My check engine light came on. The dealership said "filler neck leaking causing evaporative leak." The gas cap also says "loose cap will turn on check engine light." Would like to try a new gas cap but do not know how to turn off check engine light to see if that is the problem.

Possible causes
- Missing or loose fuel cap
- Incorrect fuel filler cap used
- Fuel filler cap remains open or fails to close
- Foreign matter caught in fuel filler cap
- EVAP canister or fuel tank leaks
- EVAP system hose leaking
- Fuel tank leaking
What does this mean? Tech notes
The P0455 it's a very common code, because the code is set if the gas cap is loose or missing. Start by tighten the gas cap and clearing the code. In most cases the code can be clear by disconnecting the car battery for about 30 minutes.
What does this mean? When is the code detected?
Evaporative Emission System (EVAP) control system has a very large leak such as fuel filler cap fell off, EVAP control system does not operate properly.
Possible symptoms
- Engine Light ON (or Service Engine Soon Warning Light)
- Possible a noticeable fuel odor caused by the release of fuel vapors
P0455 Mitsubishi Description
This diagnosis detects leaks in the Evaporative Emission System (EVAP) purge line using engine intake manifold vacuum. If pressure does not increase, the Engine Control Module (ECM) will check for leaks in the line between the fuel tank and EVAP canister purge volume control solenoid valve under the following vacuum test condition.
The vacuum cut valve bypass valve is opened to clear the line between the fuel tank and the EVAP canister purge volume control solenoid valve. The EVAP canister vent control valve will then be closed to shut the EVAP purge line off. The EVAP canister purge volume control solenoid valve is opened to depressurize the EVAP purge line using intake manifold vacuum. After this depressurization is implemented, the EVAP canister purge volume control solenoid valve will be closed.

Read more: http://engine-codes.com/p0455_mitsubishi.html#ixzz3NlVyARyX

Change gas cap.

Error messages like this are typically caused by air leaks at one of several key places: (1) at the gas tank filler neck; (2) at the charcoal canister; or (3) one of the larger vacuum hoses under your hood.

A loose hose is probably the most common problem--over time, the plastic nipples to which those hoses attach weaken and can break off. Thus, a hose can look like it's in the right place, but it's not actually creating a seal at one end.

You can try to listen for this with the engine running and the hood open. Do you hear any kind of a "hissing" sound like you might hear when using one of those spittoon-things at the dentist? That's a major vacuum leak, right there. Many leaks aren't noisy enough to hear over the sound of the engine, though, so don't consider an absence of noise a statement that everything is just OK down there.

The problem at the fuel filler neck was endemic to certain GM vehicles, but it appears in other cars, too. Believe it or not, there's a vapor recovery hose that is attached at some point of the fuel filler neck to catch evaporation off the tank. Some cars also have another pickup right at the top of the gas tank, too. If one of these has worked loose or rotted, you'll start getting strange vacuum readings and a check engine light.

Hope this helps. Finding the cause of this problem usually takes more time (and money) than the actual fix.

Good luck!

P0455 means a gross leak detected in the Evaporative Emissions System. Some call this a "gas cap" code, because the most common cause a loose or failed gas cap. But it could be caused by any vapor leak in the fuel tank, filler neck, or anywhere else in the vapor management system. If it isn't a gas cap, you will need tol take it someplace where they have a smoke machine that can find the leak using smoke.

P0496 is not alisted code.
P0455 - Evaporative Emission Control System Leak Detected Gross Leak (fuel vapors)
Symptoms - Engine Light ON (or Service Engine Soon Warning Light)
- Possible a noticeable fuel odor caused by the release of fuel vapors Possible causes - Missing or loose fuel cap
- Incorrect fuel filler cap used
- Fuel filler cap remains open or fails to close
- Foreign matter caught in fuel filler cap
- EVAP canister or fuel tank leaks
- EVAP system hose leaking
- Fuel tank leaking

The technical meaning of the code is Evaporative Emission Control System Malfunction.

P0440 EVAP System No Flow can occur during the following conditions:
Fuel filler cap might be loose

• Leak in the fuel tank, fuel filler neck or fuelsending unit 'O' ring.
• PCM might have failed.
• Purge vapor line might have clogged.
• Purge or vent solenoid power circuit is open .
• Fuel tank pressure sensor is damaged, disconnected or it failed.
• Charcoal canister is filled with fuel or moisture.

P0456 = "Evaporative emission (EVAP) control system leak detected (very small leak)."

Check that there are no other leaks in the gas filler neck or gas tank itself that would cause the gas cap error to cause the check engine light to go on.

A lack of pressure in the tank can cause the check engine light to go on, erroneously signalling that the gas cap is loose or missing.

Good luck on this repair.

Although a fuel cap can cause this error, any leaks in the gas tank including the return line to the evaporative canister, or the connections above and below the filler neck, can cause the problem you describe.

It was a good guess that the gas tank cap caused this problem, but now that you have replaced it, should look for leaks elsewhere down the line.

The evaporative emissions system uses a vaccuum pump to check the fuel system to make sure that fuel vapors cannot escape into the atmosphere. If you leave fuel cap loose the computer will detect it and set a computer code and turn on the check engine light. If the cap is tight and the light still comes on, you may have other problems. Hopefully your vehicle is still under warranty, as evaporative emission system leaks can be very difficult to locate and repair, even for the dealership mechanics.