I've had this problem before. My heater was working perfectly fine but the one day it was hot so I turned on my AC and put the temperature to low. Once I turned my heat back on it blew hot for a few seconds then goes cold. The heat blows perfectly hot from the top and bottom vents but not the centeR vents. When I turn it to the "center vents" position after having it on just upper and Lower vents position, it'll blow hot for about 15-25 seconds then go cold again. I had this problem in the summer and it just stopped doing it. What could it be? It seems that whenever I turn the temperature to low it triggers this problem.
My car has 130000 miles.
My car has a manual transmission.
Hello and thank you for contacting YourMechanic. The issue is the blend doors are not adjusting correctly when you adjust the thermostatic switch. Check the wiring in the dash for any loose contacts to the blend doors. Also check the blend doors for any debris trapped in them causing them to stick on the tracks. If you need further assistance troubleshooting your vehicle, then seek out a technician, such as one from YourMechanic, that can assist you with the heater.