I just got the blender motor replaced on my truck but my heater is still not technically working. When the fan is on level 1 the heater is just as hot as it should be but if you turn the fan to 2, 3, or 4 the air is warm for about 5 or 10 seconds and then turns cold. If the truck is in park though it works fine. Is it maybe that my heater core is going out?

 

It sounds like your heater box is loosing vacuum when in gear. Check the one way check valve by the fire wall if it is good, then you will be looking for vacuum leak in the dash. If the servo's loose vacuum then the blend door will move back to cool so it is still producing heat, but the blower motor just isn't blowing it. Let me know if this helps.

 

do you smell coolant? an did they drain the system? maybe the coolant system wasnt bled right. i know mine was doing somewhat of the same thing, and i changed the t-stat and it fixed my error. so maybe thats something you want to look into