P0442 EVAP Small Leak on Smart Fortwo 451 / 453
P0442 is a small EVAP leak. It's the same diagnostic flow as any EVAP code: start with the fuel cap. If the cap is good, a smoke test is usually needed because pinhole leaks are nearly impossible to spot by eye.
Typical Symptoms
- Check engine light with P0442
- No noticeable drivability problem
- Leak may come and go with weather changes
- Sometimes co‑occurs with P0455 or P0457
What it means
During its self‑test the EVAP system pressurizes or pulls a vacuum on the fuel system and watches how fast pressure changes. P0455 means the leak is large; P0442 means it's small – a pinhole in a hose, a cap that mostly seals or a purge valve that doesn't fully close.
Likely causes, cheapest first
- Fuel cap O‑ring slightly worn. A cap that mostly seals but leaks a little triggers P0442.
- Pinhole crack in an EVAP hose. Age and heat crack the small rubber lines.
- Purge valve not closing fully. A valve stuck slightly open depressurizes the system between drive cycles.
- Charcoal canister seam separating. Less common but real; a small crack can leak slowly.
- Filler neck rubber boot starting to crack. Similar to the cap: close to sealing, not quite.
DIY check steps
- Reseat the fuel cap until it clicks three times and clear the code. Drive two warm‑up/cool‑down cycles.
- If it returns, replace the cap with a genuine one; generic caps don't always seal Smarts.
- Visually inspect the EVAP hoses under the engine cover and around the canister for cracks, age‑hardening or popped‑off lines.
- With the engine idling, listen for the purge valve clicking. No click suggests the valve or its wiring is faulty; a continuous hiss after key‑off can mean the valve is stuck open.
- Stop there if everything looks good – pinhole leaks require a smoke machine that most driveways don't have.
When to call a shop
If a new cap, intact hoses and a working purge valve don't resolve P0442, a shop smoke test is the only way to find a small leak. If the leak turns out to be in the canister or buried in the tank‑side plumbing, the repair is awkward at home. Many owners in states without emissions testing simply live with P0442 once the cap is verified.
Related parts & typical prices
| Part | Typical price | Search |
|---|---|---|
| Fuel cap | $15-30 | Search Google |
| EVAP vacuum hose / line | $10-25 | Search Google |
| EVAP purge valve / solenoid | $30-90 | Search Google |
| Charcoal canister | $80-220 | 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
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