No Heater!?

So the other day I was driving around and realized that the engine was at full operating temp. and the the heater was blowing out cold air? I have plenty of coolant and the heater core doesnt seem to show any signs of wear. Any ideas on what this could be?? today when I drove it it seemed like it wanted to blow some hot air, but didn't really do it enough to warm the cab.:banghead:

Might need to flush the whole cooling system including the heater core.

Check both heater hoses where they go in the firewall with the truck at normal temp. They should both be about the same temp as the upper radiator hose. If one's hot and the other is not, the core is probably plugged. If both are hot it's probably a problem with the temp control door. Be nice to know year and engine?

I have a 99' with a 4.8 ltr. but im gonna have to try and figure something out quick, gets kind of cold driving here in Michigan in the mornings.

the temp control knob runs off of vacuum lines. might have a vacuum leak goin to/from the knob which isn't opening the temp regulator valve. had the same problem on a 99 s-10 and a vacuum line had come off

I wonder if thats it....I know recently one day it worked really good, now its workin at like 65%, lol. I dont know what causing this, but its making me mad.