Dreaded P0155 CEL code
 

2000 XJ here. I had the P0155 CEL code a while back (bank 2 upstream O2 sensor), so I swapped the sensor out for a Bosch and the code went away for a couple of days then came back. I read how Bosch don't heat up fast enough for the Cherokee PCM, so I changed it out for an NTK. I also changed the front bank 1 upstream sensor while i was at it. Again, code went away for a day or two, then sure enough it's back. What the hell is causing it to keep coming back? I reset the PCM each time, and the fuses are all OK. Jeep runs fine.

thanks guys

I would think that if the O2S fuse in the PDC were blown all the O2S heaters would be inop, and if the Upstream O2S Heater Relay was bad both upstream O2S heaters would be inop.

This sort of narrows it down to that one O2S.

You can check to see if you are getting voltage to the O2S heater by detaching the O2S connector and reading in the body harness end, from the Dark Green wire pin to the Black wire pin. With the engine running, you should see battery voltage (I think minimum is 11 Volts). If not try going from the Dark Green wire pin to chasiss ground. If you get battery voltage now the ground circuit (Black wire) is open. If you still don't get battery voltage swap the two O2S heater relays and try it again (why not?). Still nothing? You'll have to try and find the open circuit, + or -.

Well I put up with the CEL for a while, but was motivated to investigate further last weekend. I measured the voltage across the dark green and black wires in the plug and found less than 12 volts running. A little on the low side. I read somewhere else about grounds appearing to look OK, but not being solid, so I took the ground (C107 behind the alternator on the engine block) apart, cleaned it all up, and put it back together nice and tight.
Happy to report that the P0155 has gone, and has not returned. Looks like a ground issue. :thumbup1: