P0455 means the evaporative emission (EVAP) monitor tried to seal the fuel vapor system and could not build or hold vacuum or pressure at all. It is the gross-leak member of the EVAP leak family: P0442 is a small leak (about 1 mm equivalent), P0456 a very small leak (about 0.5 mm), and P0455 a large one, which on most calibrations means the system behaved as if it were open to atmosphere. The vapor system runs from the fuel tank through the charcoal canister and the purge valve to the intake, with a vent valve on the canister that the module closes to seal the system for the test.
Severity: low. Can you keep driving? Yes. The leak is vapor. The engine runs normally, and the consequences are a failed emissions inspection, a fuel smell if the leak is near the tank, and on a few vehicles a "tighten fuel cap" message. Nothing is damaged by waiting, but the code will not clear until the leak is fixed and the monitor has run clean.
The short explanation of the small-leak versus large-leak family lives at EVAP codes P0442 and P0455 and the fuel cap; this page is the diagnosis.
How the monitor decides it is a large leak
The module closes the vent valve, opens the purge valve to pull intake vacuum into the sealed system (or, on vehicles with a leak detection pump, pumps it up), and watches the fuel tank pressure sensor. A small leak shows as vacuum that builds and then decays too fast. A large leak shows as vacuum that never builds: the sensor stays at atmospheric pressure no matter how long the purge valve is open. Some calibrations also set P0455 when the purge valve is commanded open and the tank pressure does not change at all, which is the signature of a purge valve stuck closed rather than a leak.
That definition is why the cap is only one suspect among several. Anything that leaves the system open to atmosphere during the test, or that blocks the purge path, fails it the same way.
What the manufacturers documented
Acura RDX 2013 to 2015 (Service Bulletin 14-053, December 2014): the TIGHTEN FUEL CAP message, or P0455 and P0456. Possible cause in Acura's words: during short drive cycles the PCM may misinterpret inputs from the evaporative system. The corrective action: check the EVAP system for leaks; if no leak is found, update the PCM. The bulletin lists the vacuum gauges for the leak check, and the order is unambiguous: test first, then software, and no parts unless the test finds a leak.
Cadillac XTS 2013 (preliminary information PI0898, February 2013): P0455 caused by overfilling (topping off) the tank or by filling with a damaged service station nozzle, which floods the vapor system with liquid fuel. GM's procedure: follow the full diagnostic to find the cause, and if the purge solenoid is sticking, replace the purge solenoid and the canister, then blow out all the EVAP lines. It warns not to pry the capless filler door with a screwdriver, because a damaged flapper seal is itself a large leak. Liquid fuel in the canister and lines is a cause few owners consider and the bulletin puts it first.
Mazda, multiple models (bulletin 01-002/18, January 2018): a twelve-page procedure for determining the leak location for P0441, P0442, P0455 and P0456, superseding several earlier versions. Its existence is the point: on Mazda's own vehicles the location of the leak was hard enough to find that the factory wrote a dedicated method for it.
General Motors, all gasoline vehicles 2000 to 2023 (bulletin 21-NA-124, December 2022): diagnostic tips for P0442 and P0455. For the large leak GM lists the causes as an unseated tube connection, a damaged tube, a canister vent valve stuck open, or a purge valve stuck closed, and reminds technicians that the vent valve can seal under shop pressure yet leak under the vacuum the vehicle's own test uses.
Nissan, 2012 and newer (bulletin NTB13-097g, January 2019): a filler cap left loose for several trips stores P0442, P0455 or P0456. Nissan's instruction is to confirm with the diagnostic flow chart and not by test driving, because the monitor's conditions are too narrow for a drive to prove anything.
Causes, in the order they are found
A purge valve stuck open or leaking, which lets manifold vacuum draw air through the canister whenever the engine runs and leaves the system unsealable. This is the most common single cause on vehicles more than a few years old. It also produces a rough idle after refueling, because vapor pours into the intake.
A canister vent valve stuck open, or its hose to the filter left off, so the system can never seal. GM's bulletin names it; on trucks the vent valve sits near the tank and takes road spray and mud.
A hose or tube off or split: after a fuel pump job, a tank drop, an exhaust repair, or simply age. The line from the tank to the canister and the purge line to the intake are the usual suspects.
A canister cracked by a fill nozzle jammed in the filler neck or by liquid fuel from repeated topping off, as GM found on the XTS.
