P0304 on Mercedes-Benz: How to Diagnose & Fix
Cylinder 4 Misfire Detected
1 Mercedes-Benz model · 1 documented issue · 1 critical
P0304 on Mercedes-Benz by model: symptoms, diagnosis and fix
On the M260 2.0L four-cylinder turbo used in the A220, A250 and A35 AMG (and the CLA250, GLA250, GLB250), the exhaust valve seat rings and valve guides can wear prematurely so that the exhaust valves no longer seal. The result is a loss of compression on one or more cylinders, rough idle and misfire codes, and in advanced cases a rattling/ticking top end. Mercedes-Benz USA acknowledged the defect in August 2023 with an Emission Warranty Extension (document MC-10241534-0001, TSB 'ELW-ExhaustValve') covering 2019-2022 A-Class, CLA, GLA and GLB with the M260, extending coverage on the exhaust valve system to 15 years/150,000 miles. Owners on MBWorld and A-Class Club report dealers replacing the complete cylinder head under the extension, and UK A35 owners have documented loss-of-compression diagnoses on the same engine. This is separate from the M282 1.3L intake carbon issue: the M282 is a different (Renault co-developed) engine and does not share this valve-seat defect.
Common Symptoms
- Rough idle and misfire, especially cold
- Check engine light with cylinder misfire codes
- Low compression on one or more cylinders in a leak-down test
- Loss of power, hesitation under load
- Ticking or rattling noise from the cylinder head
How to Fix
Free repair within 15 years/150,000 miles for US 2019-2022 cars under the MBUSA exhaust-valve warranty extension: the dealer performs a leak-down/compression test and replaces the cylinder head assembly (valves, seats, guides) as needed; owners who paid for this repair before the extension can claim reimbursement with documentation. Outside the US or outside the extension, a replacement cylinder head on the M260 is a $4,000-$7,000 job, so any persistent P0300-P0304 misfire on a low-mileage M260 should be compression-tested before spending money on plugs and coils.
Sources for this issue (5)
- Mercedes-Benz Emission Warranty Extension MC-10241534-0001 (exhaust valve system, 15 yr/150,000 mi)tsb
- Warranty Repair - Cylinder Head! - MBWorld.org (GLB250, M260)forum
- Cylinder Head Extended Warranty M260 Engine - A-Class Clubforum
- A35 AMG loss of compression - MBClub UKforum
- 2019 Mercedes-Benz A220 TSBs (incl. ELW-ExhaustValve, Aug 3 2023)tsb
P0304 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0304 mean on a Mercedes-Benz?
P0304 stands for "Cylinder 4 Misfire Detected." This code means the engine computer detected a misfire in cylinder number 4 — that cylinder isn't igniting its air-fuel mixture fully or at all. The system detects this by monitoring small changes in crankshaft rotation speed as each cylinder fires. You may feel a rough idle, shaking, hesitation or reduced power, and a persistent misfire can dump unburned fuel into the exhaust and overheat or damage the catalytic converter. Since it's pinpointed to one cylinder, the problem typically lies in that cylinder's spark, fuel, or compression. On Mercedes-Benz it is documented across 1 model: A-Class.
What are the most common causes of P0304?
The most common causes of P0304 are: Worn or fouled spark plug in cylinder 4, Failed or weak ignition coil on cylinder 4, Faulty spark plug wire or boot (where applicable), Clogged, stuck or leaking fuel injector for cylinder 4, Vacuum or intake air leak near that cylinder, Low compression (worn rings, burnt/leaking valve, head gasket), Damaged wiring or connector at the coil or injector. The specific cause varies by vehicle.
How much does it cost to fix P0304 on a Mercedes-Benz?
Repair costs vary widely with the root cause and Mercedes-Benz model; each issue on this page carries its own estimate where one is published.
Looking for P0304 on a different make?
View P0304 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.