In the sketch, a sender and a receiver face each other across the opening. When something breaks the line between them while the door is closing, the opener reverses the door. This is an illustration, not measured data.
Do not move or raise the sensors. If a sensor will not stay lined up, call a technician. Your opener's manual says what the lights on the sensors should show.
Clear anything in the path of the sensors and check what your opener's manual says their lights should show. If the door still will not close, stop using the opener and call a technician. Never bypass the sensors.
Your opener's manual describes the reversal test. Chamberlain's version puts a one-inch board, or a 2×4 laid flat, on the floor in the middle of the door's path and says the door must reverse when it hits the obstruction. If the door does not reverse, stop using the opener and call a technician. Never adjust the force to make it pass.
The rule calls this inherent primary entrapment protection (section 1211.7): a closing door has to start reversing within 2 seconds of contact with an obstruction (section 1211.7 gives exceptions near the top of travel). Your opener's manual describes the test for yours.
The federal rule for residential openers, 16 CFR part 1211, requires the opener to "initiate reversal of the door within 2 seconds of contact with the obstruction" (section 1211.7). It also requires a secondary protection device (section 1211.8), such as an external photoelectric sensor or an external edge sensor (or other equivalent protection).
Springs and cables are under a lot of tension (Clopay). If a spring may be broken, do not pull the release and do not try to move the door. Stop using the door and call a trained door technician.
16 CFR 1211.9 requires a means to manually detach the opener from the door, with a red handle. Your opener's manual says how and when to use yours. If the door feels very heavy, drops or will not stay up, do not force it; call a trained door technician.
To reconnect, follow the steps in your opener's manual. Re-engage the opener before you rely on it.
An opener made before the rule took effect may not have a secondary entrapment protection device. If you are not sure what yours has, ask a technician to look.
Never bypass the sensors or raise the force to fight a binding door. If the door will not close, clear anything blocking the sensors and check what your opener's manual says their lights should show. If that does not fix it, a technician fixes it.
Reversal on contact (section 1211.7) is one protection and the secondary device (section 1211.8) is the other. This sketch shows what is left when the secondary device is out. A sensor that is not working needs a technician.
This sketch shows what is left when reversal on contact is out. A door that has stopped reversing needs a technician; stop using the opener.
Neither protection is working in the sketch. The rule requires both (sections 1211.7 and 1211.8); stop using the opener and call a technician.
Test as your opener's manual says: (1) Run the reversal test from the manual. (2) Check the sensor lights against the manual. If anything fails, stop using the opener and call a technician.