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

What happens if you don't fix it

Symptoms you can actually notice

• Check engine light
• Gas smell near the filler neck
• Failed emissions test

What the computer is actually complaining about

The computer seals the tank and pulls a vacuum, then times how long the vacuum holds. A large leak dumps that vacuum almost at once, or the system cannot pull any vacuum at all. The test aborts and retests. When it fails twice, P0455 sets instead of the smaller-leak codes.

Most likely causes, cheapest check first

What you need in your hands

• OBD-II scanner

  A $25 Bluetooth reader with a free app shows the EVAP monitor status and lets you clear the code between tests.

• Fuel cap Optional

  A sealed cap for your year and model costs $10 to $30. Buy the cap before you buy anything else.

• Smoke machine access Optional

  A shop smoke test runs about $40 to $80. It finds a large leak in under five minutes on most cars.

Test it yourself, step by step

Do these in order. Each step tells you what to do, what to look at, what counts as fine, what counts as broken, and what you will actually see.

1. 1 Open the fuel door on the rear quarter panel and take the cap off. Hold it up to the light and look at the rubber seal ring on the underside, then look into the filler neck with a flashlight.

   Look at

   Whether the cap is present at all, whether the seal is torn, and whether the cap threads are crossed or chewed.

   Fine if

   The cap is there, the seal is soft and round, and the cap screws in and clicks without wobbling.

   Broken if

   The cap is missing, the seal is split, or the cap spins without clicking. That alone sets a large-leak code.

   What you'll see A black rubber ring on the cap and a clean metal neck. A neck full of rust flakes or a seal with a chunk gone means you found it.

2. 2 Plug the scanner into the port under the dash on the driver side, above your left knee. Key to ON and read the stored codes and the freeze frame.

   Look at

   Whether P0455 is stored alone or with P0442 or P0456, and what the fuel level was when it set.

   Fine if

   P0455 is stored alone. That points at one big opening rather than several small ones.

   Broken if

   P0455 comes with P0442 or P0456. You have more than one leak, and a smoke test is the fastest way to find both.

   What you'll see One stored code on the screen, plus a freeze frame showing engine load and fuel level at the moment of failure.

3. 3 Open the hood and trace the hose that leaves the purge valve, a black cylinder the size of a film can bolted next to the intake on the engine bay side. Follow it to the firewall.

   Look at

   Whether the hose is pushed fully onto its fitting, and whether any quick-connect clip is missing or popped up.

   Fine if

   Every hose seats flush against its fitting and no clip sits proud of its slot.

   Broken if

   A hose hanging half off a fitting, or a plastic clip that will not snap down. A disconnected line is a large leak by definition.

   What you'll see A hard plastic line with a colored clip at each joint. A clip riding high on the fitting means the line is not locked.

4. 4 Look under the rear of the car, above the axle on the passenger side or driver side, for a black plastic box about the size of a shoebox bolted to the floor.

   Look at

   The canister seam, the two hose ports on top, and the vent valve for a crack, a missing cap, or a hose rubbed through on the fuel tank strap.

   Fine if

   The box is intact, both ports are seated, and no hose touches metal hard enough to wear.

   Broken if

   A visible crack across the body, a port with no hose on it, or a hose cut through where it rubs. That is the large opening.

   What you'll see A dusty black box with two lines running into the top. A clean shiny line on a hose means it has been rubbing.

5. 5 Stand at the rear quarter panel on the driver side or passenger side, fit a new cap for your exact year, make and model, screw it down until it clicks three times, then clear the code with the scanner plugged in under the dash.

   Look at

   Whether the code stays away through a full tank and several cold starts.

   Fine if

   P0455 stays off for 100 miles and one refueling stop. The cap was the whole problem.

   Broken if

   The code returns within 50 miles. Put the old cap back in the box and book a smoke test.

   What you'll see Three distinct clicks as the cap seats. A code back on the dash within two days means the cap was not the leak.

6. 6 Have a shop connect a smoke machine at the EVAP test port near the rear wheel well and watch the engine bay and the tank area with the system pressurized.

   Look at

   Where the smoke leaves the system and how big the opening looks.

   Fine if

   Smoke stays inside the system and only exits at the machine connection point.

   Broken if

   Smoke pours from a joint, a canister seam, or a hose end. A large plume marks a large leak, and the part it belongs to is the fix.

   What you'll see A thick white stream, not a thin wisp. Large leaks show up in seconds and the spot is obvious once the smoke moves.

What the repair costs

Reference range compiled from 2 source type(s), as of 2026-11. This is not a quote — actual cost depends on your vehicle, region, and which part turns out to be the real cause.

Basis: editorial reference range, not a live parts quote · Data as of 2026-11

How to confirm it's really fixed

✓ Clear the code, fill the tank, and run a full tank of driving with at least one cold start and one long cruise. The EVAP monitor has to complete before the code can return. If it stays off after that, the leak is closed. If it returns, the opening is still there and the smoke test tells you where.

Common questions