Automotive Education/Info

Understanding EVAP Systems so You Can Fix It Yourself

The Least Understood Emissions Component is the EVAP System

How many times has your check engine light come on for an EVAP leak or EVAP system failure? Problems with the evaporative emissions system are the most common cause of a check engine light in cars with OBD-II or newer.

Usually you just take the car to a local auto parts store or repair shop and have them read the code. Many times they will tell you “it’s the gas cap” or you “must have pumped gas with it running”. Although these two things can cause a check engine light for an EVAP code, the repair is often more complicated. Clearing these codes without further troubleshooting often leads to a second trip to the repair shop.

The unfortunate thing with EVAP systems is many auto technicians don’t even fully understand the way these emissions control systems work. This is especially true for auto technicians without formal training or trade school certifications. So if the EVAP system is so hard to diagnose for most technicians, how can we make things easier?

In this post, I’m going to go through some of the basic principles of EVAP and how to diagnose the evaporative emissions system. I also want to take a look at the different types of evaporative systems that manufacturers use and some of the tools you are going to need to diagnose them.

Each automaker has its own design of EVAP system, further complicating an attempt to teach people how EVAP systems work. All of the systems work on the same principles though. Im going to try to help you understand these principles and how EVAP systems work.

What is the EVAP System in a Vehicle?

parts of the EVAP system in a diagram
In the diagram provided by Emanuel Online you can see the components of a typical EVAP system.

EVAP systems have actually been required in vehicles since the 1970’s which makes the fact that some techs don’t fully understand them even more telling of how complicated EVAP systems can get. Even techs with decades of experience can find themselves in the trap of “throwing parts at” an EVAP failure code hoping to pinpoint the proper failed component. Usually this just ends up in a very costly game of troubleshooting darts for the customer and repeated comebacks for the technician.

The EVAP control system is defined as a system used to prevent fuel vapor from being released into the atmosphere, instead allowing it to be collected while the vehicle isn’t running and then released into the intake once the engine is started and meets certain conditions.

Components related to a typical evaporative emission control system
Though this EVAP setup is on an older vehicle, it effectively illustrates what an EVAP system is made up of and how it works.
Image Credit- AA1 Car

So in terms the average car owner can understand, the EVAP system just makes sure that there are no fuel vapors being released into the air. Instead, those vapors get a second chance at ignition through the intake or carburetor if the vehicle is pre-tbi.

Think of when you open a gas can or when you are pumping gas into the tank. That smell of fuel is from the vapors being released into the air. The EVAP system is designed to make sure vehicles aren’t releasing large amounts of  fuel vapor into the open air.

EVAP systems are set up to monitor the amount of fuel vapor being released and if the system finds a problem such as a leak, too little or too much pressure or faulty EVAP solenoids, it sets the check engine light.

What Components do EVAP Systems Have?

In order to get a better understanding of how the EVAP system works in a vehicle, it’s important to talk about the components involved in the EVAP system.


All EVAP systems have one thing in common and that is the charcoal canister. Without a canister to collect the vapors, the rest of the EVAP system would be useless.

After the charcoal canister, the EVAP purge valve and the EVAP vent valve are the two most common parts of any EVAP setup. This is because the system has to have a way to control the venting of fuel vapors from the fuel tank and it also needs a way to purge built up vapors from the charcoal canister.

In some EVAP systems, like the one used on many Dodge engines, there is a “leak detection pump” or LDP that pressurizes the system and uses ECM monitoring to determine how large the EVAP leak is. The system deciphers how much air it takes to pressurize the EVAP system and turns that into the “size” of a leak, if one is present.

Hoses, Vacuum Lines

In order to connect all of the different components in an EVAP system between the engine and the fuel storage and fuel delivery system, auto manufacturers use hoses, vacuum lines and clamps or quick-disconnects. Often times, these connections are the source of an EVAP leak and should always be checked using a smoke machine to determine if they are leaking.

AutoLine sells an affordable one on Amazon that you can see here.

What are some common EVAP codes?


In my experience with diagnosing and fixing EVAP systems, there are five codes that are very common to see on the scanner. In this section, I’ll outline the five most common EVAP codes and the most common cause of each fault code.

1). P0442 Evaporative Emissions System Small Leak Detected

One of the most common causes of a check engine light in any vehicle is the P0442 which is a small leak in the evaporative system. Causes for a P0442 can range from a faulty gas cap to a pin-hole in one of the EVAP lines.

In order to diagnose a small EVAP leak in the system, most techs will hook up a smoke-tester to the EVAP system and find the leak. You can buy a reasonable one from AutoLine on Amazon.

It’s important to learn how to use a smoke machine before you buy one. A common mistake that some techs make is not closing off the vent valve before smoke testing the EVAP emissions system.

2). P0445 EVAP system large leak detected

Similar to the small EVAP leak, a P0455 indicates the vehicle has a Gross” or large leak in the evaporative emission system. This code is a little more serious than the P0422 because it is indicating that there is a big EVAP leak somewhere in the system.

Whether there’s a large hole in an EVAP hose, a hole in the fuel tank or even a leaky gasket around the fuel tank sending unit, the P0455 on most makes and models tend to be the more serious emissions code of the two.

On GM and several other models, I have observed the nylon style EVAP tubing as a major weakness in the system while the cause of large EVAP leaks on Dodge systems with an LDP can be a faulty leak detection module itself.

