my check engine light has been on for a while it says gas cap but I've change the gas cap three times what could be causing this?

 

Are you using an OEM gas cap? Aftermarket ones sometimes will still throw the light. What is the exact code?

 

Other then the Gascap itself, there could be something else in the EVAP system. Check the canister for cracks. It's above the gastank though, so it might be hard to get to without either lifting the bed, or lowering the tank.

And I also agree, go with the OEM as the aftermarket ones don't always seal correctly. I would start there as it's the cheapest thing.

 

its the purge canister valve

Under the hood left of brake master cylinder on firewall.

About $ 50.00 for OEM and worth every penny