Recirculation *is not* disabled by the heater; it *is* disabled when the A/C compressor is not running. So, if it's warm enough outside ( > ~35 deg. F) for the A/C compressor to run, but for the heat to be on inside, you *can* engage recirculation. So which thermistor should be replaced by a resistor that always "reads" as 40F ??? -RDH