Knock sensor?
 

My 2006 HHR check engine light is on, and because of this it will not pass inspection. My brother, who is a mechanic, took the vehicle to his shop (he works for Infiniti) and they determined that it was the knock sensor, but they were unable to locate it. My brother turned the check engine light off but it came back on before I had left the parking lot. Does anyone have any advice on this issue? My husband is fairly mechanical and we would like to replace it ourselves if possible.

This thread should help you out....

https://www.chevyhhr.net/forums/how-tutorial-library-21/how-change-2008-2-2l-knock-sensor-34865/