I have a GMP0753 , the ac side runs okay. The heater side dose not, it will blow the fuse, have replaced the main board, and the pressure switch, have step by step verifed the hook up per the supplied print, NOTE pressure switch not in cir unit will run by passing (same) gasvalve will operate.. is the safety side designed to trip the fuse, and if so how do I determin which ros is bad.. please advise... thanks ken