What is wrong with my fuel gauge?

I have a 1997 ford explorer v8. The car runs amazing, but i just found out that the fuel gauge doesn't go past half a tank. I put 16 gallons into it and the tank is a 20 gallon tank. What could this be? It took me a week to really notice this. And one more thing, i had my car in the shop to get an oil change, i get it back and the check engin light was on, and i'm pretty sure it wasn't on when i brought it in. Should i bring it back to the shop? The car runs amazing, engin sounds amazing.

The fuel gauge registers the level with a sending unit inside the tank. Over time, yours is a '97 model, those sending units wear out from use. When they do the symptoms are like yours. Two ways to solve the problem, one is to replace the sending unit which involves removing the pump to get at it or… Simpler and less costly… Accept the fact that it doesn't register a full tank but understand that from 1/2 down to "E" its as accurate as its ever been.

As far as the checvk engine light goes it may or may not have anything to do with the oil change. However, since it did come on after they did the work take it back and have them check the codes to see what fault its indicating. It could be many things but if there's a code set you'll at least have an idea if it had anything to do with their work. They will too.

Take it back to the shop and tell the mechanic to check all the electrical wires including the fuel gauge on the dashboard and if nothing seemed to be wrong there then, try checking the gauge that's inside the fuel tank.