When I fill up my 2006 Envoy the gas gauge will register to the full mark as i start to drive the gas gauge drops down to empty. It will rise back up now and then and drop back down to empty. what could be the cause of this?

You could use two resistors, Two different ohms specs for full and empty. Place resistor between proper wire colors in diagram. Turn on the key, If gage reacts properly to full and empty resistors, then I think the problem is the sending unit in gas tank. If not, the problem could be almost anywhere, wiring circuit--pcm--instrument cluster. I did see trouble codes relating to fuel gage sending unit. I hesitate to get into that? good luck

Either you have a bad fuel level sending unit or the stepper motor that drives the fuel gauge is bad. A high end scan tool will be required to determine if it is the sending unit.

the flue sense in your tank is bad most likely but it could also be just a bad connection between it and your flue gauge witch can be caused by corrosion or a lose wire

it sounds like the fuel level sensor in the gas tank is bad,causing a misreading and fluctuation,,replace it and that should help,,

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.

The sending unit in your tank is bad replace it.

the sending unit for the gas gauge is in the fuel tank and part of the fule pump module, it is not sold seperately. price around $350.00, 2 hours labor to install the part. see photo

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

your gas gauge sensor is most likley the problem on alot of new cars they measure your fuel off an average of were your float is at. like if your not moving it stays in one spot but as you move the float moves and reads the lowest amout that it is averaging and if your float keeps reading low your going to read that you have alot less gas than you really do. have you tried filling the tank all the way and see what it does? if it stays at full until you use some gas and your float starts moving your gas gauge will probley go down in that case its the fuel sensor if this dont help let me know and ill c what i can find out for you