Cylinder 4 misfire on Mercedes — M271 Kompressor and M274 turbo petrol coil failure, OM651 diesel injector or glow plug.
Mercedes-Benz P0304 — Cylinder 4 misfire on Mercedes — M271 Kompressor and M274 turbo petrol coil failure, OM651 diesel injector or glow plug. Typical UK independent garage cost: £160–£720.
What actually causes P0304 on a Mercedes-Benz
Mercedes M271 Kompressor petrol coil packs fail commonly — when cyl 4 misfires it's almost always the coil. M274 modern turbo petrol same story by 80k miles. OM651 diesel cyl 4 misfire is usually a failed glow plug (4 minutes of cold-start hesitation) or a tired piezo injector. M272 V6 with cyl 4 misfire is often the famous Mercedes balance shaft issue if the engine has done over 100k.
Mercedes-Benz engines that log P0304
- M271 1.8 Kompressor (C/E/SLK)
- M272 3.5 V6 petrol
- M274 2.0 turbo (C/E/GLA/CLA)
- OM651 2.1 diesel (C/E/Sprinter)
How we diagnose P0304 on a Mercedes-Benz
- Xentry / Star — read per-cylinder misfire data
- Swap coil 4 with another cylinder
- Pull spark plug, check condition
- OM651: glow plug test and injector return-flow
- M272: check engine timing via Xentry — confirm not balance-shaft related
The common misdiagnosis to avoid
On M272 V6 with cyl 4 misfire, before quoting £200 for a coil, check the timing — if the engine has the balance shaft wear (pre-2008 cars mainly), the misfire will return because the timing is drifting. Quote properly or refer.
Mercedes-Benz P0304 repair cost in the UK
Expect £160–£720 including parts and labour at an independent garage. Main-dealer quotes on Mercedes-Benz 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 any M272 V6 over 100k miles with persistent cyl 4 (or any single cylinder) misfire, mention the balance-shaft history to the customer up front — better they know about the £2,500+ job potential before committing to a coil pack.
Codes usually seen alongside P0304 on Mercedes-Benz
- 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.
Reading P0304 on a Mercedes-Benz 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 Mercedes-Benz 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 Mercedes-Benz'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 Mercedes-Benz, 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.