I've been struggling with various CEL codes since I bought my 2004 Xterra 3.3l. Lately they have been converter/O2 codes.
I replaced the passenger side (bank 1) sensors up and downstream. I bought the cheap Kwiksen ones off of Amazon. The upstream sensor seemed to still have problems so I replaced it with a Bosch sensor. That seemed to help. And after that I also found a vacuum hose that was disconnected and since the the car has run much better.
However today I got a P0037 code (bank 1, downstream...Kwiksen sensor) and while waiting in line at the car wash I noticed that my temp was at 219 degrees! I'm not sure if the two could be related or not. I replaced the thermostat, water pump, hoses and coolant just a few months ago. And the fan seems to be working. I'm not sure what that was about.
Attached is a snapshot of the 02 sensor voltage. What does this say? To me it looks like the Kwiksen sensor (bank 1 sensor 2) is bad. But bank 2 doesn't look right either. Bank 2 almost looks like a bad converter.