2014 International Prostar Heater Issues

I'm willing to bet that somebody here has dealt with this exact issue. Here's your clues:

No hot air blowing from front vents, but bunk vents are working properly.

Coolant level good.

A/C is working throughout truck.

Front blower and mode selection knobs are all doing what they should.

Middle knob, (the hot/cold one), does not change temperature, just stays at ambient air temp.

Now before we start calling out blend door actuators, please specify which one of the 3 you suspect; the one I can probably get to, the one I probably can't get to or the one only the $150 an hour guy at the dealership can get to.

2014 International ProStar
Maxxforce N13.

Looking forward to the replies. Thanks, everyone.

 

I have not, mainly because I didn't know it existed until now. The only valve I was familiar with is the spiggot-type controlling the flow into the heater core, which is open of course. What do I have to remove to find the one you are referring to which must be electric-controlled, right? Fuse possibly?

 

As always I appreciate everyone's help. Crazy thing is, two days later it spang back to life and has been working fine ever since. I've stopped questioning how things like this happen. If only my credit score would fix itself like this...