Howdy y'all!
I'm leaning towards needing a thermostat, but would like to run it by anyone who might have more knowledge/ experience.

My daughter's 2009 LT, we call her Black Betty, occasionally hesitates to shift 3-4th (I believe) and shudders at about 50 mph until you get to about 52 mph. The check engine light came on and AutoZone shows P0599, P0420 repeating. I imagine my daughter just kept her foot into the accelerator thinking more pedal = more power/acceleration.

AutoZone recommends thermostat replacement. Everything I read seems to suggest the same as most likely solution.

I have read several threads here with one or other of the codes separately but not both. Black Betty did get a new catalytic convertor last summer, so I hope we don't need another. I'm wondering if P0420 is caused by whatever is going on, a symptom, but not the real problem.

Have any of you seen this?