FWIW, had one more code clear over the weekend (O2 sensor) after about 100 miles of driving. So I have 3 left:
Cat Converter
Evap System
O2 sensor heater.

I'm not opposed to replacing a thermostat, but what does that have to do with the cat. converter? I'm not making the connection there....

Picked up a set of 02 sensors, figured I'd either replace them on the X5, or use them on my kid's E39 that we're restoring, so that may be my next step. I'll try to get under it in the next day or two and check cat converter temps. I did log the output of all 4 02 sensors for a little bit of driving (and idling) yesterday, and noticed one of the pre-cat sensors was fluctuating a LOT more than the other. Car has about 130,000 miles on it, so they COULD be original, not sure. Figure replacing them isn't going to hurt anything other than possibly my knuckles.

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2001 X5 3.0, 5sp