I got a 2003 F150 and I've been having a problem with the fuel cap light coming on lately, the fuel cap is tight but the light will still come on a couple of times per week, anyone else have this problem, and /or does anyone know where to start looking for problems, I haven't started lookin but I figure this light must work off of gas tank vacuum or something, any ideas?

 

I would start by checking the entire filler neck.

 

On my '01 SCrew, the gas cap did not pass EPA inspection at the 3 year mark, but I did not have a DTC / SES. The '01 did not have the gas cap light in the instrument cluster layout, like the '03 did. This was the pressure test where it is screwed into the hose, and checked. Dad's '01 RV ( Ford chassis, less miles then my '01 ) passed first time though.

I had to get a new gas cap ( ~ USD$ 28.00 ) and went back, this time it passed .

From reading the '03 owners manual, it seems this is one example that could cause the gas cap light to illuminate, letting too many vapors to vent outside the system. I don't know how little the external venting has to be to only get the gas cap light, and not an SES / DTC.

Got an approximation on the Odometer reading ? I was at ~ 42K miles / 67.5 km at the time this happened to me.

So there is the route to start checking the vent system, or you can try a new gas cap quick, but this could be for not, without a pressure tester like the EPA has ( got me if the dealer has an adapter for the WDS / NGS to do the same thing ). When you have EPA tests in CAN, do they check the gas cap ?
If so, they might test it for you, if you go to the EPA center and ask nice.

 

Yeah, maybe a new gas cap is in order.