My rear 02 sensor took a hit from some road junk and the CEL came on. I replaced the sensor with one from an online vendor. The CEL went out for a few days. I took the car into the dealer for the thermostat fix and on the way home it came back on again. I went to autozone and here is the code.
H02 1/2 heater circuit P0036

Do you think the replacement was bad? Should I have replaced them as a set? Since I have the one ball I doubt the dealer will fix this.

 

Just because the CEL goes off doesn't mean it's no longer in memory. Did the dealer clear the code while they fixed the thermostat? I would try clearing the code and seeing if it comes back on again, assuming the dealer did not clear it.

 

I had the guy from Auto Zone clear it. Came back on after a few miles.