@dako70477 if your isolette is a Air-Shields Isolette C400QT C450QT then go to this sie [[Topic:Air-Shields Isolette C400QT C450QT]] and download the service manual. "The airflow alarm is activated by either a fan failure or a short-circuited Air Flow probe" A sensor located below deck in the normal air path of the fan controls this alarm. If air flow stops due to a fan failure, the temperature of the self-heated sensor rises causing the Air Flow alarm Indicator on the front panel to light and produce a pulsating audible tone. [br]