Gas gauge filled up gas tank and low fuel level light comes on and gauge reads empty

Check and or test fuel gauge sending unit and the gauge . Videos on youtube showing how to . Also i would have the vehicle checked for DTC's - diagnostic trouble codes . You may want to think about taking to a qualified repair shop .

Fuel Gage
The instrument panel cluster (IPC) displays the fuel level as determined by the engine control module (ECM). The IPC receives a serial data message from the BCM indicating the fuel level percent. The fuel gage defaults to empty if:
• The ECM detects a malfunction in the fuel level sensor circuit.
• The IPC detects a loss of serial data communications with the BCM.
• The BCM detects a loss of serial data communications with the ECM.

DTC P0461
The fuel level sender changes resistance based on fuel level. The powertrain control module (PCM) monitors the signal circuit of the fuel level sender in order to determine fuel level. When the fuel tank is full, the sender resistance is low and the PCM senses a low signal voltage. When the fuel tank is empty, the sender resistance is high and the PCM senses a high signal voltage. The PCM uses the signal circuit of the fuel level sender in order to calculate the total remaining fuel percent in the tank. The PCM sends the fuel level percent via the class 2 serial data circuit to the instrument cluster in order to control the fuel gage. The fuel level information is also used for misfire and evaporative emission (EVAP) diagnostics.

DTC P0462 Fuel Level Sensor Circuit Low Voltage
DTC P0463 Fuel Level Sensor Circuit High Voltage

I suggest you read the vehicle owner's handbook to be certain what the exact function of the light is. You will need to know that in order to determine whether the light is warning you or is broken.

The fuel light usually warns the fuel level in the tank is low...

I'm not sure I am understanding your question. The low Fuel indicator light comes on, depending on how sensitive it is set, when you have between 2 and 5 gallons of gas left in the tank. This also depends on how accurate your fuel level sending unit is reading the fuel level in the tank. The low fuel level light depends on the level of fuel in the tank itself but other than using that sensor it has nothing to do with the fuel gauge itself. The fuel gauge may or may not be accurately showing the fuel level.

That's got nothing to do w the O2 sensor. The float on the gas tank is a good start w the gauge issue .

very well sounds like the sending unit in the tank. Try running some octane booster and fuel injection cleaner in tank of the truck to loosen any deposits on the float.

It is most likely your fuel level sending unit has gone bad. It sits inside your fuel tank to tell the fuel guage how much gas you have in the tank.

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

You will need to replace the fuel level sensor which is in the fuel tank.They have a lot of problems because the metal fingers on the sensor wear off and gives you the incorrect readings.This will also cause the check engine light to come on.You will probably have a level sensor code.This will be a lot easier if the fuel level is down to about a quarter tank or less.Also you should be able to get just the level sensor and not have to get the complete fuel pump module.Good luck.

open circuit in the fuel level sensor. mounted in tank. as the fuel level in the tank starts to drop the arm will move past the dead spot on the sensor and the gauge will start reading again.i dont recommend diy replacement of the fuel level sender.

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