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It's Cold and I Have No Neat

And, it’s warm and I have heat. And, I have heat when AC is on. I have heat blowing when fan is turned off. Then, I have no heat at all. Volvo S-40…any suggestions are appreciated.

Thermostat in the engine. And, possibly the ventilation control unit. If the blend doors are not opening and closing properly, it is most likely a problem with the control unit.