I've searched on here for possible causes but I would still appreciate some input.
I have an 01 blazer, with the regular (non-electronic) hvac controls. The heat has been working fine up until the other day. Air blows from the vents, at whatever speed setting, without issue. So it's not a blower problem. When I switch from floor, to defrost, or to dash ventilation, it changes as it should. So no problem there either. I'm neither low on nor losing coolant. Both the inlet and outlet hoses from the heater core are nice and warm once it's up to operating temperature. So how is it there's no heat?

 

I know there are two different control units, I have the same problem with mine. Had all heat loss but everything worked. For me it was the control unit itself, the temperature adjusted on it was the problem. Swapped it out and I had everything back. I dont have the truck with me today, so I cant send a pic of what I'm talking about. Just in case your confused, im talking about the controls under the stereo lol.

I have a brand new one that in storage that i cant use. Willing to get rid of it at shipping costs if you need it.

 

Part in storage

Part in my truck.

the center **** was my problem...

 

Check to see if the blend door actuator has failed. It is an an electronic part that has a gear that can crack and when the electric motor is activated to rotate the axle for the blend door to adjust the temperature, the gear that attaches to the axle will slip on the axle not moving the door.

If an emergency, like the cold of winter, you might be able manually move the door by removing the fan motor control resistors that drop into the top of the heater box and reaching in to move it so you get full heat.


Access panel to back side of heater core and blend door.



Blend door (with arrow), and axle for door (yellow). The heater core is to the right of the yellow line. In the picture, the door is in the position for full heat.

 

Very good! Thanks everyone for the input. I will try these options when I'm off and it's not raining lol. In the meantime I'm driving the 02.
Well done on the responses!