Hello! i have just drove from Louisville, KY to Corning, NY (640 miles) and parked it. i did not start it for the next three days, and now the HHR has developed a rough idle. I already had codes for the pre-cat O2 sensor and a EVAP leak(p0455).
after hearing the rough idle, i pulled codes and got the random misfires code along with the others (p0300). my first though was to change the O2 sensor like i should have done before i left. i did that to no change. My next thought was to change the spark plugs, as it was about that time, but again no change. as of now i am on my way to pull a new coil pack as that is my next best guess. is there anything i should test for or try that i am not thinking of?

it is to note that the car is a 2006 2.2l, 193k miles with a recent timing chain job done on it.

 

P0300 is unidentified misfire, not random. What is the O2 sensor code? I would be chasing the EVAP issue first.

 

Guessing with new parts can get expensive, what plugs did you install? What O2 sensor was installed, brand names and part numbers help.
any loose wires corrosion on the ground wire connections, how old is the battery, have you load tested it lately?
MAF connection tight?

 

i cleared codes and the O2 sensor code has not come back. I used a "universal" bosch o2 sensor 15741. i then installed 4 bosch 96301 plugs. i load tested 2 months ago, and it said it was alright. MAF is secured. i pulled a coil pack from a 05 malibu and it is now running good. the only thing left to figure out is why my fuel economy dropped.

 

Should have installed AC Delco 41-103 iridium plugs, the Bosch plugs are also iridium.
the O2 sensor isn’t listed for the HHR , could be the issue

https://www.rockauto.com/en/catalog/...2)+sensor,5132

 

Keep throwing money at it; someday it will respect you. {sarcasm}

I don't know what this means Without code NUMBERS we can't help.
Clearing codes does not fix anything except turn the CEL off. Most codes will clear themselves. If the CEL is not on most codes just tell you history.