The problem Im having is that the dash temperature gauge is not moving. This happened fom one day to another, & Im affraid that if it is the temp sending unit the engine will overheat.
I got headders & changed a while ago to the jet temp sending unit.
has anyone heard of this before?

 

Temp sensor in driver's side head is the gauge sensor. The Jet sensor is (I assume) for the fan control and goes somewhere else so if you put it in place of the gauge sensor the gauge won't work. Or check the wire from the gauge sensor as those headers can burn through the insulation and short it out.