A filler cap missing, cross-threaded, or with a torn seal, or a capless filler neck with a damaged flapper. Check first because it takes ten seconds, and expect it to be the answer less often than the internet says.
A fuel tank pressure sensor that reads atmospheric no matter what, on which the monitor cannot see vacuum build; usually with P0450 through P0453.
A purge valve stuck closed, on calibrations that set P0455 when purge produces no pressure change.
A calibration that misreads short trips, as on the Acura RDX.
A fuel tank seam or rollover valve, least common.
A diagnostic order
- Record the freeze frame and companion codes. P0441 or P0496 beside P0455 point at the purge valve; P0446 or P0449 point at the vent valve; P0450 through P0453 point at the sensor. See freeze frame data.
- Cap and filler neck, ten seconds: seated, seal intact, flapper undamaged on a capless neck.
- Ask about refueling. Topping off and a nozzle that clicked off repeatedly point at liquid fuel in the vapor system.
- Command the system sealed with a scan tool and watch tank pressure. Vent valve commanded closed, purge commanded open, engine running: vacuum should build within seconds. No change at all confirms a large leak or a blocked purge path and rules out an intermittent.
- Listen and feel at the purge valve with the engine idling and the valve commanded closed. Vacuum passing through a closed purge valve is a leaking valve.
- Check the vent valve commanded closed with a hand pump or the smoke machine, and test it under vacuum as GM advises, not only under pressure.
- Smoke the system from the service port or the purge line with the vent closed, and look at every connection from the tank to the canister to the intake, including under the canister where a crack hides.
- Blow out the lines and inspect the canister for liquid fuel if the history involves overfilling, and replace the canister with the purge valve where the valve is found sticking, per GM's procedure.
- Check for a software update if no leak is found and the vehicle is used for short trips.
- Fix, clear, and run the EVAP monitor with the scan tool rather than waiting; the readiness monitors need it complete before an inspection.
What P0455 does not tell you
It does not tell you the cap is loose. It tells you the system could not seal, and a purge valve stuck open, a vent valve stuck open, a split hose, a cracked canister and a missing cap all look identical to the tank pressure sensor. On the Acura RDX the system sealed fine and the software misread a short drive. The scan tool's seal test and a smoke machine separate them; the general principle is covered in what a trouble code does not tell you.
Frequently asked questions
Can I drive with a P0455 code?
Yes. The leak is fuel vapor and the engine runs normally. The costs are a failed emissions inspection, a light that stays on, and possibly a fuel smell near the tank. If the smell is strong or there is any sign of liquid fuel, have it checked promptly.
Is P0455 just the gas cap?
Sometimes, and it is the first thing to check because it takes seconds. On most vehicles that set the code, the cause is a purge valve stuck open, a vent valve stuck open, a hose off, or a canister damaged by overfilling. Tighten the cap, then test the system sealed with a scan tool before assuming the cap fixed it.
Why did P0455 set after I filled the tank?
Two reasons in the bulletins: topping off pushes liquid fuel into the vapor lines and canister, which cannot then seal or purge properly (GM's Cadillac XTS case), or the cap was not seated after the fill (Nissan's case). A rough idle right after refueling points at liquid fuel or a purge valve stuck open.
Why does my Acura show TIGHTEN FUEL CAP with P0455 and no leak?
Because on the 2013 to 2015 RDX the PCM could misinterpret evaporative system inputs during short drive cycles. Acura's bulletin 14-053 says to check the system for leaks, and if none is found, to update the PCM.
How much does it cost to fix P0455?
A cap or a hose is inexpensive. A purge or vent valve is a modest part with a short replacement. A canister is more, and where the purge valve is found sticking GM replaces both. A tank pressure sensor on top of the tank adds labor for access. Diagnostic time is usually the larger cost, which is why the scan tool seal test comes before the smoke machine.
Related codes
- P0442 / P0456: small and very small leaks; see P0442.
- P0441 / P0496: purge flow incorrect or high, the purge valve's own codes.
- P0446 / P0449: vent control circuit, the vent valve's own codes.
- P0450 to P0453: fuel tank pressure sensor circuit; see P0451.
- P0457: EVAP leak detected, fuel cap loose or off, on manufacturers that separate the cap case.
The code number itself tells you the family and the domain; how to read an OBD2 code number explains what each character means.