P0455 straight after a fill-up: the cap-first rule

P0455 is the EVAP system reporting that it cannot hold pressure at all — a large leak. The system's biggest opening is the one you touch every week: the filler cap.

If the code arrived within a day or two of refuelling, check the cap's seal for cracks, refit it until it clicks, and drive normally. The monitor needs a few drive cycles before it retests, so the light staying on for a day or two is the system rechecking, not the fix failing.

If it comes back with the cap ruled out, the remaining suspects hold still: the filler neck lip the cap seals against (rust, on older cars), the purge valve stuck open (some tick audibly, and a stuck purge can also cause hard starts right after refuelling), the vent valve, and the small-bore EVAP hoses that crack where they bend. A smoke test finds any of these in minutes; guessing replaces them one at a time at parts prices.

Worth saying plainly: clearing the code the week before an emissions inspection does not help. The readiness monitors reset along with it, and an inspector reads "not ready" exactly the way it deserves.