Recently purchased a 2004 DTS - wife has let me drive it twice so far. On the dash information screen, I get a "check gas cap" indicator. I've checked it a thousand times and looks okay to me. Any ideas? This usually only comes on after I stop the engine and once I clear information, it goes away and doesn't come back. Any help? - Thanks -

The cap is either not tight against the ratchet or the evap. system thinks the cap is defective.

Check for trouble codes (DTCs).

Also check the rest of the EVAP system for loose vacuum hoses etc. It is a closed system, so if the system detects a leak, like a loose gas cap, it sets the warning. Since a loose cap is the most common cause, that is the default message. As JimD said, DTC's is the first step.

as a service advisor i commonly due get defective
gas caps or faulty leak detection pumps which give the wrong
signal to ecm and then sets off mil:bighead: and or loose cap light if
car is equipped

Like others have said, it's either the cap itself or something to do with the evaportive emissions system. What's happening is that the PCM calls for a solenoid under the hood to open and apply engine vacuum to the evap circuit. Simultaneously, the PCM commands a fresh-air solenoid at the evap canister (under the car) to close. By closing the fresh-air solenoid and drawing engine vacuum on the system, a sensor mounted on the fuel pump hanger should detect a vacuum. If it doesn't, it says to check the gas cap (a logical assumption by the PCM since if the gas cap is off or leaky it won't see vacuum). If it's not the gas cap, the problem can be either of the solenoids, a vacuum line or the sensor itself.

The purpose of the system is to burn fumes from your gas tank in the normal combustion process. The canister stores the fumes until the PCM decides to burn them.

Jim