Gas cap code

The gas cap code is a common diagnostic trouble code that might turn on your check engine light.

I recently put together a video that discusses a problem that my neighbor had with his vehicle after his wife filled the fuel tank. By popular demand I added a common evap system electrical diagram.

You can see the short video and diagram below. But I wanted to talk more about the gas cap code, which on some models is known as the P0440 code.

I also wanted to talk more about the evaporation emissions system that can set other diagnostic trouble codes in the P0400 range.

After posting my short video on YouTube I received many comments saying the gas cap is not the only reason that p0440 code will set.

I agree, and was just saying that the most common cause was a loose or broken gas cap. Not the only cause.

First, the function of the evap emissions control system is to reduce the amount of fuel vapors that are released into the atmosphere.

About Evap Codes

These systems will differ between make and models, but overall, most use a pressure test to confirm that no vapors are escaping from the fuel system.

The most common way that they perform this test is with a small pump connected to the evap system.

When the PCM checks the system, it turns on the pump pressurizing the system. As pressure builds the cycling rate of the pump decreases.

If there is no leak in the system the pressure builds until the pump shuts off.

If there is a leak pressure does not build up and does not shut down the pump. The P0400 series of diagnostic trouble codes are set depending on the results of this test.

Gas caps and check engine lights

It is also possible that the fuel cap is screwed on properly, but still leaks from either the o-ring gasket or the body of the gas cap itself.

Yes, sometimes the problem is much more complicated than just replacing or tightening the gas cap. In fact, small leaks can be very hard to find.

Car dealerships and the shop I work for have a smoke machine that can hook to the system and pump smoke throughout all of the lines, valves, canisters and solenoids. The idea is that you should be able to see smoke leaking from any problem area.

In my opinion the best way to solve any emission codes is to retrieve the specific check engine light code. Then follow the diagnostic repair diagram until the problem is solved. Below is the video that shows you the diagnostic tree chart for the P0440 gas cap code.




Give this gas cap code check engine light resource page a Come back and watch the video again. Or Find more auto repair information.

Here is the Actron 9410 scan tool I used in the video. The CP9410 pocket scanner reads and erases codes but it also tells you how urgent the code is.

Some codes are very important to fix right away. A good example of a code that could harm your vehicle would be an engine misfire code.

If you have a cylinder not firing, raw fuel could be dripping into your catalytic converter. This could damage the catalytic converter itself.

This is one expensive car part. Example: bad people have been stealing them right in good peoples driveway's. Also raw fuel lying in the exhaust could possibly be dangerous if not addressed.

Then you have the flip side of the equation with the p0440 gas cap code. This code is not good for the environment but it will not affect the safety or reliability of your automobile.

This is why I consider the urgency indicator on the 9410 model scan tool valuable. If you have no idea what most codes mean the scanner will give you a heads up if you should worry or not.


For more information about what else can turn on the service engine soon light visit my page dedicated to the dreaded check engine light





Do you need answers about your check engine light right now?

Ask Professional Car Mechanics

Type your car question here.

 letters left

Custom Search