P0036
P0141

Both came on at once on Friday upon coming onto the highway on Friday last week. I immediately looked up the code and bought the REAR o2 sensor (after the cat, closer to back of car?)

Is there any testing I should do before going ahead and replacing the part, or am I supposed to be replacing something different?

This car has been trouble LOL.

 

Basically, your exhaust is hot.

 

Those are both heater circuit DTC's. Check the heater circuit of the sensor in question, check its resistance value and then check the circuit when the vehicle is cold for voltage, should be whatever battery voltage is. If that checks out replace the sensor. I am sure it is just the sensor, though... They go bad on GM's all the time...

 

Based off the codes do you know if it's the sensor close to engine, or after cat?

 

Bank 1 sensor 2 would indicate after the catalytic converter . Hope all goes well in your diagnostics!