I have a 2003 4.6
I just discovered this morning that my heater doesn't work. (I live in South Florida so the need is few and far between).
The blend air door **** feels good and a visual inspection doesn't come up with anything strange.
Granted, I only use a heater (home, car, ect) about 10 times a year but, it was 50 degrees this morning so there was a need. Nothing but cold air. The AC compressor was not on but it was simply drawing the cold outside air in.
So, is there anything common I should be looking at?

 

Is your (idiot) temp guage reading normal....Usually the shy side of dead centre??
Is your rad & resovour full??
Now be careful!!!!
After it's warmed up slightly touch/grab your 2 heater hoses & see/feel they are both hot....
If they are very hot to the touch you should have circulation!!

 

Temp gauge is good. It was running on the colder side this morning (If center is 3:00, it was at the 4:00 position) but it normally runs just above normal (again, South Florida).
I didn't try the "does it burn?" test on the heater hoses. I will try that this afternoon once the truck warms up.