My check-engine light came on. I have a 2003 Ford Ranger 4.0L. I'm new to using the OBD tool.

So I used the tool and was very happy to get the result so easily. It is P0456, which means a very small leak in the evap system.

So I bought a new gas cap, to start out to see if that would fix the problem, since that is the simplest and cheapest first test. I've installed the new gas cap. The check-engine light is still on.

And so, here is my question: If the old gas cap was faulty, and the new one is good, and there no longer is a leak in the evap system, will the check-engine light eventually turn off? And also, will the code go away? Or will the check-engine light stay on until I go and erase the code?

Also, if I erase the code, but there is still a leak in the evap system, will the light come back on? If so, how soon?

  • 1
    I spent a whole lot of time on this conversation. check out my answer to this question The absolutely best practice is to find the Specific On-Board Diagnostic test for your make, model and model year of your vehicle. I was surprised how different these descriptions were. The details for the Onboard-Diagnostic test are listed in the factory service manual. Take the time to find the correct reference.
    – zipzit
    Dec 4, 2019 at 5:35

1 Answer 1


If the problem persists then yes it will come back on. Those type of fault codes are usually not instant tho so it may take some time to see it again.

After fixing the gas cap the code may clear itself without a scanner but it usually takes a few cycles or x amount of miles driven.

  • 1
    If the EVAP is the code and the gas cap is the problem, then you are exactly right in the dash light will disappear (correct itself). The code itself may remain in the computer but not set the CEL. Good answer ... +1 Dec 4, 2019 at 0:41

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