My check fuel cap light is on but i see nothing wrong with it. I made sure it was on tight. it is a 2005 ford taurus

Because you, or a technician should check the engine. There would be anything wrong, or else the light would not stay on.

Sorry, but there is no "sensor at the fuel cap". Therefore, there is no code that says "sensor at the fuel cap". There is, however, a Diagnostic Test Code (DTC) That indicates a possible problem with the fuel cap.
(HINT: When you want information on codes, it is usually wise to include the code number that you are getting - that way, it can be looked up, instead of GUESSING at what the problem is from a code description that some generic code reader is telling you..)
DTC P0457 is defined by SAE J2012 as "Evaporative Emission System Leak Detected (fuel cap loose/off)"

At this point, I am ASSUMING this is the code you got.

This DTC is set when the Powertrain Control Module (PCM) detects that there is an in-tank pressure drop that is more than 7in. H2O within 30 seconds directly after refueling, or there is excessive purge (fuel Vapor) flow more than .06 lbs. per minute. Possible causes are:
> Fuel filler cap not installed after refueling
> Fuel filler cap loose or cross-threaded

There are other malfunctions that can also cause this code to set, but it will be accompanied by other codes indicating other possible failures.
If there are no other Evaporative Emissions System codes present, then there is most likely a problem with the way your fuel cap was put on the last time you got fuel.

Check your fuel cap and make sure it is on correctly and is tight.

Make sure your fuel cap is on and tight. Other than that you might just want to have a shop scan the computer before just guessing and throwing parts at it.

THIS IS NOT A SOLUTION BUT I CAN SAY IT'S TYPICAL FOR TAURUS. I'VE BEEN DRIVING FOR THREE YRS WITH MY DASH LIT UP LIKE A CHRISTMAS TREE AND NOTHING GOES WRONG. I KNOW "MANY" OTHERS THAT HAVE THE SAME PROBLEM. GAS CAP CAN CAUSE CHECK ENGINE LIGHT TO COME ON IN OLDER MODELS. PROBABLY WHY THEY MADE A MORE SPECIFIC ONE.
GOOD LUCK...I COULD NEVER FIND A REASON.

this is an evap code. and most of the time it's a faulty gas cap or the gas cap is not tight enough try putting some vasoline on the gas caps rubber seal and tighten it at least 3 clicks. if the light does not go out. take it to a repair shop that can do a smoke test to find the evap leak

the loose fuel cap can cause what the motor calls an emmisions leak, which will cause the check engine light to appear, if you have checked the cap and it is tight and the problem with the lights persist, there could be a vacume leak on the motor causing the same issue, some vehicals see the small vacume leak and just assume it is their fuel cap.

Sounds like a compression leak from one or more injectors into the fuel system. I am guessing that you already changed both the frame and under hood fuel filters in the last 15,000 miles? Be sure the cap on the frame fuel filter is tight, any gap between the cap and housing can cause fuel to return to the tank instead of going filtered o the engine.

you will need to replace the fuel cap. The light doesnt tell you the cap is on or off. What itis doing is measuring the pressure in your tank. If it is not in a certain range then the light comes on.

P0300 maens random multiple cylinder misfire.
When was the last time the car was tuned.
Check plugs and plug leads are O.K
also check that fuel cap is on tight.

I don't think this will help but I'm willing to share my experience. My gas cap light stays on all the time. I was told to buy a new gas cap. (from different stores I bought 5). Everytime I would replace the cap the light would go away for a day or 2 but returned and is still on. The Dealership said it has been happening but nothing to worry about. I have had no troubles because of it, just annoying.