P0303 on Mercedes-Benz: How to Diagnose & Fix
Cylinder 3 Misfire Detected
2 Mercedes-Benz models · 2 documented issues · 2 critical
P0303 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 (6)
- MBUSA Emission Extended Warranty - Exhaust Valve Assembly MY19-22 A-Class (MC-10241534-0001, Aug 4, 2023)tsb
- 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
Certain M264-equipped GLC 300 vehicles can develop irregular wear of the exhaust valve guide or seat, leading to misfire and a check-engine light. Mercedes-Benz USA's January 2026 warranty notice lists model-year 2020–2023 GLC 300, GLC 300 4MATIC and GLC 300 4MATIC Coupe. A misfire does not by itself prove this condition; diagnosis must distinguish ignition, injection and mechanical faults.
Common Symptoms
- Persistent engine misfire
- Check engine light with misfire codes (P030x)
How to Fix
Ask a Mercedes-Benz dealer to diagnose the misfire using the applicable service guidance, including LI01.30-P-072328, and verify VIN eligibility for the M264 exhaust-valve-system extension. Covered conditions receive 15 years/150,000 miles, whichever occurs first, subject to the program's terms and documented diagnosis. The extension is not a blanket promise to replace every misfiring engine or head. Prior qualifying repairs may be eligible for reimbursement with supporting receipts and repair records.
Call to confirm they can assess your vehicle and perform the stated work. Search results do not confirm qualification, eligibility or coverage.
Sources for this issue (6)
- MBUSA News Channel Update (Jan 9, 2026) — Extended Warranty, Cylinder Head M264manufacturer
- M264 engine. How common is cylinder head failure? - MBWorld.org GLC-Class (X253) Forumforum
- M264 Cylinder Head Failure - MBWorld.org Forumsforum
- GLC300 2022 engine failure - MBWorld.org GLC-Class (X253) Forumforum
- For Ye Owners w/the M264 - MBWorld.org GLC-Class (X253) Forumforum
- Potential M264 Engine Defect Raises Serious Safety Concernsarticle
P0303 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 P0303 mean on a Mercedes-Benz?
P0303 stands for "Cylinder 3 Misfire Detected." This code means your engine's computer detected a misfire specifically in cylinder number 3 — that cylinder isn't burning its fuel-air mixture properly or completely. The computer senses misfires by watching for tiny fluctuations in crankshaft speed each time a cylinder is supposed to fire. A misfire wastes fuel, can cause a noticeable shake, rough idle or loss of power, and can flood raw fuel into the exhaust where it may overheat and damage the catalytic converter. Because the fault is isolated to one cylinder, the cause is usually something specific to that cylinder's ignition, fuel delivery, or compression. On Mercedes-Benz it is documented across 2 models: A-Class, GLC.
What are the most common causes of P0303?
The most common causes of P0303 are: Worn or fouled spark plug in cylinder 3, Failed or weak ignition coil on cylinder 3, Bad spark plug wire or boot (where applicable), Clogged or leaking fuel injector for cylinder 3, Vacuum or intake air leak affecting that cylinder, Low compression (worn rings, burnt/leaking valve, head gasket), Wiring or connector fault at the coil or injector. The specific cause varies by vehicle.
How much does it cost to fix P0303 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.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.