2001 Blazer heater

The heater suddenly does not heat. Blows cold air until driven for ten or twelve miles, then only very slightly warm. The temp gauge comes up to where it usually runs, radiator full, and coolant resevoir up to full mark. I did have a code once, said the coolant temp not up to normal, but the heater worked fine. The temp gauge came up to temp in the normal time it usually takes. Thanks for any info.

Auto matic or manual heater controls.?
Any recent work done on it or a dead or battery disconnect?

Are both heater hose hot to the touch?
Does moving tempt setting from hot to cold and back change heat any?
Does controls switch from defrost to vent and to floor heat?
Any check engine lights or flashing AC control lights?
Good engine vacuum to heater controls?