[wtm442]

If its a mechanical gauge, you would have to replace the whole thing (sender, capillary and gauge).

If its an electrical gauge, it could be a bad sender, bad wire or bad gauge. Check to make sure there is power being supplied to gauge.

Disconnect the wire from the sender, and measure the resistance from the sender to ground when engine is cold. Repeat measurement after driving car for an hour. If the resistance measurement is the same, replace the sender.

If the resistance measurement has changed, check wire for insulation breaks (improper grounding somewhere). Easy to just replace wire. Still have problem, test gauge by varying the resistance to the gauge using a decade box.