Gas gauge reading empty even when tank is full

You will need to test a number of things, the fuel gauge is part of a rather simple circuit.

Battery -> gauge -> tank sensor -> chassis ground to battery

If the circuit is broken at any of the steps, your gauge will read "empty". If the tank sensor is bad, it could read empty, or if the float in the tank has sunk, it will read empty.

I start at the gauge with a "test light" that glows when it sees battery voltage. Get a light at the connection to the gauge? Good. find the sensor connection, usually on top of the tank, accessible in the trunk. Probe with test light. Light on? Good. Connect wire to ground, gauge should read "full". If all that is as I said, then the tank sensor is bad. If you don't read battery at any step, you know where the problem is.

Or bad float !!

when the tank is full the resistance is low so the power through the gage make is read full
as the tank empties , the resistance increases so the less current means the gauge reads less until empty there is little or no power , the gauge reads empty
simple check , turn the ignition off, no power , no gauge reading

2005 PT cruiser, i replaced the cam sensor with a aft market sensor my fuel gauge started reading wrong, and it didn't fix my problem so i replaced the cam sensor with a factory sensor it fix my running problem but the fuel gauge still reads wrong, with a full tank of gas the gauge reads 3/4 tank,

take the sender unit out and check the float ,sounds like its full of petrol and not floating giving a reading of empty

You have a bad fuel level sensor.The fuel tank needs to come out and the sensor replaced.You don`t have to replace the pump assembly.You can get the level sensor separate.I`m not sure if the parts stores have it but I know the dealerships do.Hope this helps.Good luck.

Could be a
1. Fuse - not likely, I'd expect other instrument panel problems.
2. Broken wire from fuel gauge
3. Bad fuel sender in fuel tank. Normally requires dropping the tank.

Vehicle is not that old, would think it is still under warranty?

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

khooper, in another forum, told me that "Chevron Techron Fuel System Treatment Plus can remove harmful sulfur deposits from the sending unit and a filmer agent that can prevent the deposition of said sulfur"

My 05 Trailblazer LT 2WD.
Erratic readings, and when the fuel level was below 1/2 full, the needle would sit on "Empty" with the amber light staying on.

So I went to a dealer, they replaced the fuel level sensor (p/n 19177712 SEN KIT 3.107 $118.62) with labor $270.00 for a total of $398.86 this appears to have fixed it for now.

I also talked to another mechanic who blames ethanol for these deposits !

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