Test the gauge and sender, and assuming it is North American standard
Check for 12V on purple wire at gauge with key ON
With key ON, jump a wire from the Pink wire on gauge to ground
If meter pegs Full need to test sender.
If meter pegs empty, replace gauge
If meter pegs Full, check connections on tank sender. Pink on center post and Ground wire on side of sender
If wires good, jump across Pink and ground wire with key ON
If meter pegs Full replace sender
If meter does not peg Full check ground going to buss of Negative side battery, and Pink wire going to gauge for connections