rattle (3–5 seconds is the giveaway)
3. Inspect cam timing alignment via OBD live data
4. Swap coil 4 with another cylinder if no chain rattle
5. If chain stretch confirmed, quote chain kit + labour (£900–£1,400)

The common misdiagnosis to avoid

On Mini Cooper S with cyl 4 misfire AND any cold-start rattle, do NOT just replace the coil — investigate the timing chain first. A coil swap on a stretched chain delays the inevitable and the customer comes back furious when the chain snaps and bends valves.

Mini P0304 repair cost in the UK

Expect £140–£1800 including parts and labour at an independent garage. Main-dealer quotes on Mini are usually higher for the same job, and the spread exists because the cheap cause and the expensive cause throw the same code — pay for the diagnosis before you pay for parts.

Bob's workshop tip

Mini Cooper S R56 with cyl 4 misfire over 70k miles is one of THE moments to be honest with the customer — if the chain isn't done yet, this is the warning. A £1,200 chain job now beats a £3,500 engine rebuild later. Always sell the truth.

Codes usually seen alongside P0304 on Mini

• P0300
• P0301
• P0302
• P0303
• P052E

Symptoms drivers report with P0304

• Engine warning light flashing (not steady) under load
• Noticeable juddering at idle in gear
• Smell of unburnt fuel from the exhaust
• Misfire worse when engine is cold and damp
• Significantly reduced power

Repairs that clear P0304

• Plug & coil replacement

For the generic, all-makes explanation of this code — meaning, every known cause and UK cost bands — see our P0304 guide.

Reading P0304 on a Mini properly

A generic reader will show P0304 and nothing else. What matters is the freeze-frame data stored with it — engine speed, load, coolant temperature and fuel trims at the moment the fault was logged — plus whether any other module on the Mini has logged something at the same time. Codes rarely arrive alone, and the order they were stored in usually points at which one is the cause and which are the consequences.

Clearing P0304 without fixing it

Erasing the code resets the Mini's readiness monitors as well as the light. The fault returns on the next drive cycle if the cause is still there, and in the meantime the incomplete monitors can stop the car passing an emissions check. After a genuine repair, the car is driven until the relevant monitor completes before it is handed back.

Bob's Mechanical Repairs is an independent garage in Birnam, Dunkeld, Perthshire, with 32 years of Bob's experience on the tools. If P0304 is showing on your Mini, bring any scan results with you — we will read the live data ourselves, confirm which cause it actually is, and price the repair before any work starts.