why is my gas gauge stuck on full on my 1999 pontiac grand prix?

wants is your mileage - because your sending unit float is stuck - if you are around 80K your 160k your fuel pump going out just the facts see it all the time
 
You might have a bad sending unit inside the tank. Drop the tank, remove the in-tank assembly, and remove the sending unit inside the tank. Hook up an ohmmeter to the leads of the unit and see if the reading goes up and down when you move the float. If it's stuck at one reading or close to one reading, it is bad and needs to be replaced.
 
You're lucky I saw your question. I had several friends that work on GM vehicles tell me I had to drop the tank too. If you open the trunk and pull out the carpet you'll find an inspection cover. It took me an hour to replace my fuel pump/sending unit assembly. The only set back was the assembly was $375 at my jobber cost. If you don't want to spend the cash right now, start setting the trip odometer and fill the tank every 275 miles to be safe. My pump went out at 143,000 miles so you might want to stretch it till the pump goes.
 
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