P0420 Catalyst System Efficiency Below Threshold (Bank 1)
I was just wondering if the rear o2 sensor went bad that it will trow this code as well (previous owner replaced it with a after market sensor). I'm not noticing anything that would suggest the CAT is bad but the CEL.

Also would driving it while the CEL on be a problem?

 

it could be the sensor, but its probably the cat. You wouldnt necessarily notice the cat since it doesnt take much to throw this code. it is perfectly fine to drive on this code though. You could try using one of the cel fixes for test pipes, instal that, clear the light, if it returns its the sensor, if not its the cat.