As far as the fuel gage, there are three possibilities....cluster, fuel sender, or wiring/connection.....
Rare, that it is the cluster.....most obvious is the sender......but there is a connection that goes through the body, just rear of the driver's seat, and comes out the bottom of the body......there are two harnesses that connect to this block....one goes to the fuel tank and the other has ABS connections to the rear wheel speed sensors....
If you disconnect these two harnesses, look for green corrosion in the harness connectors and also in the pins of the block......if present, attempt to clean, and cover with dielectric grease.....in some cases, they are so bad they have to be replaced....if there is no corrosion, the likely culprit is the fuel sender in the gas tank.....
Make sure they tell you, the exact number of the codes.....they will look like P0XXX, for example, P0461, this has to do with the fuel sender circuit.........if there are ABS codes, they will be C0XXX