One of the most common engine fault codes on Mercedes petrol engines. It means the ECU has detected that Cylinder 3 is not firing properly — and it can cause serious damage if ignored.
P0303 means the ECU has detected a misfire on Cylinder 3 — the cylinder is not firing properly. On most Mercedes petrol engines, this is an ignition or fuel problem, not a catastrophic engine failure. It should be diagnosed cheapest-first.
Do not let anyone sell you a full engine rebuild before the ignition components have been ruled out. Full diagnostic order below.
The ECU monitors each cylinder's firing by analysing the crankshaft acceleration during each power stroke. When a cylinder misfires, the crankshaft slows down slightly. P0303 is set when the ECU detects a misfire specifically on Cylinder 3. Crucially, the code describes a symptom, not a cause — the misfire could be due to ignition (spark plug, coil, lead), fuel delivery (injector, fuel pressure), or mechanical issues (compression, valves, timing). That distinction is the entire difference between a £150 coil pack and a £1,500 engine repair. This code appears across the full range of Mercedes petrol engines, from the M270 four-cylinder to the M276 V6 and the M177 V8, and the diagnostic approach is essentially identical regardless of which engine triggers it.
Ordered by how often each turns out to be the real culprit — with typical fitted cost against each, so you can sanity-check any quote you've been given.
| Cause | Likelihood | Typical Cost (fitted) | Notes |
|---|---|---|---|
| Faulty ignition coil on Cylinder 3 | Very High | £150–£250 (single coil) | Most common cause — swap coils to confirm |
| Worn or fouled spark plug on Cylinder 3 | High | £25–£75 (single plug) | Often replaced as part of a full set |
| Vacuum leak (intake manifold, hoses) | Medium | £150–£350 | Can cause lean misfire on one cylinder if localised |
| Fuel injector issue on Cylinder 3 | Medium | £400–£800 | Clogged, leaking, or faulty electrical connection |
| Mechanical compression loss (valves, piston rings) | Low | £1,200–£1,500+ | Most expensive — rule out ignition and fuel first |
Cost figures [PATTERN DATA] — indicative independent-specialist pricing, not drawn from our engine replacement dataset.
Swap the Cylinder 3 coil with another cylinder. If the misfire moves to that cylinder, the coil is faulty.
If the coil swap doesn't move the misfire, swap the spark plug. A worn plug is a cheap fix.
Inspect intake hoses, PCV system, and brake booster lines for cracks or disconnections.
Ensure the fuel pump is delivering adequate pressure, and test the Cylinder 3 injector for flow and leak-off.
If all ignition and fuel checks pass, a compression test will reveal mechanical issues like worn rings or valve problems.
Our diagnostic wizard walks the same order, or confirm your engine first with the registration lookup.
Conditionally, yes — short journeys, but avoid heavy acceleration. P0303 on its own is not an engine-destroying fault, but prolonged driving with a misfiring cylinder can damage the catalytic converter, overheat the engine, and cause further mechanical wear. If the vehicle has dropped into limp mode, motorway driving is unsafe because you cannot accelerate normally. Get it looked at within a few days.
Yes, for short journeys, but avoid heavy acceleration. Prolonged driving with a misfire can damage the catalytic converter. Get it checked within a few days.
No. In most cases, it's a simple ignition issue like a faulty coil or spark plug. Compression problems are the least common cause.
Only temporarily. The code will return as soon as the ECU detects misfires again. Clearing it also destroys the freeze-frame data a specialist would use to narrow the cause.
Expect one hour of diagnostic time at an independent Mercedes specialist. Many will offset that cost against the repair if you proceed with them.
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