Volvo Repair: Volvo V90 heater output temperature, volvo v90, heater control valve

Question
QUESTION: We're finding that we can control heat only at the very highest part of the thermostatic scale, i.e., we get warmth only if it's set at 80 degrees or more.  Less than that, lukewarm to no heat.  AC works fine.

ANSWER: Peter, most likely its the heater control valve getting stuck.  May also be the lever control box by the evap case.  Roger

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QUESTION: Any diagnostics that could confirm this, or does this need to be confirmed by observation of the two controls?  Sounds like if someone could watch each of them in action, a diagnosis is possible.  What's the fix in each case?

Many thanks.

Answer
Peter, have someone watch the valve in the engine bay to see if its working when you turn the heat up. May be a split vacuum line.  As far as the inside door, you can try to lengthen or shorten the rod but I have never seen that fail.  Most likely the valve.  Roger