Nuisance Gas Cap Codes

locking gas caps

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

As you can see in the ladder diagram from the service manual, action step 1A is to check the fuel cap. Note: By popular demand I added a common evap system electrical diagram at the bottom as well. Watch the short video and review the diagram below. First lets talk a little more about the gas cap code and the possible causes.

Many mechanics and drivers consider this a nuisance code because it doesn't effect how the engine runs. The truth is this code is mandated by the government 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 air tight. On some automobiles it is known as the P0440 code.

diagram of gas cap 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 of all the function of the evap emissions control system is to eliminate or reduce the amount of fuel vapors that are released into the atmosphere as mandated by the Federal Government.

More About Emissions Codes

The car manufacturers where required to install systems that monitored the integrity of the fuel system and its ability to completely seal in fuel vapors. They were also required to notify the driver by turning on the malfunction indicator lamp (MIL) if the system has a problem.

The parameters that turn on the "check engine light" will differ between make and models. But 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 ( test passed ). If there is a leak, pressure will not build and does not shut down the pump. 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 is not sealing because the cap is loose or cross threaded the pressure that the pump supplies will escape and trigger the service engine soon light.

It is also possible that the fuel cap is screwed on properly but still leaks from either the o-ring gasket seal or the body of the gas cap itself. often gas caps are made of several parts even if they look one piece. 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.

Maybe the worst situation is small leaks that can be hard to find. Dealers and the shop I work at have a smoke machine that hooks to the system and pumps smoke into the lines, valves, canister and solenoids.

The idea is, you should be able to see a smoke monster 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.

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Evaporative emission electrical diagram

I use to recommend 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 they are. This urgency indicator is easy to understand because it uses green, yellow and red to indicate the severity of set codes. It seems no longer available?

So here is the Equus 3100 scan tool which at the time of this writing is the scan tool I recommend that 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.

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 an expensive car part, that bad people have been stealing 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. For more information about what else can turn on the service engine soon light visit my page dedicated to the dreaded check engine light.

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If you like short educational auto repair videos I have a few more you might find interesting. This next link takes you from the gas cap code to auto repair videos.