How to Diagnose Check Gas Cap Light or Check Engine Light That Has EVAP Codes

This instruction explain how the EVAP system works in simple terms, and also show you how to diagnose and fix “Check gas cap light” or check engine light that has EVAP codes on Mazda .If you don’t have a check gas cap light but instead have a check engine light that has to do with EVAP system, this article will teach you how to figure out what’s wrong.


Here we’ll tell you the background on the EVAP system that way you understand why a car has the EVAP system and how it works.On all 1996 and newer vehicles as well as many older vehicles you can get a check engine or a check gas cap light if there’s a leak detected in the fuel system.

Here are some similar topic articles for your further reference:

Jeep Cherokee P0455 P0456 Evaporative System Leak Repair

Evap Leak Test Function by G-scan for Kia Soul

Buick Enclave Can Not Start by EVAP Trouble & Rich Mixture


Why we need EVAP system?

Gasoline is volatile meaning evaporates easily and that’s why we have gas caps on containers of gas and also on our cars. If we take this cap off and we leave it off eventually this full tank of gas will evaporate out and it’ll empty completely . so we make sure that we keep a gas cap on the tank. That’s the same reason why we have a gas cap on our cars and our trucks. It prevents the gas vapors from leaking out and since gas vapors are flammable that could be dangerous. Also these hydrocarbon fumes are bad for the environment, so since 1971 gas tanks are sealed.

Because the system is sealed, there has to be a way to introduce air into the gas tank. So while you’re using your gasoline from driving around and your gas tank goes from full to empty that space has to be filled with air otherwise you’ll get a large vacuum inside the gas tank and that wouldn’t be good.

The main idea the EVAP system is to prevent the gas fumes from leaking out both due to safety reasons and environmental reasons. But at the same time we need to allow air to go into the gas tank to displace that fuel that is getting burnt.


How the EVAP system work?

Here is what the whole EVAP system looks


We have the fuel pump on the left of fuel tank which feeds the gas to the engine through the fuel lines, on the right we have the gas cap which seals the fuel tank, the most common causes for an EVAP check engine light.

The red sensor is the tank pressure sensor which tells the computer if the tank is pressurized or under vacuum. The vent that goes up the middle of the tank is what pulls the fuel vapor out of the gas tank. It has a liquid vapor separator to block out any liquid fuel from getting.


The fuel vapor gets stored in a vapor canister and when the right conditions exist the computer opens up the purge valve and allows the fuel vapors to be sucked into the intake and burned during the combustion process. As explained before the EVAP system also needs a fresh air source, so the vent valve is opened up when the tank pressure sensor shows that there’s a vacuum on the fuel tank. The fresh air prevents buildup of the vacuum and keeps the system pressure balanced. The vent valve is one of the most common problems with the EVAP system because it’s usually right under the car and exposed to all the elements. Many times the valve gets open or closed, and to fix your problem all you have to do is replace the valve.


Most cars use vacuum to test for leaks in the EVAP system but some actually use pressure instead such as dodge BMW and Subaru, there’s a pump that forces air into the tank and then sensors tests for the ability of the EVAP system to hold pressure.

There are two types of leaks, there’s a large leak which is usually the gas cap or there’s small leaks which could be a pinhole leak on one of the rubber lines in the EVAP system.

In my case, we have a large leak. The plastic that allows this to ratchet got old and brittle and broke that’s where we’re getting our air leaking in.


You can see rubber gasket, this is another common problem cause check engine light on. This gasket gets brittle, it cracks just from use wear and tear and if you don’t have this gasket it won’t seal to the surface


The easiest way to fix it is get a correct gas cap and replace it.


After replace a new gas cap, go for a test drive and make sure gas cap light is shut off.

The cars computer performs a test only once per drive cycle under certain conditions if all those conditions are met the computer will test EVAP system. The first condition the gas tank has to be between 15%-18% full, the test is also performed when there is no load or a very small load on the engine, so at low RPMs the air temperature has to be between 40 to 100 degrees Fahrenheit. And if you don’t have these conditions the light might actually shut off but you still have the problem.


More about Check Engine Light Trouble Repair,pls visit:

How to Solve Check Engine Light On?


Attached some common EVAP System check engine light codes:

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)


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