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.