P0440 Evaporative Emission System Malfunction
A general fault in the fuel vapour system — the broadest of the EVAP codes, so check for more specific ones alongside it.
Emissions
P0440 is the general-purpose EVAP fault code. It means the system failed its self-test without the ECU narrowing down which part. Because it's so broad, the most useful first step is checking whether a more specific EVAP code is stored with it — a P0442 or P0446 alongside tells you far more than P0440 does on its own.
Diagnose P0440 on your specific vehicle Add your year, make and model for ranked causes, repair videos and the exact parts you needCan I keep driving?
Driveable
Yes.
How serious
Low. Emissions failure, no mechanical risk.
Symptoms you'll notice
- Check-engine light
- Fuel smell in some cases
- Failed emissions test
- No drivability change
What causes P0440
Ordered by what to check first — how likely each cause is, weighed against what it costs to rule out. Working down the list in order is what stops you replacing an expensive part to fix a cheap problem.
Loose or faulty gas cap
The standard first check for any EVAP code, and free to rule out.
Leaking EVAP hoses or fittings
Age-hardened lines anywhere between tank, canister and intake.
Failed purge or vent valve
Either valve failing can trip the general fault when the ECU can't attribute it more precisely.
Damaged charcoal canister
Cracked, corroded, or saturated with liquid fuel from overfilling.
Faulty fuel tank pressure sensor
The sensor the whole test depends on. If it reads wrongly, the system fails its self-test even when perfectly sealed.
How to diagnose it
- Check for other, more specific EVAP codes stored alongside — they do the diagnosis for you.
- Inspect and tighten the gas cap, replacing it if the seal is hardened.
- Inspect visible EVAP lines and the canister for damage.
- Smoke test the system to find leaks.
- Check the fuel tank pressure sensor reading on live data against ambient with the cap off.
Parts you might need
Tools for the job
- Smoke machine
- OBD-II scanner with live data
Related trouble codes
- P0455 — Evaporative Emission System Leak Detected (Gross Leak)
- P0442 — Evaporative Emission System Leak Detected (Small Leak)
- P0446 — Evaporative Emission System Vent Control Circuit Malfunction
- P0441 — Evaporative Emission System Incorrect Purge Flow
Fitment matters. Causes and parts above are the common cases for P0440 across vehicles. The specific part numbers, procedures and torque specs vary by year, make, model and engine — always confirm against service data for your vehicle before buying parts or starting work.