Ever since my gas gauge hit 1/2 a tank since last fill-up, when my car idles around 500 at a stop light the gauge will fluctuate. I have to put car in neutral to stop is from happening, as well I noticed my MPG is way off on this tank of gas.
If this doesn’t correct itself you may need to replace the fuel level sending unit. The fuel gauge sending unit is attached to the side of the fuel pump module. It is located inside the fuel tank. Removal of the fuel tank is required.