Gas gauge staying at empty when tank is full

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

open curcuit guage will read full. grounded curcuit it will read empty. if wire to sending unit at the tank is broken or corroded. good-day!

The sending unit in the tank provides a ground to the gauge depending on the float level. The gauge can "rest" anywhere if the power and ground circuits are not connected.
You would need to know if the gauge has power and is getting a ground signal from the tank.

Doubt it.
The same fuse for the gauge would handle the rest of the cluster.
Probably the sending unit in the tank.

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 on the 03-06 Trailblazer 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 - Trailblazer fuel gauge problems

I suspect either the sensor was not reconnected (or may have been damaged) when you put eveything back together or the float has sunk. Either way, you will have to do the job over to fix it.

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the problem is caused by a defective gas gauge sending unit, this part is located inside the gas tank, you must remove the gas tank to replace it.

Well that tells me that someone installed the wrong fuel level sensor. Your going to have to drop the tank and pull the pump and float assymbly out. Then go to a parts store and get a sending unit that has the opposite resistance of that sending unit. Just wait until the tank is empty so its a lot lighter. You can call the parts store or go down and see if there is multiple sending units. The sending units work by resistance, when the tank is full the float moves up and turns a wheel in the sending unit, which either removes or increases the resistance causing the gauge to lower. The sending units either give full resistance at full or no resistance at full and then changes as the level goes down until it does the opposite at empty.

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