Im thinking the second code for hvac is air conditioning or heating problem. As HVAC stands for heating, ventilation and air conditioning. Its what I do... I work for the TRANE CO. one of the worlds largest mfg. of commercial and residential hvac equipment. Sounds like maybe theres a problem with the dampner that seperates the hot deck\cold deck for the climate control in your car. Maybe it modulates to control correct temperature in the cabin dependent on setpoint.