Fuel gauge is on empty and the low fuel light is on, but the tank is full

depending on your year, make and model. most are located in the fuel tank with the electric fuel pump.

need to replace the sensor

If the trouble with the gauge started after you worked on it then you likely damaged or created a restriction to the movement of the sending unit. You will have to remove the tank and double-check your work.

The problem is in the fuel sender unit in the tank. It sounds like the float part of the sender unit is binding when it's at the top (when tank is full) and hangs up there till it falls back down when tank is near empty. It will likely have to come out to be checked, fixed, or replaced. Good luck.

You have a Bad sending unit which in most cases is located in the fuel tank which is a electricial switch that sends the level of fuel in your tank to your gauge.

Bad fuel sending unit.

Sounds like the sending unit in the tank is bad.

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.

Youre gauge is controlled by a plunger in the fuel tank which is attached to the sensor and the fuel gauge. it sounds like when the plunger goes down half way its either getting stuck down lower then it should. Needs replacing. Any mechanic can do this install for you.

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