I have a 2006 hhr automatic 4 cyl with a 2.4 and 132,634 miles on it. The code reads were P0420 and P0300. I took it to a shop and he said it needed a catalytic convertor so I paid him 489.00 to install this it ran great for 2 days and then back to the same problem which is Engine light on or flashing,no power, shakes/vibration, trans doesn't want to shift. I have installed new plugs with boots. Please Help thanks in advance Rick

 

I have a 2007 and I had to take out the throttle body and clean it and change the gasket. It was easy and helped! Hope it works out ☺️