So many car problems are easy to diagnose. Low air in your tire? There’s an easy way to figure that out. Issues with your HVAC system? You’ll know it. Don’t let those “easy” issues lull you into a false sense of security, though. Some maintenance issues don’t present themselves so readily.

That’s the case with EVAP leaks. Although they’re not the most serious problem you can have, they can be difficult to diagnose and fix, if you don’t know what to look for. Car Bibles’ editors have spent enough time reading a code scanner to know an EVAP leak when they see one, so we’re here to help you get started on the right foot.

Let’s get rolling.

What Is The EVAP System and How Does It Work?

The evaporative emission control system, also known as EVAP, exists to prevent gas fumes from the fuel tank from escaping into the atmosphere. Leaking fumes and vapors are bad for a whole bunch of reasons, mainly because they can damage the environment and may even be harmful to the vehicle’s passengers. The EVAP system works in three major ways: containing fumes, purging fumes, and monitoring the system for leaks and problems. It does this with several sensors and valves that contain and move fuel vapors as needed during normal vehicle operation.

What Is an EVAP Leak?

An EVAP leak just means that the fuel vapors are escaping from the closed system somewhere along the way. If there’s enough vapor leaking, the EVAP monitor will pick up on it and may flag the issue with a check engine light.

Evap Leak Symptoms, Causes, Issues

Your car can communicate an EVAP leak to you in several ways:

  • Drop-in fuel efficiency
  • Rough idle
  • Drop-in performance, such as sluggish acceleration and trouble climbing hills
  • Difficulty starting engine
  • Check engine light

Depending on the type of leak your car is experiencing, you might see error codes when scanning with an OBD2 tool. These can include:

  • P0442: Small leak detected
  • P0455 System leak detected
  • P0440 EVAP System
  • P0446 Vent solenoid valve control system
  • P0411 System control incorrect purge flow

Can I Diagnose The Problem Myself?

You absolutely can. You’ll need an OBD2 scanner to read the codes that are being generated by your car’s computer, and you can also observe the vehicle’s performance for signs of a leak. If nothing is obvious from there, you might need to perform a visual inspection to observe any signs of physical damage. In some cases, you’ll be able to see the failing seal or hose that is causing the leak. The challenge with these things is that it’s impossible to see air as it escapes, so if there’s no obvious physical damage you’ll need to perform additional testing.

If you’re savvy with a vacuum pump, you can test for leaks while the engine is off. This can be done with a hand or power pump, but you’ll need to refer to a vehicle-specific maintenance manual to find the correct testing points. The best way to test for leaks, however, is a smoke test. The idea here is to pump smoke into the system and observe any that escapes as it passes through.

The Car Bibles Glossary of EVAP Leak Terms


Emissions, and emissions control systems, are your car’s way of controlling its own waste and byproducts. Burning fuel in an internal combustion engine releases all sorts of harmful gases and chemicals into the air, so it’s important for a vehicle to mitigate that pollution as much as possible.

Purge Valve

The purge valve is an electrically-powered solenoid that is part of the EVAP system. It is closed when the engine is off and is opened by the car’s ECU as the engine comes up to temperature. Its job is to allow some unburned fuel vapors to move into the engine for combustion.


OBD2, or on-board diagnostics, is a system through which a vehicle owner or repair tech can scan for errors or issues that have been detected by the computer system. If there is an issue, the system generates a code with a specific number that corresponds to the problem at hand.

The EVAP Leak Questionnaire

Your questions, our answers.

Q: Can I Drive With an EVAP Leak?

A: It might be possible, but it’s certainly not a good idea. You could be spewing harmful fumes into the atmosphere and may end up breathing some of them in if they’re able to enter the vehicle’s cabin.

Q: How Much Does It Cost To Fix an EVAP Leak?

A: Depending on the vehicle, you could spend upwards of $600 or so for a repair, but many fixes are cheaper than that. The type of leak, including its location, will dictate some of the cost as well.

Q: Is There a Way To Prevent Leaks?

A: Sure, with regular inspection and maintenance. Sometimes, though, things just happen, and no amount of checking will help. As seals, O-rings, valves, or hoses start to age, leaks can occur even in the best-maintained vehicles.

Video on EVAP Leaks

Car Bible’s Favorite EVAP Leak Related Products

It can be tough to figure out what you need to diagnose and fix an EVAP leak. That’s why Car Bibles’ editors have grabbed a few of their favorites to get you started. They include Mechanix Work Gloves, Pro-Lift Jack Stands, and the Innova CarScan OBD2 Scanner.

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