Bob's Mechanical Repairs

BMW P0304

Bob's Mechanical Repairs — independent family-run garage in Birnam, Dunkeld, Perthshire. Call 01350 727 276 or email [email protected].

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Cylinder 4 misfire on BMW — N47 diesel cyl 4 injector or glow plug; N20/N52/N55 petrol coil pack failure very common.

BMW P0304 — Cylinder 4 misfire on BMW — N47 diesel cyl 4 injector or glow plug; N20/N52/N55 petrol coil pack failure very common. Typical UK independent garage cost: £120–£580.

What actually causes P0304 on a BMW

BMW petrol coil packs (DENSO/BERU) commonly fail by 60k–80k miles — N20 4-cyl turbos and N52/N55 inline-6 affected equally. Cyl 4 is heat-stressed on N20 (rearmost on 4-cyl) and central on inline-6. N47 diesel cyl 4 misfire is usually a worn glow plug (cold-start hesitation) or piezo injector failure.

BMW engines that log P0304

  • N47 2.0d (320d, 520d, X1)
  • N57 3.0d (330d, 530d, X5)
  • N20 2.0 turbo petrol (320i, 528i, F30 328i)
  • N52/N55 6-cyl petrol (cyl 4 on inline-6)

How we diagnose P0304 on a BMW

  1. INPA / ISTA / BimmerLink — read per-cylinder misfire counters
  2. Swap coil 4 with another cylinder to confirm
  3. Pull spark plug, inspect, replace if worn
  4. On N47, glow plug test and injector smooth-running values
  5. Replace component and clear adaptations

The common misdiagnosis to avoid

BMW main dealer will quote a 6-coil set on inline-6 with one misfire — refuse, do the diagnostic, replace what's failed. Coils are NGK/BERU genuine spec, generic eBay coils fail within months.

BMW P0304 repair cost in the UK

Expect £120–£580 including parts and labour at an independent garage. Main-dealer quotes on BMW 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

On N20 turbo petrol with cyl 4 misfire, ALSO inspect the timing chain at the rear of the engine — N20 timing chains stretch by 70k miles and the early warning is a slight rattle on cold start plus occasional misfire. Catching it early avoids a £3,000+ engine job.

Codes usually seen alongside P0304 on BMW

  • P0300
  • P0301
  • P0302
  • P0303

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.

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