Mercedes-Benz P0304

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

1. Xentry / Star — read per-cylinder misfire data
2. Swap coil 4 with another cylinder
3. Pull spark plug, check condition
4. OM651: glow plug test and injector return-flow
5. 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.