Denali had cold air blowing from the vents when heat was on. REplaced thermostat flushed rad and heater core and lines. Still blowing cold air. Checked Damper door by the glove box, I can move it manually but just changes the air flow from blowing by your feet to blowing out the defrost vents. I'm wondering where the Temp blend door is and if something is wrong with it?
My car has 87000 miles.
My car has an automatic transmission.
If your engine is not overheating and you’ve flushed the core and coolant correctly, it’s possible that you may have an electrical issue causing the heater to not function correctly. I’d start by checking fuses and relays powering the heater core first before you begin to remove components. If the issue does not clear up after checking electrical, there may be a problem with your heater controls. You might want to purchase a service manual for your 2003 GMC to get specific locations of all control vents before you remove the dash; as it’s very complex under there and can be difficult to repair and replace. If you’d like to have a professional mobile mechanic assist you, contact our team and set up a heater is not working inspection.