Nuisance Gas Cap Codes

locking gas caps

The gas cap code is a common diagnostic trouble code that might turn your check engine light on. I put together a video discussing a problem my neighbor had with his vehicle after they filled the fuel tank.

It's a common scenario I see all too often. As you can see in the ladder diagram from the service manual, action step 1A is to check the fuel cap. By popular demand I added a common evap system electrical diagram at the bottom.


The truth is the government mandates this code to protect the atmosphere from fuel vapors. The check engine light will come on and set a code if the fuel system isn't sealed airtight. On some automobiles it's known as the P0440 code.

diagram of gas cap code

I also want to talk more about the evaporation emissions system. This sensitive arrangement of parts sets other diagnostic trouble codes in the P0400 range.

After posting my short video on YouTube I received many comments saying the gas cap isn't the only reason that a p0440 code will set. I agree, and was just saying the most common cause is a loose or broken gas cap. Not the only cause.

When dealing with these issues lets remember the basic function of the evap emissions control system. It eliminates or reduces the amount of fuel vapor released into the atmosphere. This became part of the clean air act amended by our Federal Government.

More about Emissions Codes

These laws required car manufacturers to install malfunction monitoring systems. Now carmakers automatically check the integrity of the fuel system and its ability to completely seal.

They also had to notify the driver of any problems by turning on the malfunction indicator lamp (MIL) if the system has a failure. The parameters that turn on the check engine light will differ between make and models. However, many use a pressure test to confirm that no leaks are present from the tank and lines.

The most common way manufacturers perform the test is with a small pressure pump connected to the system. When the control module runs an automatic diagnostic, it turns on the pump, pressurizing the system. As air pressure builds the cycling rate of the pump decreases.

If there is no vapor leaks the pressure builds until the pump shuts off and the test has passed. If there is a leak the pump runs too long and the test fails. The P0440 - P0459 series of diagnostic trouble codes are set depending on the results of this automatic test.

Gas Caps and Check Engine Lights

If the rubber seal on the fuel cap isn't sealing, because the cap is loose or cross threaded, the test fails. The pump never reaches its desired pressure and therefore triggers the service engine soon light.


To the left is an inexpensive scanner that just reads and clears codes. Plus here is a link to popular replacement gas caps. If you want to try replacing the cap and clearing the codes its worth a try, but there are no guarantees it'll solve all your problems.

Yes, sometimes it is much more complicated than just replacing or tightening the gas cap. I have seen crazy things like spider nests in vent lines cause a p0454 code.

The Hard to Fix Gas Cap Code





In my opinion the best way to solve any check engine light is to first retrieve the specific diagnostic trouble code. Then follow the ladder repair diagram until the problem is solved. Below is the video that shows you the tree chart for the P0440 gas cap code. If you put it in full screen mode you can actually see the individual steps. Automotive Diagnostic and Repair Information.

Evaporative emission electrical diagram

A Bargain Scanner with Advanced Features


So here is the Equus 3100 scan tool which at the time of this writing is the scan tool I recommend. It's an older model, but provides a good balance between price and features. For around 100 bucks this thing has on screen definitions and freeze frame or snapshot capabilities. Remember, some codes are important to fix quickly.



