My son purchased a 93 Sedan Deville yesterday and he has no heat. I replaced the thermostat. The upper hose as well as the two heater hoses are now too hot to hold on to, but the temp inside the car does not get above 65 degrees. It appears that the fan speed changes with the change in temperature settings but the air does not get warmer when the setting is set above 65 degrees. For example, I would expect that changing the temp from 65 to 75 would result in an increase in fan speed as well as an increase in the temperature of the air. I can feel that the air temp is not increasing butthe fan speed does increase. Are we suffering from a fan speed problem or a vent door problem? I have access toAlldata information via the library.

 

Check for F codes. Air mix door is behind glove box. Check operation of door. Door is controlled by programmer. When temp is set at 60 or 90 degrees temp sensors are overridden. If you get heat at 90 you could have sensor issue.