Door Closes Then Reopens? It Is Almost Always the Safety Beam

How the safety beam works and why it reverses the door

Two units sit about 100mm above the floor on either side of the opening. One transmits an infrared beam and the other receives it. If the beam is interrupted while the door is closing, the opener reverses immediately. That is required on automatic doors and it is what stops a door closing on a child or a car.

The catch is that the opener cannot tell a real obstruction from a misaligned sensor. A bracket knocked 5mm out of line by a wheelie bin produces exactly the same behaviour as a person standing in the opening.

Most units flash their courtesy light a set number of times to signal a beam fault, and counting those flashes usually identifies the problem before we are through the door. Two flashes means one thing on a Merlin and something different on a B&D, so the code gets read against the right chart.

Alignment, wiring and afternoon sun

Alignment is the most common fix. Both LEDs should be solid rather than flickering. We loosen the brackets, sight them in, tighten them and confirm with a slow close cycle rather than a single test.

Wiring is the next suspect. The light-gauge wire running along the wall gets pinched behind shelving, nicked by a nail, chewed by rodents or corroded where it enters the sensor housing. Intermittent faults that come and go are almost always wiring rather than the sensor itself.

Perth garages facing west get direct low sun into the receiver from about four in the afternoon in summer, which washes out the beam. Shields, repositioning, or swapping the transmitter and receiver sides fixes that permanently.

• Opener flash code read before anything is touched
• Both LEDs confirmed solid through a full slow close cycle
• Wiring traced end to end when the fault is intermittent

When a sensor needs replacing

Sensors are replaced as a matched pair, because transmitter and receiver are tuned to each other and mixing brands or generations rarely works. A cracked lens, water inside the housing, or a unit that stays dark with confirmed power all mean replacement.

We also test the door's own reversal force once the beam is working, by obstructing the door with a solid object and confirming it backs off cleanly. A working beam and a badly set force are two separate safety systems, and both have to pass.

Sensor work is typically 30 to 60 minutes, including running new cable where the existing run is damaged. If the original eyes are obsolete we fit a matched aftermarket pair wired into the same terminals, which costs far less than replacing an otherwise sound opener over a pair of failed sensors.