Emission System

I Have An Evaporative Emission System Leak. What Is Going On?

Mike Cross
Updated Oct 24, 2021
Evaporative Emission Control System

You’re driving down the road and that pesky check engine light comes on. ‘What could it be this time?’ you ask yourself.

One of the more common issues, especially with older vehicles, is an evaporative emissions system leak. Fortunately, despite its commonality, these issues are pretty easy to fix. They are also easy to diagnose, often being done by the driver themselves when they are confident that the engine system is in good shape.

Let’s take a look at what an evaporative emissions system leak is and what you should do if you have one. With immediate response, these situations are easy to fix.

Evaporative Emission Control System
Evaporative Emission Control System

What is an Evaporative Emissions System Leak?

Whenever there is any gas in the tank, your vehicle is emitting vapors. Typically, these are consistent and aren’t harmful to the engine or driving. It’s when the vapors are being emitted at higher levels, quicker than normal, or in other unusual ways that you have an issue.

This evaporative emissions system leak is extra vapor leaving the vehicle, more so than during normal operation. The EVAP is supposed to keep gasoline fumes from emitting outside the vehicle, and this leak means that the emissions are pushing through the system.

How long can I drive with a misfiring cylinder
How long can I drive with a misfiring cylinder

Leaking EVAP systems mean extra pollution, extra gas costs, and added wear and tear on your engine. They are part of the reasoning behind emissions tests in many major metropolitan areas around North America and make the air quality worse for residents.

These evaporative emissions are monitored by the Power Train Control in your car, which is programmed to pick up on faulty emissions. When it picks up on a fuel vapor leak in the EVAP control system, the vapor is being emitted in a harmful way.

Any punctures, loose joints, or disintegrating parts might be picked up by the Powertrain Control Module. This will trigger the check engine light to come on and notify you that there is a problem. We all hate that stressful moment, but it’s part of owning a vehicle!

What are the common causes?

There are a number of causes for this. As an experienced mechanic will tell you, some are more serious than others. Here are some of the most common:

  • Your gas cap is missing or loose
  • Your gas cap is broken or built for a car other than one you own
  • The vapor hose or vapor tube has been punctured
  • Your O-Ring has a broken seal, or is just old and needs to be changed
  • Other leaks in the system that are altering the emissions of the vehicle
  • Added pressure to the EVAP system that can stem from the vehicle being old and/or poorly cared for

What should I do when I have an Evaporative Emissions Systems Leak?

There are more than 1,000 different causes of a check engine light coming on. 

The first step is to stop by a car parts store or a mechanic and have them run an OBD test on your engine. Before you walk into the store, double check to make sure your gas cap is firmly secured – save yourself the embarrassment of the mechanic calling you out right away for this!

It is literally the most common cause of an EVAP leak warning. If the cap is not fully tightened or closed all the way, the Check Engine Light may come on. Before taking your vehicle to a mechanic, check your gas cap to make sure it is positioned on the filler correctly and closed tightly.

If the gas cap is on correctly, there may be a more serious issue, such as a leak in the fuel system.

If your Check Engine Light comes on and you suspect a leak in your EVAP system, it is best to have a mechanic inspect the vehicle. EVAP problems can be challenging and the mechanic will have to use advanced troubleshooting techniques to determine where the leak is, as well as how severe it is.

Assuming that it’s a more serious case, the OBD reading should produce a code that identifies the problem. In this case, that code signifies the EVAP leak and the mechanic or parts employee can advise on the probable issue.

If you are driving when the check engine light comes on, there is a much better chance that this is the issue than if you were parked. The engine monitoring system in your car typically won’t pick up on these leaks when the car is turned off.

While the car is driving, there may be a smell or temperature change that triggers the light to go on.

Is it safe to drive with an EVAP leak?

Technically, it is ‘safe.’ But it certainly isn’t advised – at most, you should drive to your destination where the issue can be looked at or to a mechanic. Certainly, you don’t want to be driving all over town or on a road trip while emitting all these extra fumes.

Let’s reiterate this point - check the gas cap. Open the hood and have a look (and a smell!) around. If you can’t figure out what the issue is, get it fixed right away. We can’t advise driving more than ten to twenty miles with an evaporative emissions system leak.


If the OBD EVAP monitor detects a leak when it runs the EVAP leak check, it will set a fault code in the P0440 to P0457 range if you check it with obd2 scanner on the market:

P0440 --> Evaporative Emission Control System Fault

P0441 --> Evaporative Emission Control System Incorrect Purge Flow

P0442 --> EVAP Emission Control System Leak Detected (small leak)

P0443 --> EVAP Emission Control System Purge Control Valve Circuit

P0444 --> EVAP Purge Control Valve Circuit Open

P0445 --> EVAP Purge Control Valve Circuit Shorted

P0446 --> Evaporative Emission Control System Vent Control Circuit

P0447 --> EVAP Emission Control System Vent Control Circuit Open

P0448 --> EVAP Emission Control System Vent Control Circuit Shorted

P0449 --> EVAP Emission Control System Vent Valve/Solenoid Circuit

P0450 --> Evaporative Emission Control System Pressure Sensor

P0451 --> EVAP Emission Control System Pressure Sensor

P0452 --> EVAP Emission Control System Pressure Sensor Low Input

P0453 --> EVAP Emission Control System Pressure Sensor High input

P0454 --> EVAP Emission Control System Pressure Sensor Intermittent

P0455 --> EVAP Emission Control System Leak Detected (gross leak)

P0456 --> EVAP Emission Control System Leak Detected (small leak)

P0457 --> EVAP Emission Control System Leak Detected (fuel cap)


Like other issues that cause the check engine light to come on, it is important that you get an evaporative emissions system leak looked at right away.

By letting it sit, you’re only making the problem worse and improving the chances of more serious damage down the line.

For the optimal life of your vehicle, important engine care is absolutely essential. Most parts stores and mechanics can fix, or sell you the materials to fix, this problem at a reasonable rate and you shouldn’t have a repeat.

If you have enjoyed this article, please give it a share on social media. Who knows – you might be saving a friend or relative from a major engine repair just be giving them this information! Have a particularly crazy story about EVAP leaks to share? Put it in the comments below and let’s get a discussion going.

Mike Cross
Life is too short to drive with stock audio

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One comment on “I Have An Evaporative Emission System Leak. What Is Going On?”

  1. The dialogue here is well intended but is exaggerated.

    The worst is the FEAR TACTIC used to suggest auto owners need to make an hasty visit to the auto repair shop. AS IF you do not know....when the customer goes to an auto repair shop expressing unusual concern for this issue....the mechanic WILL take the customer to the 'cleaners'!!!

    Bottom line - You set up a person to be victimized. There is already enough cheating, dishonest auto repair shops out there without you making it worse.

    FACT: out of the 12 auto repair shops on one street in Harrisburg, PA - there is only one shop that proved honesty but they are Foreign auto repair only.

    This is not just business, as some say, ......it is stealing.

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