# P0455 code: Evaporative emission system large leak detected

Source: https://codeandclear.com/blog/p0455-code
Published: 2026-10-09
Publisher: Code & Clear

**Short answer:** P0455 means the engine computer ran a self-test on the fuel vapor (EVAP) system and could not hold pressure or vacuum, so it decided there is a large leak. The most common cause is a loose, missing or worn gas cap. Other causes include a cracked EVAP hose, a stuck vent valve or purge valve, or a damaged charcoal canister. It is not an emergency, but your car will not pass an emissions inspection with it.

This page is for drivers who pulled P0455 with a scan tool or got it read at a parts store and want to know what it means and what to do. P0455 is one of the most common check engine codes, and it is often one of the cheapest to fix. Below you will find a plain-English explanation of the code, the symptoms to expect, a table of causes from most likely to least likely, a step-by-step diagnosis, and answers to the questions people ask most. If you are new to reading codes, start with our guide on [how to read OBD2 codes](https://codeandclear.com/blog/how-to-read-obd2-codes).

## What P0455 means

P0455 is a generic SAE code that reads **Evaporative Emission System Leak Detected (large leak)**. On some vehicles the description is "gross leak" or "no flow". The P means powertrain, the 0 means it is a generic code shared by all makes, and the 4 places it in the auxiliary emission controls group. You can break down any code with our [code decoder](https://codeandclear.com/tools/code-decoder).

The EVAP system traps gasoline vapor from the fuel tank and stores it in a charcoal canister. When the engine is running under the right conditions, the purge valve opens and the engine burns those vapors. To make sure vapor is not escaping into the air, the engine computer seals the system and checks whether it holds vacuum or pressure. If it cannot build or hold any meaningful vacuum or pressure, it sets P0455. A small leak sets [P0456](https://codeandclear.com/blog/p0456-code) or [P0442](https://codeandclear.com/blog/p0442-code) instead.

## Symptoms of P0455

In most cases the only symptom is the check engine light. The car usually drives normally. Some drivers notice other signs:

- Check engine light on, often a day or two after refueling
- A faint fuel smell near the car, especially near the filler door or rear of the car
- Some vehicles show a "check fuel cap" or "loose fuel cap" message on the dash
- Failed emissions inspection, since the EVAP readiness monitor will not complete or the code is stored

## Common causes of P0455

Start at the top of this table. The causes are listed from cheapest and most likely to least likely. The gas cap is the first thing to check every time, especially if the light came on after a fill-up. See [check engine light after getting gas](https://codeandclear.com/blog/check-engine-light-after-getting-gas) for more on that pattern.

| Cause | How to check | Typical fix |
| --- | --- | --- |
| Loose, missing or wrong gas cap | Make sure the cap clicks and the seal is not cracked or flattened | Tighten or replace with an OEM-style cap |
| Damaged fuel filler neck | Look for rust or cracks at the top of the filler neck where the cap seals | Replace the filler neck |
| Disconnected or cracked EVAP hose | Visual check of hoses near the canister, purge valve and tank; smoke test | Reconnect or replace the hose |
| Canister vent valve stuck open | Command the vent closed with a scan tool and check if it seals; smoke test | Replace the vent valve or solenoid |
| Purge valve stuck closed or not working | Command the purge valve with a bidirectional scan tool and listen for clicking | Replace the purge valve |
| Cracked charcoal canister | Visual inspection and smoke test | Replace the canister |
| Faulty fuel tank pressure sensor | Compare live data reading to actual conditions with the cap off | Replace the sensor |

P0455 causes, checks and typical fixes

Note: Gasoline vapor is flammable. Never use an open flame or a cigarette smoke source to look for EVAP leaks. Use a proper EVAP smoke machine or let a shop do it.

## How to diagnose P0455

A basic code reader tells you the code. A scan tool that shows live data and can run active tests on the EVAP valves makes the job much faster. Our guide to the [best bidirectional scan tools](https://codeandclear.com/best/best-bidirectional-scan-tool) covers tools that can command the purge and vent valves.

