The Evaporative Emission Control System (EVAP) on late model vehicles with OBD II can detect fuel vapor leaks that allow gasoline fumes to escape from the gas tank and EVAP system. The EVAP monitor runs a self-test that checks the integrity of the system to make sure there are no leaks. If the fuel tank and EVAP system fail to hold pressure during the test, it indicates a leak.
A large leaks such as that caused by a loose or missing gas cap will set either a code P0455 (large leak) or a code P0457 (loose gas cap). Small pinhole-sized leaks will set a different code: P0456 (small leak).
If your Check Engine light is on and your vehicle seems to be running normally, and there are no other warning lights on, the problem might just be a loose or defective gas cap.
NOTE: The Check Engine light will NOT come on immediately following a fill up if you forgot to put the gas cap back on or did not get the cap fully tightened. The Check Engine light will not come on until the OBD II system runs its EVAP leak self-test.
The EVAP leak self-test only runs under certain circumstances. The fuel tank must be 15 to 85 percent full (not near empty and not completely full). The outside temperature must not be too cold (less than 0 degrees F) or too hot (over 90 degrees F). And the vehicle has to have been sitting overnight before the EVAP leak self-test will run.
If your Check Engine light came on within a day or two after filling your tank, check your gas cap. Turn the engine off, open the flap over the gap and check to see if it is tight. If the cap is obviously loose or is missing, you have found the problem. Tighten or replace the cap.
How tight should the cap be? On some vehicles, the cap only needs to be turned until you hear ONE click for it to seal tight. On other vehicles, the cap must be turned until you hear THREE clicks before it is tight.
If the cap seems to be tight, remove it and closely inspect the gasket or seal on the underside of the cap. A cracked, deformed or damaged gasket or seal may not seal the cap tightly, allowing vapors to leak past the cap. You need a new gas cap. Replacement gas caps are available at most auto parts stores or online, and are relatively inexpensive to replace in most cases ( $10 up to $40).
Also, carefully inspect the sealing surface on the top of the gas tank filler neck. If the surface where the cap seals against the neck is dented, deformed, badly corroded or otherwise damaged, it may prevent the cap from sealing tightly. This may require replacing the filler neck or gas tank to fix the problem.
If the cap and filler neck appear to be in good condition, retighten the gas cap the recommended number of clicks (ONE or THREE depending on your vehicle). Then drive your car for a couple of days to see if the Check Engine light goes out.
Remember, the EVAP leak self-test does not repeat on every drive cycle. It may take a few days before the EVAP leak self-check will run. If no leaks are found, the Check Engine light should go out. However, if the Check Engine light remains on, the cause may be something else that will require further diagnosis with a scan tool.
Plugging a scan tool into the OBD II diagnostic connector will allow you to read out any codes that are causing the Check Engine light to come on. You can also use a scan tool to clear any codes (after writing them down) to see if the codes come back.
If the cap and filler neck appear to be sealing tightly but you find a large leak code (P0455 or P0457), the problem may be a loose or cracked fuel tank vent hose, or a fault with the EVAP system vent solenoid or purge solenoid. These kind s of faults will usually set additional codes that can help you or a technician pinpoint the problem.
Vapor hose leaks in the EVAP system can be very DIFFICULT to find because the hoses are often routed inside body panels and behind other components. A special smoke machine that produces low pressure mineral oil vapor may be needed to find small leaks. The smoke (which may also contain UV dye) is fed into the EVAP system and will seep out any place there is a vapor leak.
Most professional grade scan tools allow the EVAP leak self-test and other diagnostic tests to be run so the fault can be found. This type of capability is NOT included in DIY scan tools for safety reasons, so you may have to take your vehicle to a repair shop to fix the problem.