Climate Control

98 Envoy (Jimmy) with auto climate control - limited heat unless I turn the temperature knob all the way to the override position, then gets HOT so that seems to rule out heater core and heater valve problems..
Happens in both Auto mode and in manual operation mode. Seems to maintain reasonable temp control when cooling so temp sensor seems to be OK. 101K miles, 4X4. Can't find anything on this control system in Hayes. Suggestions on where to start?
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Sounds like a weak vacuumn actuator or sticky door hinge for the heater/blend door. It sounds like it's not opening unless you open it all the way. Try adjusting the heat with the fan set to MAX (in manual mode) and see if that works.
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