A Hyper Tough HT300 scan tool resting on the gauges of a 2005 Mercedes E55 at our shop.

Written By Working Mechanics, Sources Checked November 2026

What Your EVAP Leak Code Means And What The Fix Costs

Type the code from your scan tool to see the leak size, where to look first and a fair price range.

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P0442 Evap Emission Control System Small Leak Detected

The computer found a small leak somewhere in the fuel vapor system.

.04 inch hole. Left is true size, right is 10 times bigger.

$61 to $90diagnosis labor at RepairPal

Look First: The gas cap first, then a smoke test

Safe to drive. Usually yes for now, but get it fixed soon and check for open recalls.

Read The Full P0442 Guide

Code wording from carmaker bulletins. Prices from published sources.

What An EVAP Leak Is

Your car stores gasoline vapor in a charcoal canister and later burns it in the engine. The computer tests that system for leaks to the air, and an EVAP code means it found one or found a part that is not working.

Cars from 1996 on must watch the whole EVAP system for vapor leaks, says the California Air Resources Board.
Since model year 2017 federal rules call for finding a leak as small as 0.020 inch, per 40 CFR 86.1806.
A loose gas cap can set P0455, P0456, P0457 or a maker code, and it is the first thing to check.
The light goes off after three drive cycles in a row where the test runs and passes.

Most of the time the car drives the same. The problem shows up as a check engine light, a failed smog check, or sometimes a fuel smell. The fix can be a $20 cap or a canister that runs past $500, so the code and a real test matter before anyone sells you a part. Read the answer in plain words from the people at what a check engine light means, from our shop and the sources below.

Where Vapor Goes, From The Cap To The Engine

Tap A Part

Each box opens the guide for that part. The orange line is the path fuel vapor takes. The purge valve lets it into the engine and the vent valve lets fresh air in when the canister empties.

See Every EVAP Part

Three Ways A Leak Gets Found

The Car Tests Itself

The computer seals the system and watches pressure. Ford's test seals it with the vent valve, pulls vacuum with the purge valve and times how fast it bleeds back, per Ford's OBD summary.

A Shop Runs A Smoke Test

A machine fills the system with harmless smoke and the leak shows where it comes out. Mazda's bulletin uses a smoke tester. How A Smoke Test Works

You Check The Cap First

Take the cap off, look at the seal, and put it back until it clicks. Ford names a cap that was not installed right as a P0455 cause. Gas Cap Guide

What EVAP Repairs Cost

Each row is one published source with its date. We never average across sources.

Repair	Price	Source	What The Number Covers
Gas Cap	$138 to $150	RepairPal	national average estimate, parts and labor
Purge Valve	$210 to $251	RepairPal	national average estimate, parts and labor
Vent Valve	$182	YourMechanic, 2011 Dodge Nitro	vehicle specific estimate, parts and labor
Fuel Tank Pressure Sensor	$356 to $457	RepairPal	national average estimate, parts and labor
Charcoal Canister	$558 to $643	RepairPal	national average estimate, parts and labor
Leak Detection Pump	$201	YourMechanic, 2009 Dodge Charger	vehicle specific estimate, parts and labor
Filler Neck	$492	YourMechanic, 2010 Toyota Camry	vehicle specific estimate, parts and labor

CarMD's 2026 report puts the purge valve at number 6 of the top check engine repairs, at an average of $172 (CarMD). Our own shop billed a smoke test at one hour, $150. See Every Price Row

Will It Pass Smog

In California any stored trouble code or a check engine light that is on fails the car. Gas cars from 2000 on may pass with only the EVAP monitor not ready, per the Bureau of Automotive Repair.

Lake Havasu City sits in Mohave County, which is outside the Arizona emissions areas, so cars registered there get no emissions test. That comes from Arizona law.

Smog Rules By State

An OTOFIX scan tablet showing a fault stored in the engine computer of a 2013 Mazda 3. From a real job in our Shop Desk.

From Real Jobs At Our Shop

A Hyper Tough HT300 scan tool plugged in and resting on the gauges of a 2005 Mercedes E55. From a real job in our Shop Desk.

The check engine light lit on the dash of a 2013 Mazda 3 that came in for a diagnosis. From a real job in our Shop Desk.

Questions People Ask

Can I Drive with an EVAP Leak

Several carmaker bulletins about EVAP leak codes say drivability is not affected. CARB says most makers advise service as soon as it is convenient, and a blinking check engine light needs fast service. A few recalls are different. A stuck purge valve could stall some Ford Focus engines, so check for open recalls on your car.

Will an EVAP Leak Fail a Smog Check

In California, yes. A car fails the OBD part of smog check if the check engine light is commanded on or any trouble code is stored. Many other states fail a car when the check engine light is on with a stored code. Check your state in our emissions table.

Can a Loose Gas Cap Turn on the Check Engine Light

Yes. Ford lists a cap that was not put on right as a cause of P0455, and has a separate P0457 code for cap off. Acura, Mazda and Honda bulletins also tie EVAP codes to caps that do not seal. Fueleconomy.gov names a loose gas cap as one possible reason for the light.

How Long After I Tighten the Gas Cap Will the Check Engine Light Go Off

It depends on the car. CARB lets the light go off once the system checks that the cap is tight. Otherwise the light goes off after three drive cycles in a row where the EVAP test runs and passes. Ford says the code and light clear as soon as normal purge vacuum passes a stored level. The EVAP test only runs under certain conditions, so it can take days.

What Is the Difference Between a Small and a Large EVAP Leak

The rules use test holes. A small leak is about the size of a 0.040 inch hole (often P0442). A very small leak is about 0.020 inch (P0456). A large or gross leak (P0455) means the system could not even hold its test vacuum, which Ford says can come from a loose cap or a disconnected line.

How Much Does It Cost to Fix an EVAP Leak

It depends on the part. RepairPal lists about $138 to $150 for a gas cap, $210 to $251 for a purge valve, $356 to $457 for a tank pressure sensor and $558 to $643 for a canister, with parts and labor. CarMD's 2026 report lists an average of $172 for a purge valve. See our cost table for each source and date.

See Every Question