Help Please - 2009 CRV temperature control (red/blue knob) problem

Suddenly the red blue knob adjustment yields no heat. All other A/C-temp functions/switches appear normal. Knob feels lose - like it's not connected to anything. If I had to guess I would say
something is wrong with the linkage. Any thoughts. Will probably take it to dealer, but would like to know what I am talking about/looking at for repair $$$ TIA starrin
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Looooong shot but cheap to verify: Check the coolant level in the radiator. Level a bit too low and the hot water does not make it to the heater efficiently.
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