Heater will only blow cold air. Why will it not get hot? on 1999 Chevrolet Silverado 1500

It got hot last winter but when I tried it this year it will not get hot. Engine temp is running 190 and there is no sign of anything being wrong like leakage from heater coil or anything. Air conditioner works fine and Fan works fine.

Asked by for the 1999 Chevrolet Silverado 1500
I had that problem..took off the hoses and blew compressed air through core in reverse direction as water several times and now it works. Found that on you tube
Any way you could post a link to that video? I have the same problem. Just replaced the blend door actutor under the drivers dash, but now it just blows cold air all around instead of just half the vents. I found that if I cycle power to the system by turning the truck off and back on, that it fixes the issue temporarily.
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core could be clogged with debris. blend door motor may be bad as well. take it to a shop and have them check it for you and give you a bid.