P0440
- Powertrain
- Generic (SAE)
- Emissions
Diagnose soon
Emissions only. The engine will run normally and nothing is being damaged, but the vehicle will fail an inspection. If the fault turns out to be a leak rather than a blockage or a sensor, vapor that should be captured is venting to atmosphere. The exception is a fuel smell strong enough to notice, which deserves same-day attention because it can mean liquid fuel, not vapor.
- Safe to drive
- Safe to drive normally. Get it inspected promptly if you smell raw fuel or see wet fuel anywhere.
Evaporative Emission Control System Malfunction
The computer tested the fuel vapor system for leaks and the test failed. It seals the tank and captures fuel fumes so they burn in the engine instead of escaping. This code says the seal did not hold, but it does not say where.
P0440 is the generic EVAP system fault. Depending on the design, the PCM either closes the vent valve and pulls engine vacuum on the tank through the purge valve, then watches the fuel tank pressure sensor for how fast the vacuum bleeds off, or it pressurizes the system with a leak detection pump and watches for pressure decay. A result outside the calibrated window sets the code without identifying which component or joint failed.
How the car detected itThe PCM runs the EVAP monitor only inside a narrow enabling window: fuel level within a manufacturer-defined mid-range window (commonly somewhere between about 15 and 85 percent, but check the service information for the vehicle), ambient temperature within a band, engine off long enough for a cold soak, then steady cruise. It commands the vent valve shut to seal the system, uses purge to draw vacuum, then closes purge and watches the tank pressure signal. Vacuum that bleeds away faster than the calibration allows, or that never builds at all, sets P0440. Most manufacturers set a more specific code when they can characterize the failure: P0442, P0455 and P0456 grade the leak by size, and vent or purge circuit codes point at a particular component. A bare P0440 usually means the module saw an out-of-range result it could not classify. Note that even a leak-size code tells you how big the leak is, not where it is.
A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.
- Safe to drive
- Safe to drive normally. Get it inspected promptly if you smell raw fuel or see wet fuel anywhere.
- Urgency
- Diagnose soon
- Will it pass emissions
- No
- If ignored
- The vehicle keeps running as it is and fails emissions testing. If the fault is a leak, vapor that should be captured vents to atmosphere. A canister saturated by repeatedly topping off the tank at the pump can pass liquid fuel forward into the purge valve and intake, which turns a cheap repair into a stalling complaint. The light also masks any new code that appears later.
- Check engine light
- Solid
- Clearing the light
- A completed drive cycle is normally required before the monitor re-runs and the light can clear.
- Possible smell of fuel near tank or engine bay
- Failed emissions inspection
- Loose-cap warning may appear
Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.
These are the causes this code can have. For the order to work through them in, see what to check first.
| # | Cause | How often | Component |
|---|---|---|---|
| 1 | Fuel filler cap loose or worn | Very common | — |
| 2 | Purge valve stuck open or closed | Common | — |
| 3 | Cracked EVAP hose | Common | — |
Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.
In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.
The fuel filler cap
Free to check and a genuinely common cause. Confirm it is the correct cap for the vehicle, that it clicks and seats, and that the rubber seal is not hardened, cracked, or missing. Check the filler neck sealing surface for rust and debris too.
Freeze frame data
Fuel level, ambient temperature, and engine coolant temperature at the moment of failure tell you which monitor branch ran and whether the conditions were marginal.
Any companion EVAP codes
A purge or vent circuit code stored alongside P0440 usually points straight at the failure and saves you a smoke test.
Visible EVAP hose and canister damage
The canister and its lines sit under the vehicle on most designs and get hit by road debris, rust, and rodents. A cracked line or a smashed canister is often visible from underneath in a few minutes.
Recent fueling habits
Topping off after the pump clicks pushes liquid fuel into the charcoal canister. A saturated canister causes repeat EVAP faults and kills replacement valves.
Pull all codes and freeze frame before clearing anything
Tools OBD-II scan tool with freeze frame access
Record every stored and pending code across the powertrain module, not just P0440. Note fuel level, ambient temperature, and engine run time from freeze frame. Clearing first destroys the only evidence of what the monitor saw.
ExpectedP0440 alone, or P0440 with a purge or vent circuit code that redirects the diagnosis
Inspect the cap, filler neck, and accessible EVAP lines
Tools Flashlight, basic hand tools
Remove the cap and check the seal for cracks, flat spots, and hardening. Look at the filler neck sealing lip for corrosion. Follow the vapor lines you can reach, checking for chafed rubber, brittle plastic, disconnected quick-connects, and rodent damage near the canister.
