P0303 Cylinder 3 Misfire on Smart Fortwo 451
P0303 (cylinder 3 misfire) on the 451's M132 three-cylinder is most often the coil or spark plug. Swap them to a different cylinder first — only suspect a burned valve if both swap tests fail.
Typical Symptoms
- Check engine light with code P0303
- Rough idle, especially when cold
- Hesitation or stumble under acceleration
- Slight loss of power; you can feel the cylinder dropping
- Fuel economy slightly worse than usual
What it means
P0303 is an OBD-II generic code: cylinder 3 is misfiring — the ECU saw incomplete combustion events on cylinder 3 often enough to trigger the fault. On a 451, cylinder 3 is the cylinder closest to the exhaust side of the 1.0L M132 three-cylinder engine.
Cylinder 3 also runs the hottest. That matters for the worst-case scenario down the page (burned exhaust valve), but it is not the place to start.
Likely causes (cheapest first)
The diagnostic order matters. Owners burn through hundreds of dollars in parts when they jump to "must be a burned valve" before doing the 5-minute swap test.
- Ignition coil failed. The most common cause. Coils fail intermittently, often heat-related — the misfire shows up after a long highway run or hot day.
- Spark plug worn / fouled. Cheap and easy. If plugs are over 30k miles, replace all three regardless.
- Fuel injector partially clogged or weak on cylinder 3. Less common, but worth checking after the coil and plug.
- Burned exhaust valve on cylinder 3 — the model-specific failure mode. Cylinder 3 sits closest to the exhaust manifold and runs hottest, making the exhaust valve the most failure-prone on the M132. Only suspect this if both the coil and plug swap tests leave the misfire on cylinder 3. Confirm with a compression test. Fix is a head pull / valve job — not DIY for most owners.
DIY check steps
- Coil swap test. Pull the coil pack from cylinder 3 and put it on cylinder 1 (or 2). Move cylinder 1's coil to cylinder 3. Clear the codes. Drive the car for a normal cycle. Re-read codes. If the misfire moved with the coil to cylinder 1, the coil is the problem — replace it. If the misfire stayed on cylinder 3, the coil is fine.
- Spark plug swap test. Same procedure with the spark plug. Move the cylinder 3 plug to cylinder 1, cylinder 1's plug to 3. Drive, recheck. If the misfire follows the plug, replace the plug. If the misfire stays on cylinder 3, the plug is fine.
- Visual check on the plug while you have it out. Wet, oily, or carbon-fouled tells you something specific. Dry and tan = normal wear. Wet with fuel = injector or coil. Oily = headed toward 453 territory (valve cover gasket leaking into the plug well), but rare on the 451 M132.
- Compression test on cylinder 3. Only if both swap tests came up clean. Low compression on cylinder 3 with the other two normal points at the burned valve. This needs a compression gauge ($30-50 from any parts store) and 30 minutes.
When to call a shop
Compression on cylinder 3 is below ~120 psi or 75% of the other cylinders, and you don’t want to pull the head yourself. Valve job runs $1,200-2,500 depending on shop and what else needs doing while the head is off. If you’re considering it on a high-mileage car (150k+), price a used engine swap as a comparison — sometimes it’s the better number.
If you’ve done the coil and plug swap tests and the misfire moved — congratulations, that's a $40 fix. Do that first.
Related parts & typical prices
| Part | Typical price | Search |
|---|---|---|
| Ignition coil pack (single) | $25-60 | Search Google |
| NGK / Bosch spark plug | $8-18 each | Search Google |
| Fuel injector | $80-180 | Search Google |
Prices are rough community-reported ranges, not quotes. Aftermarket vs. genuine Mercedes parts swing the spread; call a Smart-experienced shop for an actual quote.
Manual references
- Workshop / service manual PDF — full procedure with torque specs, hosted on smartcarmanuals.com.
- Browse Smart manuals on smartcarmanuals.com — pick your chassis code section on the home page if a specific manual isn't listed above.
Community references
- Evilution: OBD-II error code reference
- Evilution: Specific error codes (Smart-known patterns)
- SmartCarOfAmerica: P0303 / cylinder 3 misfire threads
Stuck on this one?
SmartDiag-AI runs through the cheap-first checks with you, weighted to community-known patterns for your exact model. The link below pre-fills the code and model.