The gas guage on my 2003 Bonneville SSEi appears to be malfunctioning. The guage reads full until about a quarter tank and then just reads slightly off full?

Is this likely a sensor problem or presumably the part in the trunk?

The car is still under warranty so I will take it to the dealer and my only hsitation is all the effort required to convince them of the problem!!! No doubt they will proably want to drive the car for a tank before they will be convinced and correct the situation.

Incidently, the amount of fuel used appears to be working properly.

Thanks

The dealer will use a signal generator to diagnose the fuel sender which is in the fuel tank (can be accessed via a porthole in the trunk).
The resistance changes with the sender as the fuel level drops, a piece of debris or loose connection can affect the gage reading.
E 40 to 77ohms
1/4 129 ohms
1/2 165 ohms
3/4 204 ohms
full 233 to 250 ohms
Low fuel indicator On at 100 ohms
Low fuel indicator Off at 106 ohms

The only other thing that it could be is the instrument panel (fuel gage itself) and again the dealer will need the signal generator to diganose.

Mine had to be replaced last spring.
It is a common problem, GM and the dealer are aware of.
You should have no problem getting it fixed.

Thank you both - I will contact my dealer.