Those two codes have a common denominator of air leaking near the second O2 sensor.
It could be a clogged catalytic converter.
I’ve gone through the replace one piece at a time, so, if you can get under your HHR ( safely on jack stands) take a 7/8 inch open end wrench and tighten the rear O2 sensor.
If that fails, then you could replace the rear , then the front, then the catalytic converter down pipe, either one at a time , or in one fell swoop.
I wish I knew then what I know now, when I was chasing a P0420 code a year ago.
I would have simply ordered all three parts and be done with it.

 

One of the ways too much oxygen can get into the system is a cracked flex pipe or a hole in the pipe. Is it cracked or rusted through?

 

My Florida born and raised HHR still has all stock exhaust components from header to the tip of the muffler, 220,000 miles and counting.