5 days ago I put a half a tank of gas in my 2005 Mustang.Today when I was driving the check fuel cap light came on.I checked to see if the cap was on tight enough and took it off and put it back on but the check engine light came back on.Anyone know what else could be causing the problem ? Thanks.

 

My money is on a bad seal or a leak somewhere. Next guess would be a sensor.

 

How would I check to see if there is a bad seal and if it is a sensor is it an easy fix ? Thanks

 

easiest way is to hook up a scan tool and see what codes are present. if they're "evap" codes (evaporative emissions faults) then it's likely related to the gas cap or gas tank filler system.

 

the check fuel cap light will set an evap code, this only means that the leak is large enough to be a loose gas cap(it could be anything else that leaks as much), the only real way to see if there is a problem is to command the vent valve closed with a scan tool, and use a smoke machine to pressurize the evap system, this will tell you if it leaks, and if it does where it leaks. anything else is just a guess