2004 GMC Sierra 1500 4x4 5.3 Heater Not Warming

[Chad_has_an_04_Sierra]

Other than my thermostat being stuck open..

Causes?

[txab]

Plugged heater core

[CamGTP]

Non working blend door is also possible if the coolant temp is a non factor.

[Sharpz]

inspect the HVAC hoses coming in and out of your firewall in the engine compartment (after truck is up to temp) if both hoses are hot then the problem is as the previous posters have said. If one hose is hot, the other is luke warm then the problem is a plugged core.  If both are hot, then most likely it will be as Cam said, although I havent heard of this happening on the heat side, only the AC side.  You can move your climate control down 1/4 the way and see if that gets you heat.  This method works for the dual climate control on AC setting, so might work for the heat setting too.