P0440

P0440 is the generic catch-all EVAP code — the ECU has decided something in the evaporative emissions system isn't sealing, purging, or venting the way it should, but it hasn't narrowed down where. The EVAP system is a closed loop that captures fuel vapors from the tank, stores them in a charcoal canister, and feeds them into the intake under controlled conditions; any leak, blockage, or stuck valve in that loop will throw P0440 or one of its more specific cousins (P0441-P0455). The canonical first move on a shop floor is the cheapest one: check the fuel cap. A worn O-ring, a cracked cap, or an owner who didn't click it three times is the cause of P0440 in roughly 30-40% of cases, and the part is $5-30. If the cap is good, the next step is a smoke test — pump low-pressure smoke into the EVAP service port and watch for it to escape; this is the diagnostic that catches 80% of remaining causes in one sweep, and it's why every shop doing EVAP work owns a smoke machine. Common leak sources behind the cap: degraded charcoal canister (the plastic cracks with age and heat), a purge solenoid stuck partway open, or debris in the vent solenoid holding it from sealing.

Vehicle-specific patterns worth knowing: 2005-2015 Toyota and Lexus models throw P0440 most often from a tired fuel cap O-ring or a sticky canister vent solenoid, with the vent solenoid being a known wear item that Toyota sells as a discrete part for about $80-150. 2003-2010 Ford 4.6L applications (Crown Vic, F-150, Expedition) commonly fail at the purge valve — the diaphragm cracks and leaks vacuum into the intake, which the ECU sees as an incorrect purge flow that eventually trips P0440. 2007-2014 GM full-size trucks (Silverado, Sierra, Tahoe) have a notorious canister vent solenoid problem because the vent line ends near the rear wheel well and gets packed with road dust and mud; a $40 vent solenoid and a clean vent line clears years of intermittent P0440 returns. 2008-2015 VW and Audi vehicles use a Leak Detection Pump (LDP) that fails internally and triggers P0440 even when nothing else is leaking. Repair range: $5 fuel cap to $400 for purge/vent solenoid replacement to $600+ if the canister itself is cracked.