Everything on my HVAC works fine, but the temp **** has no effect on either hot or cold. I noticed one time, in the spring, that no hot air came out. Naturally, I didn't need the heat again and forgot about it.
Now it is getting cooler, and I noticed that regardless of where I set the ****, the air is at the coldest setting.

I can even turn it to hot while on defrost, or try to warm the A/C temp a little, with no results.

Does the **** control go bad or does this indicate a deeper issue?