I have a 2007 chevy HHR LT he has had a misfire on cylinders 2&3 for quite some time now. It only happens when I hit the gas. The coil packs/spark plugs/wires have all been changed numerous times and it keeps happening. Has anyone else out there had this problem? How did you fix it?

 

I'd perform a running compression test on it.

 

Which engine, 2.2 or 2.4?

 

Could be head gasket, fuel injectors, valves, rings. Yes, a compression test is in order.

I've never performed a running compression test, do these engines start and run on just 3 cylinders?

 

I recommend just a good old compression test.
check the fuel injectors for signs of fuel leaks

 

Do you mean there are codes? If so what specific numbers, like PXXXX?