What An EVAP Leak Is And What To Do First

An EVAP leak means gasoline vapor can escape from your vehicle somewhere it should not. Your vehicle stores that vapor in a charcoal canister and burns it later in the engine, and the computer checks the whole system for leaks. When it finds one, the check engine light comes on.

By Enrique Reyes, who works at a family run auto repair and smog check shop in Delano, California. The shop has been open since 1972. Author Source

Call (866) 389-3808

0.020 Inch

Very Small Leak Test Hole

0.040 Inch

Small Leak Test Hole

1996

First Model Year With OBD II

Passing Drive Cycles To Turn The Light Off

The Basics

How The EVAP System Works

Three parts do most of the work in the evaporative emissions system. The charcoal canister stores the gasoline vapor, and the purge valve opens while the engine is running, so the engine can pull the stored vapor in and burn it. The vent valve lets fresh air into the canister while that happens.

All 1996 and newer gasoline cars and trucks must have OBD II. California's regulation says the OBD II system must watch the whole EVAP system for vapor leaks to the air, and you can read the exact wording in the California OBD II regulation.

The canister has another job during refueling, because it catches the gasoline vapor that gets pushed out of the tank. The EPA says this equipment has been required on nearly all new highway vehicles since 2006.

Instrument cluster of a 2013 Mazda 3 with red and amber warning lights lit under the speedometer — The dash of a 2013 Mazda 3 that came in for a diagnosis

The Codes

The Three Leak Codes

The computer sorts leaks by size, and the regulations use test holes to define those sizes.

P0455, Large Leak

The system could not hold its test vacuum at all. On Ford's test, this code sets when the system cannot pull 8 inches of water vacuum.

P0442, Small Leak

A leak about the size of a 0.040 inch hole on Ford's engine running test.

P0456, Very Small Leak

A leak about the size of a 0.020 inch hole. The federal regulation names this code for that test.

The names change from one manufacturer to another. Volkswagen calls P0455 a gross leak, General Motors and Mazda call it a large leak, and a 2022 Acura bulletin calls it a very large leak. Go by the code number instead of the name.

What Sets The Large Leak Code

Ford's list of P0455 causes is long, and it includes a fuel cap that was not installed correctly, a large hole, an overfilled tank, vapor lines that are disconnected or kinked, a vent valve stuck open and a purge valve stuck closed. For the causes listed by each manufacturer, open the P0455 large leak page on evapleak.com.

First Steps

What To Do First

Smell or see raw gasoline? Have the vehicle inspected right away, because leaking fuel near a spark can start a fire.
Check the gas cap. Honda's bulletin says to tighten it at least three clicks, and a cap that was not installed correctly is on Ford's list of causes for the large leak code.
Stop when the pump clicks off. General Motors says more than three extra clicks is too much, because it can push liquid fuel into the EVAP system.
Give the test time to run. The warning light goes off after three drive cycles in a row where the test runs and passes, as long as no other fault is present.
Look up open recalls. Several EVAP faults are safety recalls and not only emissions problems.
Light still on? Have the code read and the leak located, which the Smoke Test page in the menu explains.

Handheld scan tool plugged in and resting on the gauge cluster of a 2005 Mercedes E55 — A scan tool plugged into a 2005 Mercedes E55, the usual way a stored code gets read

Safety

Can You Keep Driving

Several manufacturer bulletins about EVAP leak codes say drivability is not affected, and the California Air Resources Board says most manufacturers advise service as soon as it is convenient. A blinking check engine light is different, and it needs service quickly.

None of the sources behind this guide give a set number of miles or days. Two Ford recalls show why the repair should not be delayed for too long. A stuck purge valve could stall the engine on some 2012 to 2018 Focus vehicles, and a leaking purge valve could crack the fuel tank on some 2010 to 2011 Fusion and Milan vehicles.

It Can Fail An Inspection

In California, a vehicle fails the OBD part of Smog Check if the check engine light is commanded on or any trouble code is stored. In 19 of the state program rows behind this guide, an EVAP code that turns on the warning light means the vehicle does not pass.

Timing

Why The Light Is Slow To Clear

The EVAP test does not run on every drive, because manufacturers may skip it when the tank is over 85 percent full or during refueling. Ford's test needs the fuel level between 15 and 85 percent, and on Toyota's key off system, the check runs after the engine has been off five hours, according to MOTOR Magazine.

A first failure stores a pending code, and if the fault shows again on the next drive cycle where the test runs, the warning light turns on and a confirmed code is stored. A confirmed code is erased after 40 warm up cycles with no repeat of the fault.

Clearing the code with a scan tool has a cost, because it resets the monitors to not ready. That condition can fail an inspection until you drive enough to complete them again.

Have A Code And Want To Talk It Through?

Call (866) 389-3808 and have the code number from your scan tool ready.