Front blower fan turns off & on, rear blower fan is fine. Sensor or Fan issue?

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The front defrost-heat blower fan will not blow air until the car warms up to 180-200 degrees then the blower fan starts blowing air but then continually stops & starts intermittently. However, the rear blower fan does not seem to have a problem. Could it be a sensor or the front fan-blower be the problem?