I would first recommend checking the connector that plugs into the fuel pump module on top of the fuel tank. If there are any signs of corrosion, heat, or damaged terminals replace this for sure. Gm has had a real problem with these causing problems with the fuel pump and the fuel level. This system works on a 5 volt reference so you may even want to verify that you do have 5 volts at the black wire as the sensor reference wire, and you may also want to check for erratic readings on the signal wire (purple with white stripe) If this reading is erratic, you may still have a faulty fuel level sensor. From here testing gets tough as we must verify that the signal is getting to the vehicle control module. We must also verify that the connections at the vehicle control module are clean. There is a real good change that this could be a problem with the instrument panel, vehicle control module, or any of the associated wiring, but at this point we need further diagnosis to be certain. If you can do these tests i can further assist you. Here is how this system is wired. Feel free to contact me with any questions or problems and I will further assist you! Good luck!