This fell loose by the core support (in front of radiator) after I took off the plastic splash shield to gain access to the lower rad hose.
I'd like to know what it is and more importantly where it exactly snaps into a small hole. It has a plastic push pin for mounting.
TIA

 

Outside air temp sensor.
You don't say what year HHR. Mounts in different places depending on year.

 

This is what Mitchell says for 2011. It was on the driver side between the bumper cover and the radiator on my 2011.

Any hole that it fits is fine, as long as it can read the ambient temp.