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Intermittant heater/defroster

I recently got a 2000 Ford Taurus w/ 80,000 miles for my step-son about to graduate college. We drove the car from Houston Texas to Iowa for his spring semester. During the second hour of a 3-hr drive to take me to the airport in Omaha the heat and defrost stopped working although the fan kept running. After driving a half-hour without heat we pulled off at a gas station for a break. When starting the car back up the heat came back on. We had no further trouble that trip. I’m very worried about having the heat/defrost failing during a drive in the middle of winter. The local mechanic did not seem to know what the problem might be. Any suggestions?

Did the airflow continue and just turn cold, or did the airflow stop? If it stopped, the evaporator may have iced over.

The airflow continued but just turned cooler. The temp outside was probably in the mid 40’s. The heater was set to use interior air, not exterior in order to keep windshield fogging to a minimum. It became cool enough inside the car that thoughts turned to using a blanket over our laps to keep warm. Thanks for any help.

The main things to check would be:

  1. Air trapped in the cooling system, or air being added due to a bad head gasket.
  2. Faulty thermostat.
  3. Faulty blend air door or motor.