ExpectedA cap that seals cleanly and lines with no visible cracks, cuts, or disconnections
Command the purge and vent valves and watch tank pressure respond
Tools Bidirectional scan tool
With the engine idling, command the vent valve closed to seal the system, then command purge open and confirm the fuel tank pressure sensor moves toward vacuum. Command purge closed and confirm the reading stabilizes. Then command the vent open and confirm the tank returns to atmospheric. Testing purge with the vent still open proves nothing, because the tank simply draws fresh air through the canister. A valve that does not respond electrically or does not move mechanically is your fault.
ExpectedBoth valves respond audibly, tank pressure moves toward vacuum only with the vent commanded closed and purge open, and returns to atmospheric when the vent is opened
Verify the fuel tank pressure sensor is believable
Tools Scan tool live data, service information
Key on, engine off, with the cap off, the sensor should read essentially atmospheric. The exact scaling and units differ by manufacturer, so compare against the vehicle's own specification rather than a generic number. A sensor stuck at a fixed value will fail the monitor no matter how well the system seals.
ExpectedAtmospheric reading with the system open, and a smooth change when vacuum is applied
Smoke test the sealed system
Tools EVAP smoke machine, scan tool with bidirectional control
Do this after the valve and sensor checks, because those cost minutes with the tool already connected and a dead valve or a stuck sensor explains the code without any leak being present. Introduce low-pressure smoke at the EVAP service port with the vent valve commanded closed by the scan tool. Without closing the vent, smoke simply escapes through the canister and you learn nothing. Watch the fuel filler neck, canister, purge and vent valves, tank seams, and every hose joint.
ExpectedNo smoke escaping anywhere once the system is sealed
Inspect the canister and its vent path
Tools Flashlight, service information
Check the charcoal canister for impact damage, cracks, and liquid fuel weight. Check the vent filter or vent inlet for mud, spider webs, and insect nests, which are a routine cause of a blocked vent and a failed monitor. Location varies by vehicle, so consult the service information.
ExpectedAn intact, dry canister and a clear vent path
Repair, then run the drive cycle and confirm the monitor completes
Tools Scan tool with readiness monitor status
After the repair, clear the code and complete the manufacturer's EVAP drive cycle, which usually needs an overnight cold soak, a partial fuel level, and steady cruising. Confirm the EVAP readiness monitor changes to complete. A cleared light is not a verified repair.
ExpectedEVAP monitor reads complete with no return of P0440
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Replace the fuel filler cap with the correct part for the vehicle | A visibly damaged, hardened, or wrong-fitment seal, or smoke escaping at the filler neck during a smoke test | $20–$70 | DIY |
| Replace or repair a cracked, disconnected, or rodent-damaged EVAP hose or line | Smoke escaping at that specific joint or section during a sealed smoke test | $90–$400 | DIY |
| Replace the EVAP purge valve | The valve fails to hold vacuum when de-energized, does not respond to a scan tool command, or leaks smoke | $110–$320 | DIY |
| Replace the EVAP vent valve | The valve does not seal when commanded closed during a smoke test, or does not click when commanded | $120–$380 | DIY |
| Replace the charcoal canister | Visible cracks or impact damage, or a canister heavy with liquid fuel | $250–$750 | DIY |
| Replace the fuel filler neck or tank sealing component | Smoke escaping from a corroded filler neck or a failed tank seal after other sources are excluded | $250–$900 | Hard |
| Replace the fuel tank pressure sensor | A stuck or implausible pressure reading that does not respond when vacuum is applied, with the system proven sealed | $130–$400 | DIY |
US independent shop ranges, parts and labor. EVAP work varies more than most repairs because the canister, vent valve, and tank sensor are often mounted above or behind the fuel tank, and a tank drop can double the labor on some vehicles. Smoke testing typically adds one diagnostic hour.
- Replacing the gas cap, clearing the code, and calling it fixed. The cap is worth checking first but is far from the only cause, and the repair is unverified until the monitor completes.
- Smoke testing without commanding the vent valve closed, so the smoke escapes through the canister and the real leak is never found.
- Smoke testing before checking the valves and the tank pressure sensor, then spending a diagnostic hour proving a system that has no leak in it.
- Assuming a fuel smell means an EVAP leak. Raw fuel odor can be a liquid fuel leak, which is a fire risk and a different investigation.
- Replacing the charcoal canister for what is actually a purge or vent valve fault.
- Clearing the code and driving away without completing the drive cycle, then failing an inspection because the monitor is not ready.
- Repeatedly topping off the tank after the pump clicks, which saturates the canister and destroys replacement parts.
- EVAP Canister
- EVAP Purge Valve
- Fuel Filler Cap
- EVAP Vent Valve
- Definition
- Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
- Diagnostic guidance
- Written and reviewed in-house. See our editorial policy.
- Review status
- Reviewed
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