P0303 Cylinder 3 Misfire on Smart Fortwo 453 (Valve Cover Gasket Oil Leak)
P0303 on a 453 is most often a valve cover gasket leak — oil pools in the spark plug well, fouls the plug, and damages the coil. Inspect the well first; the fix is the gasket, not just plugs.
Typical Symptoms
- Check engine light with code P0303 (sometimes also P0301, P0302 if multiple cylinders)
- Rough idle, stumble at low RPM
- Oil-soaked or wet spark plug when pulled
- Visible oil weeping at the valve cover edges
- Misfire returns within weeks of a plug or coil swap
What it means
P0303 on a 453 looks like a normal cylinder-3 misfire on the surface, but it has a specific cause that catches owners off guard: the valve cover gasket on the 0.9L turbocharged three-cylinder (Renault-Nissan H4Bt / TCe 90, shared with the Twingo III) integrates the spark plug tube seals. When the gasket starts to age or crack, engine oil pools in the spark plug wells. Cylinder 3 is the first to suffer because it sits closest to the hot exhaust side.
That oil fouls the plug, and over time it works its way into the ignition coil pack and degrades it too. Owners replace the plug, the misfire comes back. Replace the coil, the misfire comes back — because the source of the problem (the oil leak) wasn’t addressed.
This is the dominant misfire pattern on the 453, well-documented across SmartCarOfAmerica, the Smart Owners Group on Facebook, and Renault TCe communities (the Twingo and Clio communities see the identical failure mode).
Likely causes (cheapest first)
- Valve cover gasket failure with oil in the plug well. Almost always the cause on a 453. The integrated plug tube seals harden and let oil through. Do this check first.
- Coil already damaged by oil exposure. If you’ve been driving with oil in the well for a while, the coil pack may need replacement after the gasket fix.
- Worn or fouled spark plug. Always replace the cylinder 3 plug at the same time as the gasket fix — cheap insurance.
- Carbon buildup on intake valves (DI engine quirk). The H4Bt is direct-injected, so intake valves can carbon up over time. Less common cause of P0303 specifically, but worth knowing about.
DIY check steps
- Pull the cylinder 3 ignition coil and spark plug. Look down into the plug well with a flashlight before you do anything else.
- Is there oil in the well? Wet, glossy, smells of engine oil = valve cover gasket. The plug itself will likely be oil-soaked. If yes, the gasket is the fix — not just plugs and coils.
- Inspect the coil boot. Oil-saturated boot = the coil is degraded too and should be replaced with the gasket.
- Check the valve cover edges from above. Look for weeping or oil staining around the valve cover seam. Confirms what the well already showed.
- If the well is dry, you’re back to the standard misfire diagnostic — coil swap, plug swap (see the P0303 451 page for the procedure; same approach on the 453).
When to call a shop
The valve cover gasket job on the 453 H4Bt is doable for an experienced DIYer but it’s tight, the engine bay is small, and torque specs matter (the cover bolts will warp the cover if over-torqued). Plan on 3-5 hours and budget $60-150 for parts. A Smart-experienced shop will charge $300-600 for the job, plus coil and plugs if needed.
If you’ve already swapped the plug and coil and the misfire came back — stop swapping parts. Pull the coil and look for oil in the well. That’s the actual answer.
Related parts & typical prices
| Part | Typical price | Search |
|---|---|---|
| Valve cover gasket kit (with plug tube seals) | $60-150 | Search Google |
| Ignition coil pack | $40-90 | Search Google |
| Spark plug set (3) | $25-50 | 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
- Colin Leung: DIY Smart Car valve cover gasket and spark plug seal replacement (YouTube)
- SmartCarOfAmerica: P0303 valve cover gasket oil-fouled plug threads
- Reddit r/smartcars: oil in plug well discussions
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