1. Read all stored and pending codes. Note any other EVAP codes like P0446 (vent control) or P0449 (vent solenoid circuit), since those point to a specific part.
2. Look at the [freeze frame data](https://codeandclear.com/blog/freeze-frame-data-explained). It shows the fuel level, temperature and engine conditions when the test failed. Many EVAP tests only run with the fuel level between about a quarter and three quarters full.
3. Remove the gas cap, inspect the rubber seal and the filler neck lip, then reinstall it until it clicks. Replace the cap if the seal looks worn.
4. Check the hoses and connectors around the charcoal canister and purge valve. Many are near the fuel tank or in the engine bay.
5. If your scan tool supports it, command the vent valve closed and the purge valve open and watch the fuel tank pressure reading. It should move toward vacuum. If it does not move at all, look for an open leak or a valve that is not working.
6. If nothing is obvious, a smoke test is the standard way to find EVAP leaks. Smoke is pushed into the system and you look for where it escapes.
7. After the repair, clear the code and complete a [drive cycle](https://codeandclear.com/blog/drive-cycle-after-clearing-codes) so the EVAP monitor can run again.

## How to fix P0455

Fix what the diagnosis points to. If it was the cap, a quality replacement cap is usually all you need. Avoid cheap caps that do not seal well, as they can bring the code back. If a hose is cracked or disconnected, replace it with the correct EVAP-rated hose. Valve and canister replacement procedures vary by vehicle, so check the service information for your make and model.

Clearing the code without a repair only turns the light off for a while. The EVAP monitor will run again and set the code. The monitor also needs specific conditions to run, so it can take several days of normal driving before it shows complete. Our page on [readiness monitors](https://codeandclear.com/blog/readiness-monitors-explained) explains why.

## Can you drive with P0455?

Yes. P0455 does not affect how the engine runs on most vehicles, and it will not damage the engine or catalytic converter. Fuel vapor does escape into the air, and you may lose a small amount of fuel to evaporation. Fix it soon because it will fail an emissions test, and a lit check engine light can hide a new, more serious problem. See [can you drive with the check engine light on](https://codeandclear.com/blog/can-you-drive-with-check-engine-light-on) for the general rules.

[See OBD2 scanners for DIY mechanics](https://codeandclear.com/best/best-obd2-scanner-for-diy-mechanics)

## Related codes

These EVAP codes often appear with P0455 or describe a similar problem: [P0456](https://codeandclear.com/blog/p0456-code) (very small leak), [P0442](https://codeandclear.com/blog/p0442-code) (small leak), [P0446](https://codeandclear.com/blog/p0446-code) (vent control circuit), [P0449](https://codeandclear.com/blog/p0449-code) (vent solenoid circuit) and [P0496](https://codeandclear.com/blog/p0496-code) (purge flow when it should be closed).

## Frequently asked questions

**Can a loose gas cap really cause P0455?** Yes. A cap that is not tightened, is missing, or has a damaged seal is the most common cause. Tighten it until it clicks, clear the code, and see if it comes back after a few days of driving.

**Will P0455 clear itself?** If the leak is fixed, the light usually goes off on its own after the EVAP test passes on several drive cycles. Clearing it with a scanner is faster, but the monitor still has to run before an inspection.

**What is the difference between P0455 and P0456?** Both are EVAP leaks. P0455 means a large leak or no ability to build vacuum. P0456 means a very small leak. The large leak is often easier to find because it is usually something obvious, like the cap or a loose hose.

**Do I need a smoke machine?** Not always. Check the cap and hoses first. If you cannot find the leak, a shop with an EVAP smoke machine can usually find it quickly.

## Gear guides for this topic

- https://codeandclear.com/best/best-obd2-scanner-for-diy-mechanics
- https://codeandclear.com/best/best-bidirectional-scan-tool
- https://codeandclear.com/best/best-wireless-bluetooth-scan-tool