The present disclosure includes a heating, ventilation, and air conditioning (HVAC) door diagnostic system. The system has a rotatable hard stop lever that contacts, and restricts movement of, both a first airflow control door and a second airflow control door beyond a stop position when both the first airflow control door and the second airflow control door are present and moving in sync. When one of the first and second airflow control doors is not present or the first and second airflow control doors are not in sync, only one of the first and second airflow control doors will: contact the hard stop lever at the stop position, rotate the hard stop lever, and move beyond the stop position. The HVAC door diagnostic system generates a failure notice when the first airflow control door or the second airflow control door moves beyond the stop position.