00' jimmy v6 no heat

A few years ago, the heat would not work. I took it to the dealer as it was still under warrenty. When I got there, the heat was working fine. They flushed the radiator and heating core anyway and everything worked fine, until last winter.
My question is, could it be the temperature control knob? I'm not sure it it is vacuum opporated or electrical. My dad thinks it should be vacuum, but when I turn the knob from hot to cold and back, I don't hear any actuators or the tell tale hiss of vacuum. Any insight would be greatly appreciated.

thanks

Also, when I turn my controler, I don't hear anything. But it does work right. Are you getting any change at all?.. w/ temp or airflow? If air flow slows, possibly dirt in heat core air intake. I had that happen before. Check coolant flow to the core also. Maybe you can get some idea of that just touching the coolant line to and from the core after the truck is warmed up. If line is unusually cool, I'd look further into a plugged up core.

I checked the heater hoses and they are getting hot both going in and coming out. When I turn the control between hot and cold, there is no difference in temp or air volume.

Just had the same problem, had it flushed by the dealer, But it turned out to be the heater core anyway. its a real pain though, had to pull the dash, steering column and all.

hmm Are you getting any heat at all?? or is it just cold air blowing? because sometimes the thermostat is the cause of it all, I had that happen once, the car would heat up just fine, but all that came out was cold air. Come to find out, the previous owner was a horrible mechanic and they had put the thermostat in backwards! i replaced it, and had heat ever since. soo assuming your heater core and control unit are fine, thats my guess on the next step

First flush the core - I buy 3' of 5/8 and 3/4 hose and just do the core at the firewall. If defrost works or blowing there check the inderhood vac lines drivers side that end in a plastic line to the heater controls - all lines need to be good and not rotted also to the can in the fender.

If auto controls with the temp in degrees on the dials you also have an additional electric diverter to maintain correct temp - spin those knobs a few time and see if the contacts clean up and set to full hot and not a temp (move controls off auto settings as a test). Motor on diverter has plastic gears.