my son told me that it has been going on for awhile but did not think to let me know until the coldest day of the year. anyway he has a 2000 trailhead with a 3.0 and the truck gets to temp fine with new thermastat but the heater takes 15 to 20 minutes to blow warm air. what do i need to check first.

I'm having the same problem; if you get an answer, please forward to me also, OK? Thanks, BMP My A/C works great, blowere motor runs great but unless I close everything, nothing but cold air comes in. It just started recently. I replaced the heater core last year and the temp control gets to the middle and engine is very warm but no warm air comes into the cab? What's going on?