Q&A

The rear Oxygen Sensor ( the one at the end of the CAT ) does not have anything to do with starting the engine. It is only for checking the efficiency of the Catalytic Converter. ( usually the test is run at highway speeds after the vehicle is warned up ) Are you getting a Check Engine Light that indicates that the rear Oxygen Sensor was setting a fault code? The rear Oxygen Sensor should read at about .700-750 volts much of the time at highway speeds. It is not supposed to go up and down like the front Oxygen Sensor. As far as long cranking, I have replaced a lot of fuel pumps for this reason because the pressure holding check valve wears out and it takes about 7-10 full cranks before system pressure is built up enough to feed the injectors. One way to test this is to see if the car takes longer to crank as it sits for 1/2 hour, then 1 hour, then 1.5 hours etc or have a technician put a fuel pressure gauge and then see if the rest pressure goes way down. It should hold within 10 psi of system pressure for 24 hours. Hope this info is a help, if not, email back and I will respond.

The fact that your rear Oxygen sensor is going back and forth does not necessarily mean that it has a problem. Can you recall the specific code? Some Jeeps use a different Oxygen sensor system that uses a 5 volt scale. Was the 4.5 volt reading from a scan tool or a digital volt meter? And, always be sure that the scale is absolutely correct because 4.5 could be .45 which is a very normal reading. If it's going back and forth too much, it can mean that the CAT is getting 'tired'. The rear Oxygen sensor should move VERY slow and NOT mirror the front, or it means that the CAT is weak. This is the purpose of the rear O2, to test the Cerium bed of the CAT ( which stores Oxygen to burn up the pollutants ). The code P0420 is a code for the CAT getting weak though many people Misinterpret it to mean that the rear O2 is bad. Let me know what you find

5 volt Oxygen Sensors typically read between .5 to 4.5 volts. The PCM 5 volt supply will only show 5 volts when the sensor is disconnected.