Engine temperature is really stable, never really looked into that other than a pesky coolant loss. I think my scanner can monitor it real time...

when we looked at the O2 sensor the readings just jumped every now and then, which seemed to imply the sensor was bad. But the sensor was replaced and the fault code remained. Then the sensors were swapped and the code remains, so I was thinking bad connection. But your suggestion makes sense, I'll look into that!