Pull the wire/terminal off of the top of the sender terminal & ohm the sender metal body (not the terminal at the top) to the nearby block to see if there is continuity or plug the wire/terminal back on & hold a jumper wire from the block to the body, which is the same deal & see if the gauge straightens out then I would pull the wire/connector back off of the sender & ohm the sender terminal at the top to the block with eng cold & again with it warmed up & see what ohms you get. always a good idea to pull a batt cable before switching the meter to ohms or at least confirm that what you are testing is isolated/unplugged from voltage/battery.