O2 sensors ( especially the heated ones) are a funny animal
They are constantly switching, ( toggling if you will) to monitor the changes in the exhaust
We can thank the EPA for this
Any way, it does not take much to foul a reading, this is why IMO once you clear a code it may come back sooner than later
A wise man said think of it this way, it is akin to a fouled spark plug
I hope this makes sense
PS, there is ALWAYS an underlying reason for codes and for their return
I suspect an oil fouling on the rear O2 sensor, that is where I would start AFTER you get the air cleaner a PV done JMO and good luck