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I have a 2006 Saturn Ion, and a few months ago I started getting a warning about my gas cap. Specifically, as I was driving the dash display would beep and say "Check Gas Cap." I bought a new cap to rule out that issue, and I've been diligent about making sure it's screwed on and clicking to be on tightly.

In spite of that the message still pops up every few days, and will eventually throw a check engine light in response.

I live in an area with emissions testing, so I'm trying to figure out if this is a real problem, or simply that I somehow got another bad gas cap. The cap wasn't specifically a Saturn/GM branded cap, but the computer systems at the auto-parts store indicated it was a part compatible with my car.

Is there any way for me to test if it is indeed the gas cap, or find out with any more certainty what the actual problem is? If I need to take it into a mechanic, will they actually be able to help me, or am I libel to end up paying over and over again for uncertain outcomes?

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  • Welcome to Motor Vehicle Maintenance & Repair!
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Nov 17, 2018 at 23:47
  • The fuel system in vehicles does one of two things to check for fuel tank integrity: it either pressurizes it or puts it under vacuum. This is part of the evap system which is for emissions. If there is any leak in the system, be it from the gas cap or whatever, where the system cannot create the pressure or vacuum, it blames it on the gas cap. The leak can come from any place which would normally be sealed, such as a soft line, evap canister, the seal around the intank fuel pump, or the filler neck.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Nov 17, 2018 at 23:58
  • Possible duplicate of Why would a new gas cap cause the "check engine" light to shut off?
    – Solar Mike
    Nov 18, 2018 at 5:41
  • There's air getting into your fuel line and fuel vapor is seeping out, you have a hose that probably came detached, smell for a strong gas odor around your vehicle, happened to my mom's car.
    – user38183
    Nov 18, 2018 at 10:29

3 Answers 3

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The evap test system creates a vacuum in the fuel system in order to check for integrity. The cap check light comes on when there is air leak somewhere in your system - it might be tank or cap or something else. If you have replaced the cap, sometimes new caps don't fit properly and leak some air inside the tank, or you could actually have a hole in the tank, the inlet pipe, or one of the breathers.

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A very common issue on one of my cars. My gas cap has a rubber seal which cracks and causes the light to come on. You said you bought a new cap, was it a universal one or an OEM one?

If that doesn't solve the problem, look into your evap system as suggest by @AsenM. There should a rubber hose that goes from the top of your gas tank to the engine bay.

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I had what sounds like a similar problem. (Codes p0440 P0442 p0445 p0455 P0496 at different times.) Cause was a leak at the top end of the filler neck, where the cap screws on. You can test it by taking the neck off and blowing into the tank end, with the cap end in a bucket of water.

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