When ac is on it drinks fuel like crazy! And ac only works on high everything else is heat. All the settings r heat except max ac. I found a leak on the pressure switch and fixed it. At first there was no heat now reg ac is heat and the thermostat does nothing, settings vent placements work
Have your Freon level checked or performance tested then check for a vacuum leak I tmay be not closeing the door for the heater. Start at engine and go all theway to heater under dash as it may be a bad vacuum motor.