I have a 2004 XLT X-Cab 5.4 4x2 . The check fuel cap message kept coming up and then ceck engine light came on . Autozone checked and cleared light ,said to make cap click several times for a good seal. Message came back on so I bought a new cap from Ford and had AZ clear it again. After about 3 weeks the check engine light came back on with codes of PO 455 and PO 457. The Autozone clerk said that the first code(PO 455) showed a large emission leak and the other(PO 457) was the fuel cap. The truck runs great with good gas mileage ;what is the cause of the codes and how do I correct them?

 

Check the fuel filler neck for any damage.

Observe whether your fuel gauge is indicating properly.

If nothing relevent is observed, take it to a good independent shop or a dealership that has a smoke machine and a technician who knows how to use it.

Steve

 

I had a similar problem. Turned out to be a cracked charcoal canister (after two new gas caps).