Gas Gauge

Most likely will have to replace the sender in the fuel tank.

If you go to the trouble of replacing the sender unit you might want to replace the fuel pump at the same time. I brought a used 2000 Seville with a erratic fuel gauge. After six months the fuel pump went, when I replaced the fuel pump the gauge worked fine. The fuel pump is also in the gas tank.

First I would check the signal wire. It could be shorted to ground. Prefered method would be to check continuity of the wire with a multimeter. Disconnect the wire from the tank and measure both the wire to ground and the signal lead on the tank to ground ohms. There should be no reading from the wire to the ground O/L or infinite. If you get a reading the wire is shorting and needs to be fixed or replaced. If the wire is good concentrate on the sensor. GM's generally operate between 0-2 empty and 80 Ohms full. If you have a low reading and fuel in the tank then it's time to pull and replace the sensor. You can also use a 12 volt test light but I recommend the multimeter. The negative lead on the test light is to be connected to the + of the battery (use wire to lenghten and tape all connections to avoid shorts). Test the light on the frame to gauge it's brightness to a short. Check the disconnected sending wire if the light lights up at all check the wire. If the wire is good check the sensor. If the light goes as bright as the test and there is fuel in the tank replace the sender

my 1994 reads out consumption as well as what's in the tank so if you start with 20gal and your consumption says you used 10 gal (but maybe your dash doesn't display consumption) Personally I wouldn't spend a minute of my time nor a dollar of my money on a gas gauge, you must have a pretty good handle on your gas mileage so just set your trip meter (if there is one) or keep an eye on odometer. But that's just curmudgeonly old me.

Follow the suggestions above for checking your ground (usual suspect in many electrical problems), then check your resistance readings as noted above. My guess is that the float in the tank has a pinhole (my personal experience), which has allowed the float to fill with fuel. It therefore no longer 'floats' when the tank is filled. It is down in the 'empty' position, and that is what it reads, regardless of the quantity of fuel in the tank. When you go to the work of dropping the tank, and if you plan to keep the vehicle for a while, replace everything in the tank so you do not have to do it again.

Disconnect the positive battery wire in the front and then reconnect it. Your gauge will work again. No need to spend unnecessary money on a mechanic for him to charge you for something you don't need.