I have a 2005 Envoy, the fuel gauge does not work when full, always reads empty. The stereo only works when it is over 100 degrees and dry outside, the warning sounds (i.e. seatbelt, lights etc) are not audible anymore, the speedometer does not work at times, the oil and battery gauge do not work, always on one end or the other and the dashboard lights went out on one side. ARGHHHH.

It's probably your speed sensors or your whole speedometer cluster

Ensure that all of the wire connections are clean, secure and corrosion-free.

2 Check your fuel gauge fuse if the gauge's indicator stays on empty when your turn on the ignition. If the fuse is good, join the sending unit connector to ground. Replace the sending unit if the gauge indictor moves to the full line.

3 Ground the terminal located on the back of the fuel gauge with a jumper wire if the indicator doesn't move. If it does move, consider replacing the wire between the fuel gauge and the sending unit. A poor ground is frequently responsible for fuel gauge problems.

4 Release the wire that connects to the fuel tank's sending unit. Confirm that there's no worn insulation that would permit wire contact to the body or frame. A fuel gauge reading of full generally indicates a problem with the tank unit, not the wiring.

Common problem is for the tiny stepper motor to go out in the back of the guage. Can buy off of E-bay, very cheap.

When testing a fuel guage you should disconnect the wires to it,guage should read over full,ground the float wire,turn off key then turn back on should read empty.You have to cycle the key for guage to move.If still inop the wire is broken in the harness between the tank and the dash.Voltage on the float wire is low.When tested with a test light it will barely light the light and pulsate.This test indicates the wiring is good.

COULD BE THE FUEL SENDER OR YOU HAVE A POOR GROUND POINT IN THE CABIN.
WEIRD AND MULIPLE MALFUNCTIONS ARE USALLY A POOR GROUND.

Common problem on the 03-06 Envoy instrument clusters for the fuel gauge to go out. It's usually caused by one of two main problems, either the stepper motor on the cluster's circuit board is going out or the sending unit on the fuel pump is going out. Check out this blog post about the problem - Envoy fuel gauge problems

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

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.

the float on the fuel pump is stuck or need to be replaced

You don;t say whether the fuel gauge is reading accurately, or whether it's always on empty. If the gauge is always showing empty, it's most likely a problem with the sending unit in the fuel tank. Check the resistance across the two terminals of the sending unit.

On a C900, the fuel sender level unit is separate from the fuel pump and located on the right side top of the tank, accessible by removing the trunk floor lift up panel (the pump is located on the left side top of the tank.
Possible problems are: electrical line off or broken, float not working (it's fairly delicate); or gauge not working.

Test the resistance of the sending unit with the lines disconnected, the resistance should be 2.6 to 3.8 ohms full tank, 63.3 to 67.5 ohms empty tank; and test for voltage across the top of the sender unit, if no voltage, that's likely the problem!

If the gauge is otherwise reading accurately (showing 'full' for a full tank, etc) then the problem is likely a gauge malfunction and can be simply replaced.

Good luck!

Saabreur