Have a 2000 Impala with first 2 speeds on fan swith not working.
Gas gauge keeps moving to empty even when there is gas in it, then jumps back to proper level. Anyone know a fix for this.

 

Sorry, Can't help you with the fan switch.
As far as the fuel gauge my 2000 had the same problem. There is a sensor on the outside of the fuel pump that sends a signal to the gauge on the dash. I guess being on the outside and in the fuel it wears down. I'm not sure.
Anyway, it's easy to fix. I ordered the FUEL LEVEL SENSOR from a website called "Rockauto.com" It was about a $100. Get a chiltons book or something similar to follow. I do not remeber the exact details of it.

1) Relieve the fuel pressure from the engine. There should be a schrader (bicycle tire like valve) on the fuel rail. Use a fuel pressure gauge to do it or cover the engine and use something to push it in. The fuel will shoot far and high so stay clear and keep kids away. Clean any off of the engine.
2) open the gas cap.
3) Remove the trunk lining. In the back of the trunk there is a access panel that you unbolt and the pump will pop up. Again plenty of rags. It may be easier thru the back seat.
4) The gauge is on the pump. It's an armwith a float.Follow the directions to remove it. It wasn't that hard and attach the new one.
5) then reinstall the fuel pump.