Hi, I’ve been having an issue with my 2007 S550. There are only 22,000 miles on it, so I’m not sure if it’s just old equipment or parts drying up etc. I will be driving and all of a sudden the heat will come blowing out instead of the air. Sometimes it will just be on 1 side, and the other side will still have the air on. Any thoughts on what this may be, and how expensive it could be. Thanks for any insight.

 

Sounds like a typical heater control valve failure. I have replaced plenty of these on 2007 vehicles. Probably due to age....

 

That heater control valve behind the firewall, is probably done, change it before the plastic falls apart and sends coolant everywhere. The part is less than $100 and fairly easy to replace.

 

I feel like I have this same thing happen with my rear AC (S600). It seems to just go on and off randomly. The other day it continued to blow after I had turned off the car for several minutes.