Check Engine Light and EVAP Faults

By Jim Taddei - March 6th 2012

I am sure you have all seen it, that yellow Check Engine Light (CEL). And now it’s on again, for perhaps no apparent reason.

There are many different faults that can cause the CEL to turn on; each will set a different “fault code” in the engine control computer. These codes aid the technician in diagnosing why the light has come on.

Just because the engine seems to be running okay is no reason to ignore the light. The light is on because an emission related fault has occurred and the vehicle is polluting the air more that it should.

One of the most common fault codes is P0440. This code is commonly caused by a small leak in the evaporative (EVAP) emission system. The EVAP system collects and stores gas vapors in a charcoal canister when the vehicle is off. When the engine is running, these vapors are purged from the canister and burned during the normal combustion process. EVAP systems have been around since the 1960s, but they have only been monitored by the engine computer since the mid 1990s.

The computer checks the EVAP system first by sealing it, meaning the vent and purge valves are closed. With the system sealed, the computer will monitor the system via the fuel tank pressure sensor. If the pressure or vacuum (depending on the vehicle) varies by more than a predetermined amount over a certain period of time, a code P0440 will be triggered. If two such failures occur with forty vehicle “trips,” the Check Engine Light will come on.

The most common reason for this code to be set is a loose or worn gas cap. Oftentimes, the first time this code is set, the technician will recommend replacing the gas cap to see if it’s the problem. If replacing the gas cap fails to correct the issue, this next step is a very lengthy process of checking the EVAP system for leaks by using a “smoke machine.”

A smoke machine will generate real smoke, which is introduced at a specified pressure, into a sealed EVAP system. When the EVAP system is filled with smoke, the fuel tank, charcoal canister, fuel fill pipe, and all related hoses and valves are inspected for leaks (smoke will be seen coming from the location of the leak). The leaking part is repaired or replaced as necessary and the system is retested.

If no leak is noted, the fault could be intermittent, which adds another level of complexity and the possibility of a “no trouble found” diagnosis. In this case, it may be necessary for the vehicle to be driven until the fault becomes more consistent. This can require a lot of patience on the part of both the vehicle owner and the service technician.

The technician may also recommend replacing the vent or purge valve as a “best guess.” Obviously, guessing is not the best way to go about repairing an emission system fault. But in some cases, this is the only way to begin to eliminate some of the components that could be causing an intermittent problem.

Does anyone have a story to share about an EVAP system leak?

9 User Comments

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Thanks for the information on the "Service Engine Soon" light coming on. I just experienced this problem and it was checked out at an auto repair shop. Everything checked out fine and they did not changed any parts. Was told if it comes on again to bring it in. This article helped to understand what the cause could be and what types of problems could cause it. My gas cap was replaced last year and it also checked out OK. Thanks for the article - Lillian
a vacum line came off on my 98 exp check that it is on the right side of eng and looks like a wire it gos to a canister by the rad
I am having the same issue with my 98 Bonneville se. P0440. Older gas cap kept turning & never clicked & car would act-up sometimes, i replaced cap and havnt had any hic-ups but light came on again. Im determined to resolve this issue & if & when i find my problem i will post again so others can fix too.
I had radiator changed in 2015 I had the mass air flow meter replace my engine is still on I am being told that the automatic starter has defected my ecm I need a engine compurer nedds to be replace
I have a 98 Toyota Corolla le and check engine light has been on steady for at least 3 months. I've gotten codes p0770, p0441. I've changed my gas cap, pvc valve, and evap solenoid sensor (near the air filter). I know it's not dangerous for me to drive my car but would like to know what's wrong already. I'm thinking, since there are 72 possibilities, I should get a smoke test. Please send me your input before I continue my quest. Thanks.
I have a 2002 Olds Bravada (SUV) This problem is driving us crazy. Tech replaced the EVC close to the engine. He thinks there is a second one near the gas cap. Replaced gas cap cover with off market then put a new Brand one on. Is there a EVC near the gas cap on this car? Goes into the shop next Thursday. 9/17