I have checked power & ground to gauges,w/temp works,yel/blk wire is good from gauge to sending unit,swapped out sending unit. With key on gauge sweeps to 140lbs,with engine started gauge slowly drops to below zero and stays there. Is it the gauge?? and if so,can I change only the oil press side?? Appreciate any advise on this problem,Thanks,Aloha from Honolulu