Hello, I have a 2001 F150 and for some reason my heater does not blow hot air. Now I've read a lot about the heater cores being bad in them and before I go tearing out the dash I was looking for a little more help. The heater when you have it on defrost does blow as cold of air as outside but it is definitely not warm at all! After awhile of driving though I can feel really hot air coming out of the panel vents on the left side of the steering wheel. This makes me think that maybe its not bad but I don't know. I need some help with my problem Please!!

 

Could possibly be your blend door has broke

 

Could be blend door or the blend door auctuator. Best way to find out is remove glove box and carefully cut into your hvac box for inspection hole. There is a you tube video showing where to cut box. I did that and just used the foil hvac tape to repair.
Also make sure both heater hoses are hot. If one is not hot then your core has a blockage.

 

Check the hose's first..