P0455

00:00What this code means

Code P0455. Evaporative Emission System Leak Detected (Large Leak).

In plain English: the fuel vapour system failed a sealing test by a wide margin, and about half the time the cause is the fuel cap.

Nothing about this will strand you and most people notice no difference in how the car drives at all. What it will do is fail an emissions test and vent fuel vapour to the atmosphere. Tighten the cap, and if the light clears after a few drive cycles you are done.

00:32How the test actually works

Here's what your engine computer is actually measuring.

Petrol evaporates in the tank, and letting that vapour escape is both a fire risk and an emissions problem. So the tank vents into a charcoal canister that traps the vapour, and once the engine is warm and running the computer opens a purge valve to draw it into the intake and burn it. The whole path from the fuel cap to the purge valve is supposed to be sealed.

To test it, the computer closes the vent valve to atmosphere and either pulls the system into vacuum with the purge valve or pressurises it with a small pump, then watches whether that pressure holds. A sealed system holds. If pressure returns to atmospheric almost immediately, the leak is large, and you get P0455.

So the code is not a part failure report. It's a performance measurement that came back under spec.

01:27Symptoms

What you'll usually notice, beyond the light itself.

Check engine light, and usually nothing else at all. This is the distinctive part. Most people notice no change in how the car runs, which is why it often gets ignored for months.

Failed emissions or smog test. Usually the thing that forces the repair, since there is no driveability complaint to prompt it.

A faint smell of petrol, mostly after refuelling. Vapour escaping where it should be trapped. Strongest near the filler or the rear of the vehicle.

The light appears shortly after a fill-up. The classic pattern for a cap left loose or cross-threaded. If the timing matches your last refuel, check the cap before anything else.

Very slightly worse fuel economy. Vapour that escapes is fuel you bought. The effect is small and easy to miss.

02:27Most likely causes, ranked

Now the part you came for: what actually causes this, ranked by how often it turns out to be the culprit.

45 percent of the time, it's this: Fuel cap loose, cross-threaded or with a perished seal. Nearly half of all cases, and the only one you can fix in ten seconds in a car park. Check it before anything else.

20 percent of the time, it's this: Cracked or disconnected EVAP hose. Rubber lines near the canister go hard and split with age. Look where hoses bend or pass near heat.

13 percent of the time, it's this: Purge valve stuck open. A valve that will not close means the system can never seal, so the test fails every time.

10 percent of the time, it's this: Vent valve stuck open or its filter blocked. Often mounted near the canister under the vehicle, where road salt and grit reach it.

7 percent of the time, it's this: Cracked charcoal canister. Usually from impact or corrosion. More common on older vehicles and in salted climates.

5 percent of the time, it's this: Damaged or rusted filler neck. The cap can be perfect and still not seal if the neck it screws onto is corroded or distorted.

Notice that the most expensive repair is not the most likely one. That matters, because the cheap checks come first.

03:56The measurement that decides it

This is the single measurement that tells you whether you need the expensive part or the cheap one.

If your scan tool can display EVAP system pressure, watch it through a monitor run. The shape of this curve tells you the size of the leak, and the size of the leak tells you where to look.

A sealed system pulls down to around ten inches of water and stays there. The line goes down and then flattens. That is what passing looks like, and if you see it after a repair, the repair worked.

A large leak looks like this. The system barely pulls down at all before returning straight to atmospheric, because air is coming in as fast as the pump can remove it. That is P0455, and it means a big opening — a cap, a disconnected hose, or a valve stuck wide open.

A small leak is the middle line. It pulls down properly and then bleeds back up slowly. That sets P0442 instead, and it usually means a pinhole or a marginal seal rather than something obviously open.

05:01Diagnostic order

So here is the order to actually test this, cheapest and most likely first.

Step 1. Is the fuel cap the correct part, tight, and is its seal soft and intact? Take it off and look at the rubber seal. If it is hard, cracked or flattened it will not seal. Refit it until it clicks several times, clear the code, and drive it for a few days. This alone resolves close to half of these.

Step 2. Are all EVAP hoses intact from the tank to the canister to the engine? Follow the lines with a torch. Look for splits at bends, hoses knocked off during other work, and rodent damage, which is far more common than people expect. Anything obviously off or split is your leak.

Step 3. Does the purge valve hold when it is closed? With the engine off the purge valve should be shut and should not pass air. Apply light vacuum or blow gently through it. If it passes air when closed, the system can never seal and the test will fail every single time.

Step 4. Does the vent valve close when commanded? The vent valve is normally open and must close for the test to run at all. It usually lives underneath near the canister, where it collects road salt and grit. A scan tool with bidirectional control can cycle it while you listen.

Step 5. Does a smoke test reveal where the leak is? Last, because it needs equipment. Low pressure smoke is pushed into the system and you look for where it escapes. On a large leak it is usually obvious within a minute, and it is the fastest way to find a cracked canister or a rusted filler neck.

Check the cap, then the hoses, then the valves. Nobody should pay for a smoke test before spending ten seconds on the cap.

07:00Repair costs

What this costs to put right, in US dollars, parts and labor.

Fuel cap: 10 to 35 dollars in parts, plus about 0.1 hours of labor. The most likely repair on the list and the cheapest thing on this entire website.

EVAP hose or line: 10 to 45 dollars in parts, plus about 0.7 hours of labor. Cheap unless the split section is buried above the tank.

Purge valve: 25 to 130 dollars in parts, plus about 0.7 hours of labor. Usually under the bonnet and straightforward to reach.

Vent valve: 30 to 150 dollars in parts, plus about 1.0 hours of labor. Often mounted near the tank, so labour depends on access and rust.

Charcoal canister: 120 to 480 dollars in parts, plus about 1.5 hours of labor. The expensive end, and genuinely uncommon as the cause.

The realistic spread here is a ten dollar cap or a five hundred dollar canister, and the odds are heavily on the cap. This is the one code where the cheapest possible fix is also the most likely one.

08:15Recap and related codes

Quick recap. P0455 means the fuel vapour system failed its sealing test by a wide margin. Check the fuel cap first, because it is roughly half of all cases and costs nothing to rule out. Then hoses, then the purge and vent valves, and only then pay for a smoke test.

If your scan tool showed a related code alongside this one, check those too: P0442, P0456, P0446, P0440.

Codes are on screen and linked below.