Like a bad injector and/or fuel pressure regulator. The CEL may come and go, but the codes can still be in memory. There are 3 varieties of codes; current, pending, and historic. Some need to have consecutive occurrences to light the CEL.

Anyways P0420 means that something killed your cat. If you just replace the cat the new one will get killed.

 

Awesome. Thank you for the information! I'll definitely keep you all updated. Taking it to the dealer to have it checked out, plus have the ignition recall fixed. :)

 

I have had various vehicles (not my HHR) have catalyst codes. I have never replaced the catalyst. The poster above had problem with bad head gasket leaking. My friend had a similar problem with coolant in the engine, replaced the engine (well paid a shop to do it) and waited. The catalyst had been fouled and overwhelmed. She waited long enough and the CEL went off and stayed off. On other cars, cleaning the injectors or "top end" made the code go away. Even a dealer (Subaru) mis-diagnosed this one time. So, in my experience the CEL from a catalytic converter is the effect, not the cause. Fix the underlying problem and leave the catalyst alone until it's clear that it still needs replacement.

Doctuh / Harlan

 

New follow up: I put 4 new injectors in. It has now passed the catalyst test and cleared the permanent code.