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Discussion Starter #1
So I'm narrowing down my O2 sensor issues. Here's the story:

  1. Bought Jeep knowing I had work to do including fixing the CEL, codes were all 4 O2 sensor heater circuit low.
  2. Replaced PCM with new (remanufactured) one from Solo Auto
    This left me with one O2 Sensor Voltage High code for the driver's side downstream - the one O2 sensor that had not been replaced​
  3. Replaced this O2 sensor with a new one (Bosch) because the other 3 were all new
    This then left me with the same code (heater voltage high), but also curiously occasionally threw a voltage low for the passenger side downstream​
  4. Checking all the sensors, I found that the three that had previously been replaced were all NTK. I switched out the passenger side NTK with the old Bosch, and also tried switching sides with the various different combinations of brand. I always got voltage low with NTK and high with Bosch when they were combined, with both Bosch I got both circuits low.
    This makes me wonder if I have to ensure that all the O2 sensors are the same brand so that they all have the same drain on the heater circuits - seems a bit sensitive if so, but at 4:30 today I'll have my sweaty paws on a new NTK sensor to match the other 3 and we'll see if that does the job.​
Anyway, any recommendations on O2 Sensor brand that's reliable? Is it surprising that the Jeep is so sensitive to brand?

Update to follow later.

Ian
 

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Discussion Starter #2
So I'm narrowing down my O2 sensor issues. Here's the story:

  1. Bought Jeep knowing I had work to do including fixing the CEL, codes were all 4 O2 sensor heater circuit low.
  2. Replaced PCM with new (remanufactured) one from Solo Auto
    This left me with one O2 Sensor Voltage High code for the driver's side downstream - the one O2 sensor that had not been replaced​
  3. Replaced this O2 sensor with a new one (Bosch) because the other 3 were all new
    This then left me with the same code (heater voltage high), but also curiously occasionally threw a voltage low for the passenger side downstream​
  4. Checking all the sensors, I found that the three that had previously been replaced were all NTK. I switched out the passenger side NTK with the old Bosch, and also tried switching sides with the various different combinations of brand. I always got voltage low with NTK and high with Bosch when they were combined, with both Bosch I got both circuits low.
    This makes me wonder if I have to ensure that all the O2 sensors are the same brand so that they all have the same drain on the heater circuits - seems a bit sensitive if so, but at 4:30 today I'll have my sweaty paws on a new NTK sensor to match the other 3 and we'll see if that does the job.​
Anyway, any recommendations on O2 Sensor brand that's reliable? Is it surprising that the Jeep is so sensitive to brand?

Update to follow later.

Ian
Well bugger. I popped the new NTK O2 Sensor into the Jeep tonight, and now that new one is throwing the heater circuit high code, regardless of which side I install it into (I tried both). I am beginning to get a bit frustrated by this process.

Any ideas as to what to try next?

Ian
 

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Discussion Starter #3
Scored - I got rid of the CEL. It turns out that the heater circuit high code on the new O2 sensor was actually being caused by the other side downstream sensor having failed open. I pulled both O2 sensors and used my trusty multi-meter to check the resistance of the new O2 sensor, the Bosch sensor I had pulled earlier, and the much newer NTK sensor I assumed was good. Both the new NTK and the older Bosch were both showing between 4 and 6 ohms across the heater circuit. The NTK that had been installed was showing an open circuit. I put the old Bosch and the new NTK back in the Jeep and lo-and-behold, no CEL.

This article was very helpful: P0038 – Heated oxygen sensor (HO2S) 2, bank 1, heater control -circuit high – OBD2 Trouble Codes

This has been quite the journey. I need to complete the drive cycle to get my code reader to tell me it's ready to be smogged, then I'm running off to get that certificate before something else goes pear-shaped.

Ian
 

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Discussion Starter #5
I don't understand why the other side o2 sensor caused the code.....


My theory is that because it had failed closed it was dragging the voltage down across both and the PCM was then non-deterministic in terms of which it thought had failed.


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