P0300 Random / Multiple Cylinder Misfire on Smart Fortwo 451 / 453
P0300 means the engine is misfiring on more than one cylinder or the misfire is moving around. On a Smart, that usually points to worn spark plugs, a failing coil pack or a vacuum leak downstream of the mass‑air sensor. Chase the cheap stuff first; fuel‑pressure problems are rarer but possible.
Typical Symptoms
- Check engine light, often flashing under load
- Rough idle that doesn't smooth out
- Power loss, especially on hills or during merging
- May appear with P0301, P0302 or P0303
- Noticeable drop in fuel economy
What it means
A misfire code points at incomplete combustion in one or more cylinders. P0300 is the umbrella code for random or multiple misfires; the ECU can't assign the misfire to a single cylinder. On a three‑cylinder Smart that typically means all the plugs are worn at once, the coil pack has a weak channel, or unmetered air is entering the intake.
Likely causes, cheapest first
- Spark plugs all worn. If they're over 30,000 miles, replace all three as a set.
- Coil pack failing intermittently. On variants with a single coil pack feeding all cylinders, one weak channel randomizes which cylinder misfires.
- Vacuum leak after the MAF. A cracked intake boot, torn PCV hose or leaking gasket lets unmetered air in and leans out the mix across cylinders.
- Air filter or filter housing leaking. Less common, but a torn housing seal has the same effect as a vacuum leak.
- Low fuel pressure. Fuel filter, pump or regulator issues are rarer but possible.
DIY check steps
- Pull all three plugs and inspect them. Wet with fuel suggests injector or coil weakness; oily suggests the valve cover gasket issue documented on P0303 (453). Crusty deposits mean it's simply time for new plugs.
- Check live fuel trims using a scan tool; a long‑term trim above +10% is a vacuum leak fingerprint. Near‑zero trims with misfires still present point at ignition or fuel pressure.
- Swap or replace the coil pack. On models with individual coils, move one coil to a different cylinder and see if the misfire follows.
- Hunt for vacuum leaks by spraying carb cleaner around intake gaskets, hoses and the PCV system. A bump in idle speed indicates a leak.
- If everything else checks out, a shop smoke test and possibly a compression test are next.
When to call a shop
If new plugs, a known‑good coil, no vacuum leaks and clean fuel trims still leave P0300, it's time for a shop. A compression test or fuel pressure check can be done with the right tools but most owners hand it over at this point. Persistent random misfires on a high‑mileage Smart can also be an early sign of intake valve carbon on a 453; that's a shop intervention.
Related parts & typical prices
| Part | Typical price | Search |
|---|---|---|
| Spark plugs (set of 3, NGK / Bosch) | $15-35 | Search Google |
| Ignition coil pack (single) | $25-150 | Search Google |
| Air filter | $15-30 | Search Google |
| Fuel pressure regulator | $50-130 | 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
- Evilution: Specific error codes (Smart-known patterns)
- SmartCarOfAmerica: misfire troubleshooting threads
Stuck on this one?
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