The #2 O2 sensor monitors the health of the cat. If you cored the cat you wasted money on the sensor since it now has nothing to monitor.

So, what happened to the P0300? Any other codes present now?
What about oil consumption? Did you fix whatever killed the cat?

 

P0420: Cat below threshold (gutted cat).
P0128: thermostat (about to be replaced)P0300 went away on its own.
Theonly current codes are the ones listed above (P0420 & P0128).
No oil consumption yet.
CAT started breaking up after nearly 150K miles and destroyed the O2 sensor. The old spark plugs were bad and were replaced.

 

The correct ACDelco for an 06, 2.2L. I forget the number, but it's the correct plugs. I'm aware of the varieties available and there was some discussion in the other thread with a consensus being I installed the correct ones.

 

Cats don't go bad for no reason. There should have been codes long before it died.

 

There's a first for everything.
I got a code for the downstream O2 sensor and replaced it within a week. The sensor was bent sideways by pieces from the cat breaking apart.
I'm unsure why your focusing on the cat.
Am I missing something from which I could learn?

 

I have to ask.
Doesn't a random misfire sometimes come a lean air/fuel ratio. Like a vacuum leak ?

 

Threads merged.

 

dammit, I hate when that happens. I was writing a dissertation while you were merging; I think my long involved post is in the ozone now.

 

This is a short version:

I am not focusing on the cat. I am focusing on the reason for the failure; if it was not fixed then it is still there.

For the cat to break into pieces it takes a physical blow, on an HHR it is pretty much out of the way of road hazards.

From Mitchell OnDemand:

The IC is peculiar to the 2006. The igniition is a "wasted spark" type where 2 plugs spark each cycle regardless if it is on the power stroke or the exhaust stroke. It sense which one is igniting anything by the resistance of the plug.
The P0300 means The ECM has sensed a condition that is consistent with a misfire, but can't identify which cylinder. NOT random misfire.
This is the reason for relearning crankshaft sensors on new engines, even if you don't get a code.

 

Thank you for the education.
The shuddering nearly disappears (once a minute) with the fuel injector cleaner in use after a jaunt on the Interstate.
I will post an update once the thermostat and coolant have been replaced.
If the thermostat doesn't address the issue, it seems an ignition coil would be the next thing to check, correct?