I have a 1999 Jeep Grand Cherokee with inline v6 4.0 L ENGINE. Today I will start my jeep and the oil pressure gauge mark 40 The middle oil pressure with the engine off and the key switch in acc. When I start the engine at idle speed the gauge marks 80 the maximun oil pressure. I dont know why he does that the day before the jeep was fine all work excelent. I think that may be the oil pressure sensor is failing but I try to find it and I dont know where it is. please if you have any idea how to repair this let me know thanks

I think sending unit as well.