P0301 Cylinder 1 Misfire on Smart Fortwo 451 / 453
P0301 is a misfire on cylinder 1. On a Smart three‑cylinder engine, cylinder 1 runs coolest, so compression problems are rare; it's almost always an ignition problem. The diagnostic order is the same as P0303: swap the coil, then the plug, to see if the misfire moves.
Typical Symptoms
- Check engine light with P0301
- Rough idle when cold
- Hesitation under acceleration
- Slight loss of power
- Slight drop in fuel economy
What it means
P0301 means the ECU saw enough misfires on cylinder 1 to set a code. Cylinder 1 sits opposite the exhaust manifold on both the 451 1.0L and the 453 0.9L turbo, so it's the coolest cylinder. That makes compression issues unusual here; the dominant causes are ignition – coil or plug.
Likely causes, cheapest first
- Ignition coil failed. The most common cause; coils tend to fail intermittently, often heat‑related.
- Spark plug worn or fouled. If plugs are over 30,000 miles, replace all three.
- Fuel injector connector loose or its wiring chafed. Less common; check after coil and plug.
- Fuel injector partially clogged or weak. Rare on a Smart with regular fuel-system maintenance.
DIY check steps
- Perform a coil swap test: move the cylinder 1 coil to another cylinder and put that cylinder's coil on 1. Clear the codes, drive normally and see if the misfire moves with the coil.
- If the misfire stays on cylinder 1, swap the spark plug in the same way and re-test.
- While the plug is out, examine it: wet with fuel suggests an injector or coil problem; dry tan is normal; oily indicates a valve-cover leak (more common on the 453).
- If both swap tests pass, inspect the injector connector for looseness or corrosion.
When to call a shop
If new plugs and a known‑good coil and a clean injector connector still leave P0301, that's shop territory. A compression test or leak‑down test can rule out the unlikely scenario of a mechanical issue. Compression problems on cylinder 1 are rare on these engines; if they show up, they're usually part of a wider high‑mileage engine condition where a rebuild or replacement makes more sense.
Related parts & typical prices
| Part | Typical price | Search |
|---|---|---|
| Ignition coil pack (single) | $25-150 | 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
- Service manual on Manualslib — external mirror (we don't host this specific document).
- 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 (P0301)
- Curated: Smart cheap-first diagnostics (cylinder misfire swap test)
- SmartCarOfAmerica: cylinder misfire troubleshooting
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.