I have a 2005 GMC Envoy and recently every time I fill up the gas tank, it shows that it is full and then drops down to empty and then will keep bouncing around to half full, full, empty. This has happened the last two times that I have filled up. Any ideas/suggestions? It just started after my last oil change.

Most likely fuel level sensor which is located in fue tank.

the float is punctured

The new design filler neck on newer gm vehicles don't have a flap. The flap was just to keep stuff from falling in. I've had problems with the fuel sending unit in the tank on these vehicles.

The oxygen sensor has nothing to do with the fuel gauge. There are two possibilities for the gas gauge error, first it could be the in-tank gas level sensor or it could be a defective stepper motor which drives the gauge. A professional scan tool can determine the fuel level being sent by the in-tank sensor, if that disagrees with the gauge, then the stepper motor needs replacing.

u need to change this gauge or try to dissasseble and clean it very well then try to install it back again

The sending unit in your tank is bad replace it.

You will need to remove the fuel tank from the truck and replace the fuel pump/fuel level sensor module that is inside the fuel tank. Use extreme caution when doing this as there is a real fire hazard.

Sending unit. Its all one unit including fuel pump, float, ect. The check engine light is because of the high or low resistance in the unit. Let me know if you have already fixed.. You have to drop the tank to get to it also.

most likely a fuel sending unit problem in the tank. could also be problem in dash but sending unit is first suspect

The sulfur content in the fuel is contaminating the contacts of the sending unit.

Some people have had good luck with some fuel system cleaners solving this problem. The cheap stuff doesn't seem to do the job.

Not that GM isn't completely exonerated from fault but the gas stations don't do a very good job of watching what the stations sell.