pull the wire off of the sender on the engine and put it to earth turn on your ignition ............ the gauge should go all the way over to hot ......... if this happens your gauge is ok .....  you then have a knackered sensor ...... replace it !

All i can say to you mate is simple..... if your trying to change "spark plugs" on a diesel engine ya best of forgetting doing the job yourself and taking it to a garage as you havent got a clue.

Diesel engines........... DONT HAVE SPARK PLUGS!