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. Three things come to mind; sending unit, wiring, and the guage itself. My suggestion would be to find the sending unit, unplug the wiring from it and see if the condition changes( gauge drops from full to zero). If nothing changes you can either trace the wiring back to the guage or test the guage itself inside the cluster. Not sure about the diesels specifically, but the gas VW's do loose the temperature sending units occasionally.