06 HHR LT throwing P0420 and P0138 codes
 

Long story very short, I have a cheap POS aftermarket catalytic converter on the car, its been throwing a P0420 code for a little while, but my mechanic advised until I need to get an Etest its ok, but I've now got one coming up and was going to have it replaced with a proper OEM part. Today I got a P0138 code thrown as well, just confirming before I spend a ton of money, this IS most likely the cat that's gone, not just the O2 sensor?

P0138 means the #2 O2sensor is either bad or the wire is shorted out. Could be the wire is laying on the exhaust pipe. Could be just a bad O2 sensor for both codes.

You'll be needing a new sensor anyway with a new cat. Might as well give it try. P0420 will not go away by itself it must be cleared.

So is it worth it to just replace the O2 sensor and see what happens, or would I need to replace it again if it turns out I need the cat as well? the sensor is obviously way less expensive, but I'm pretty resigned to having to replace the whole shebang. The P0420 code has been coming back pretty relentlessly, every time I clear it I get maybe 50 kms and it comes back...the P0138 is only in the past day or so.

If I was doing the work myself I would replace the sensor. Then Re use it. Shops will probably not let you Re use parts.

Maybe a $70 sensor will fix everything. It's your bet.