When the car is started the gas gauge goes to full, doing a system check i am guessing, but soon after drops to empty. No matter how much gas is in the car it drops to empty.
My first thought was the fuel sending unit. However....
With the gauge showing full then empty, this is making me think it is not the sending unit. Or i could be totally wrong.
Any help would be appreciated.
Thanks

 

From 2003 to 2006 GM had a problem with the stepper motors in all of the gauges. There was also a recall on it for some of the vehicles. They will probably all go bad eventually. If you can solder then you can buy the stepper motors and do it yourself. The dealer wants about $600.00 for the job. You can also send out your old cluster and have it repaired or have it exchanged.