3.) P0441 Evaporative Emissions Control Incorrect Purge Flow

Another common problem with the EVAP system that may appear and can not be solved simply by replacing the fuel cap is the code P0441 which stands for “evaporative emissions control system incorrect purge flow”. As the description of the P0441 suggests, this code has little do with leaking fuel vapors and a lot to do with the “purge” process of the EVAP system.

In every EVAP emissions system, the ECM has a way to control the flow of vapors being purged from the charcoal canister. Causes that can lead to symptoms pertaining to the purge flow can range from a faulty purge valve to a hole in the tubing that leads to and from the purge valve or even damaged and worn o-rings.

In rare cases, the purge valve can also fail internally and cause a clicking noises like is commonly seen on the Chevy EcoTec engine. More often though, the P0441 engine code is caused by a failed purge valve or a broken wire leaving the purge valve inoperable and incapable of purging vapors from the canister back to the intake.

When diagnosing a P0441 it is important to use a probe tester to verify power and ground before utilizing a smoke tester like the AutoLine models sold cheaply on Amazon to make sure a hole in the lines is not causing a lack of flow from the purge valve.

A common symptom of a stuck purge valve is a rough-running vehicle, most commonly right after the tank is filled with fuel. This symptom of a failing or already failed purge valve is due to the engines inability to properly regulate the “purge” of fuel vapors back to the intake for ignition.

4.) P0446 EVAP Emission System Vent Control Circuit

As I noted previously when trying to explain how the evaporative emissions system works, one of the most important components is the vent valve. In most EVAP systems, the vent valve allows the fuel tank to release pressure as needed and also closes when the system needs to perform leak tests.

EVAP vent valve p0456
This vent valve is a common design used in GM vehicles. All manufacturers use a different style, but they all have the same concept.

Similar to how an EVAP leak detection pump works, the vent valve closes off and measures pressure using the fuel tank pressure sensor. The ECM them uses this data to determine the integrity of the EVAP system. How long it takes for the EVAP system to de-pressurize will tell the PCM if a leak exists and if it does, how severe is the fuel vapor leak?

In 99% of the cases that I have diagnosed with a P0446 DTC, either the solenoid portion of the vent valve was faulty (internal short) or there was a broken power or ground wire to the solenoid preventing the valve from opening and closing.

When trying to determine the cause of a P0466, it’s important to first check the integrity of the wiring to the solenoid and if that checks out, try bench testing it with a probe tester. If the solenoid does not click when given power and ground, the EVAP vent control solenoid is likely faulty.

5.) P0456 Very Small EVAP Emissions Leak Detected

Perhaps the most common EVAP system DTC- the P0456 can also be the most frustrating code to diagnose in terms of the emissions system. Why it’s so hard to find the source of a very small EVAP leak is actually fairly obvious when you think about it. At least with a large EVAP leak, the smoke tester has a good chance of leaving a noticeable trail of white smoke when hooked up.

Unfortunately when it comes to those very small EVAP leaks, which are less often caused by a bad gas cap than the auto parts store would have you believe, diagnosis can be a real pain- even with a smoke machine.

I always recommend using a UV light like the ones used to find leaks in the air-conditioning system when dye is applied to the system. Streamlight makes a great one, of course but there are others out there. Using a UV light to detect smoke from an EVAP leak is highly effective and extremely helpful on the really small leaks that are hard to see without visual help.

How Can I Prevent EVAP Codes in my Car?

Figuring out how to prevent a check engine light for EVAP is difficult
Ignoring the check engine light for EVAP can cause problems to accumulate

With EVAP being one of the hardest to diagnose emissions control systems, it’s natural that you might want to know how to prevent evaporative codes from ever coming up in the first place. Is there a way though to prevent those troublesome DTC’s related to the emissions control system or are we doomed to dealing with check engine lights until all vehicles become electric in the distant future?

Maintenance/Inspection is the Only Preventative Measure for EVAP

Unfortunately, there is no magic trick when it comes to preventing DTC’s related to the evaporative emission system. There is no oil to change and there are no spark plugs to replace that will help keep the EVAP system healthy in your vehicle. Visual inspections of the components related to the EVAP system are the only real way to make sure the system stays healthy and prevents leaks.

It’s never a bad idea to run quality fuel additives in your vehicles fuel system, but alone these will not prevent problems with the EVAP system. Most importantly when trying to keep the check engine light off for EVAP reasons is visual inspections of all the lines and components. Even this will not be fool-proof and you can bet that at some point you will have to replace things like the purge valve and EVAP vent valve.

Now that you have a better understanding of how the EVAP system works and what components are most likely to fail and why, you will be in a much better position to make the right choices regarding the emission control system known as EVAP.

Author- John Green


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By John Green

I’m John Green. I’m a 33 year old auto technician from Upstate New York. I have 18 years of experience as an automotive light duty and heavy duty truck mechanic. Cars, trucks and anything with moving parts are my passion in my professional life.

Aside from my life as a technician, I am also a seasoned investor and consider myself very financially literate. I use this other passion combined with my passion for cars, trucks and tools to look for ways to save money for my technician friends.

Raising my three girls and teaching them the proper way of life is my personal passion in life. If you want to know more, just ask! I’m on Twitter, Facebook and YouTube as well!