2009 Chevy impala heater problem

IMO the most likely causes are either a defective positioning switch or a bad mixer solenoid. I'm assuming that you have separate controls for driver and passenger ... if you do then the position switch has both electrical and vacuum functions ,,, in some areas the heat/cold over the years causes the position switch to fail ... That's easy compared to changing the vacuum positioner .. on some GM's you have to be very flexible to even see it ...(more than once I had to change the actuator by feel because I couldn't see it)