⚠ P0440 LOW SEVERITY

P0440: what it means

Officially? A wall of jargon. To most people? Absolutely nothing. That's exactly why we built the app.

The official definition vs. an actual explanation

Plug into any code reader and you'll get the same line every other app shows you. Technically correct. Practically useless.

What other apps tell you

"P0440: A fault in the fuel vapour system, the one that stops petrol fumes escaping the tank."

What OBD Mechanic tells you

"Something's not right in your EVAP system. That's the sealed plumbing that catches petrol vapour from the tank and feeds it to the engine to burn, instead of letting it into the air. P0440 is the broad version: the car knows the system isn't behaving but hasn't pinned it to a leak size or one part. Start with the free check, the fuel cap. After that it's usually a hose, the purge or vent valve, or the charcoal canister. Petrol cars only."

What actually causes it, cheap to dear

Worth checking in roughly this order. Don't jump straight to the expensive fix:

Fuel cap loose, missing or with a worn seal
Cracked or perished EVAP hoses
Purge valve stuck open or shut, or its wiring
Vent valve stuck, or its filter blocked
Charcoal canister cracked or full of fuel (regularly topping up after the pump clicks off can flood it with liquid fuel)
On some makes, a leak-check pump in the system has failed

usually cheap depends pricey

Suggested checks

Check the fuel cap seal for cracks and refit it until it clicks
Look for other EVAP codes alongside it. A P0442, P0455 or a valve code narrows it right down
Eyeball the EVAP hoses you can reach: near the canister (usually by the tank) and the purge valve (engine bay)
If nothing obvious turns up, a garage smoke test finds leaks quickly and beats swapping parts on spec

Which dials to watch for P0440

The code itself is a snapshot. The fault sets after the car's watched the numbers over a whole drive cycle. Here's what's worth keeping an eye on:

Evap purge

Shows how much purge the computer is asking for. That's the request, not proof the valve opened. If the short fuel trim shifts as purge rises, vapour is flowing, which points away from a valve stuck shut.

Not simulated live Our free demo drive doesn't model this reading yet. Watch it live on your own car in the app.

Short fuel trim

A purge valve stuck open lets vapour in even when Evap purge reads zero, often at idle, and that can upset the mixture and the idle. Trims that move about with purge at zero hint at the valve rather than a leak.

Not simulated live Our free demo drive doesn't model this reading yet. Watch it live on your own car in the app.

Illustrative panels. These readings aren’t in our free demo drive, but the app shows them live from your own car.

It spots things you'd never see in the numbers

Codes rarely tell the whole story. The clever part is reading them together, and against your exact car. Here's the kind of thing it catches:

AI Mechanic

What our mechanic says

P0440

Something's not right in your EVAP system. That's the sealed plumbing that catches petrol vapour from the tank and feeds it to the engine to burn, instead of letting it into the air. P0440 is the broad version: the car knows the system isn't behaving but hasn't pinned it to a leak size or one part. Start with the free check, the fuel cap. After that it's usually a hose, the purge or vent valve, or the charcoal canister. Petrol cars only.

P0440: quick questions

Can I still drive it?

Yes, it's safe to drive. A vapour fault doesn't change how the car drives. Check the fuel cap is on tight. A strong fuel smell is never something to sit on, so if you smell fuel, get it checked straight away.

Is it just the fuel cap?

Often, but not always. Fit the cap properly or replace a tired one, clear the code, and drive normally for a few days. EVAP tests only run under certain conditions, so give it time before you call it fixed.

How much will it cost to fix?

A fuel cap is cheap. Hoses and valves are usually modest. The canister is the dear end but less often the culprit. A smoke test costs a bit, but it often saves you buying parts you didn't need.