I have a 2007 HHR 2.4. A/C works fine then intermittently starts blowing hot air. When this happens I can hear a change in the air coming from the vents and then the air gets hot. I can duplicate this condition by adjusting the temperature knob to about half way to heat. Also, sometimes turning it to full heat and back to full cool it will start cooling properly again. I am thinking the temp controller, [Cool/Heat] is failing causing this. Thanks.

The compressor continues to run when this is occurring.

 

Does the smell of the air change? I'm thinking the evaporator is freezing up. That would be from low freon.

 

Have you inspected or replaced the cabin air filter?

 

I haven't noticed any smells, I will check. Thanks

 

I haven't changed the cabin filter, but will order one. This started all of a sudden.

 

Plausible that the Freon is low, but that would indicate a leak as you explained it happened suddenly.

 

When it starts doing this find a place to stop for 10 to 15 minutes and park where you can see under and if you get a lot of water draining the evaporator is freezing up.