Customer FAQs Discover answers to the most frequently asked questions from ADT customers with the click of a button. 1. How does a door or window sensor work? A door or window sensor is typically a two-piece magnetic contact sensor. When the system is armed and a door or window is opened, the two magnetic sensors are separated from each other, thereby tripping the alarm. If a protected door or window with a sensor is open, you will not be able to arm the system until the door or window is closed or the zone is bypassed. If your door or window sensor is causing false alarms, the door or window may not be securely closed or the sensor may be damaged. Swelling or contracting of the door or window due to changing weather may also result in false alarms. If you are having problems with your door or window sensors you may try troubleshooting the problem from our Sensor Troubleshooting page to determine if a service visit is necessary.