What worked for me was putting the system in heat mode. Windows up, temp high & Fan on high. In my 2007 Coupe, only the Driver's side blew hot air. After you do the above, let the car run for 15-20 minutes. Hopefully the heater Valves in the engine bay will unclog. Then switch your system back to full cool & see if the problem went away. I'll probably do this PM every year or 2 going forward.