The Safety Sensors
The two small units near the floor either side of the opening are the reason a garage door does not close on a child, and they are the most commonly defeated safety device in the house.
Short answer
A beam runs between them across the opening. Break the beam while the door is closing and the opener reverses. They misalign easily — a knock, frost heave, a bracket working loose — and the usual symptom is a door that will not close and an opener light blinking.
How they work and why they misalign
One unit transmits an infrared beam and the other receives it. Breaking the beam during closing reverses the door.
They sit low, usually within fifteen centimetres of the floor, which is where things get knocked.
A bike, a bin, a shovel, a vehicle door or a foot knocks the bracket and the alignment goes.
Frost heave and slab movement over a winter shifts them.
Spider webs, dust, condensation and road salt film on the lens stop the beam.
And low sun shining directly into the receiver can defeat it, which is a seasonal and time-of-day fault that baffles people.
The indicator lights
Both sensors have a small light. On most units one is steady when powered and the other is steady when it sees the beam, and a blinking or off light on the receiving side means alignment or obstruction. That is the diagnosis, it takes ten seconds, and it is on the units themselves rather than needing any tool.
Realigning them
Clean both lenses first, which solves a surprising proportion.
Check both brackets are tight to the track or the wall.
Loosen the bracket slightly and move the sensor gently until the indicator light goes steady, then tighten.
Check the height matches on both sides and that they are square to each other.
Check the wiring, which is a low-voltage pair stapled along the track and which gets pinched, cut by a staple or chewed.
And test it properly afterwards by breaking the beam with an object while the door closes, rather than assuming.
Why not to defeat them
They are the device that stops a closing door on a child, a pet or a bicycle, and that is not an abstract risk.
Taping them facing each other at the top of the track, or wiring them out, is something people do when they are frustrated and it removes the protection entirely.
A door that keeps reversing has a cause, and that cause is frequently something worth knowing about — ice at the threshold, a binding door, a failing spring.
Openers sold since the early 1990s are required to have an entrapment protection device, which is why they are there.
The mechanical reverse on contact is a secondary protection and is not a substitute, because it only works after contact.
And testing both the photo-eye and the contact reverse a couple of times a year is a two-minute job worth doing.
Common questions
Why will the door not close?
Usually a misaligned or dirty photo-eye sensor. Check the indicator lights.
Can I bypass the sensors?
No. They are the device that stops the door on a child or a pet.
Why does it only happen in the afternoon?
Low sun into the receiving sensor can defeat the beam.