When the heater comes on, it seems to stay on and almost run you out of the car at times. Sometimes it doesn't and works perfect. It seems there is a valve or damper which is sticking in the heat mode. Adjustment of the temp control does not make a difference when it sticks.

Does the system use a heater control valve or a damper to direct air over the heater coils and what do you think is sticking. I would like to know how the system works so I can trouble shoot.
Thanks in advance