P0303 on Mercedes-Benz
Cylinder 3 Misfire Detected
P0303 on Mercedes-Benz vehicles indicates cylinder 3 misfire detected. Au7o has documented this code across 1 Mercedes-Benz model — most commonly on GLB. 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.
Common Causes of P0303
- •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
P0303 on Mercedes-Benz by Model
Mercedes-Benz GLB(1 issue)
- M260 Cylinder Head Exhaust Valve Seat/Guide Wear (Misfires, Compression Loss)2020-2022
The 2.0L turbo M260 four-cylinder used in the GLB 250 / GLB 35 (and the M264-related variants) suffers from premature wear of the exhaust valve seats and valve guides. The exhaust valves stop sealing, causing loss of compression in one or more cylinders, rough running and misfires. Mercedes acknowledged this as a manufacturing defect and issued a voluntary extended warranty in the US and Canada covering cylinder head replacement for up to 15 years / 150,000 miles on affected 2019-2022 A-Class, CLA, GLA and GLB vehicles. A 2026 NHTSA-posted TSB (MC-11028267) documents the valve seat/guide wear on M260/M264 engines. Left unaddressed it can progress to catastrophic piston/secondary damage that is NOT covered by the warranty extension.
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View P0303 across all makes →Frequently Asked Questions
What does P0303 mean on 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 specifically, this code is documented across 1 model.
What causes P0303 on Mercedes-Benz vehicles?▼
Common causes on Mercedes-Benz: 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. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P0303 on a Mercedes-Benz?▼
Repair costs vary widely depending on the root cause and specific Mercedes-Benz model.
Which Mercedes-Benz models have P0303 documented?▼
Au7o has documented P0303 on 1 Mercedes-Benz model: GLB.