Every garage door opener sold in the United States since 1993 has two separate systems whose only job is to stop the door killing someone. They are independent of each other on purpose, because the failure of one is exactly when you need the other.

Both are required to work. Neither announces when it stops working. And in a decade of garages, the number of homeowners who have tested either is small enough to be startling.

The test takes two minutes. Here it is, and here is what it is actually protecting against.

Why there are two of them

The law changed after a run of child fatalities in the 1980s. Doors of that era closed until something stopped them, and a small child under a closing door does not stop a several-hundred-pound panel. The federal requirement that followed specified two independent mechanisms:

  • A non-contact sensor — the photo eyes — which reverses the door before it touches anything.
  • A contact-based force reversal — the opener detects resistance and reverses after touching something.

The second exists because the first can be blocked, misaligned, or aimed over the top of whatever it should have seen. The first exists because the second requires contact, and contact with a toddler is already a bad outcome. Neither is a backup for the other in the casual sense; they cover different failures.

Test one: the photo eyes

Two small units mounted near the floor on both sides of the opening, facing each other, with a wire running back to the opener. One usually shows a steady light and the other a different colour; a blinking sensor light almost always means they are not seeing each other.

The test: press the button to close the door, and while it is moving, break the beam — a foot, a broom, a cardboard box. Do not use your hand at floor level with a door in motion, and do not use a pet.

Pass: the door stops immediately and returns to fully open, usually with the opener light flashing.

Fail: it keeps closing. Stop using the opener until it is fixed.

Common causes, in order: a sensor knocked out of alignment by a bike or a bin; a lens filmed over with dust, which is genuinely most of the calls; direct low sun into the receiver at certain times of day, which produces a fault that appears and disappears on a schedule and drives people to distraction; a chewed or corroded wire; and, occasionally, sensors that have been mounted high on a wall to stop them "being a nuisance", which defeats them entirely.

Test two: the mechanical reversal

This is the one nobody knows about.

The test: lay a solid object flat on the floor in the middle of the door's path. The traditional item is a length of two-by-four laid flat, which is about an inch and a half tall. A roll of paper towel is a reasonable substitute — something firm enough not to simply squash.

Close the door onto it.

Pass: the moment the door meets the object, it reverses and goes back up.

Fail: it sits on the object, keeps pressing, or grinds and stalls. The down force is set too high.

Here is why that failure is so common: when a door gets heavy — springs fatiguing, rollers binding, a track slightly out — the opener starts struggling and someone turns the force up to make the symptom stop. It works. It also raises the threshold at which the door decides it has hit something, and it does so by exactly the amount needed to hide the original problem.

A failed force test is therefore usually two faults wearing one coat: a door that is too heavy, and a safety setting that has been turned up to compensate.

The third thing to check while you are down there

Sensor height. Photo eyes belong no more than about six inches off the floor. Higher than that and a crawling child, a dog, or a foot passes underneath the beam entirely. It is a five-minute correction and it is one of the most common installation faults we find.

And the wall button should be mounted high — the guidance is at least five feet — and sited where the door is visible from it. A button a child can reach, in a spot where the door cannot be seen, is how people press the button on something.

What not to do about a failing sensor

There is a well-known trick where the two sensor wires are twisted together at the opener so it believes the beam is intact. It makes the fault light go away and the door close normally.

It also removes the entire non-contact safety system from the house. We find it a few times a year, usually left behind by someone who meant to come back to it. If your door has been "fixed" and the sensors are still lying on the floor unbolted, that is what happened.

How often

Both tests, once a month. The manufacturers all say monthly and they are right, for once — these systems degrade silently and the only symptom of a broken safety feature is that it does not work when it is needed.

Two minutes. A foot through a beam and a block of wood on the floor.

The part where we talk ourselves out of a job

Of the safety failures we are called to, the clear majority are a misaligned sensor or a dirty lens, and both are within reach of anyone willing to kneel down with a cloth and loosen a wing nut. The force settings are a screwdriver and a manual.

Call someone when the door itself is heavy, when a spring is involved, or when the tests still fail after the obvious has been ruled out. Everything before that, you can do on a Sunday, and it is the single most useful two minutes anyone spends on a garage.

Garage Door Doctor’Z · (833) 991-3899 · doctorzgaragedoor.com
CSLB C61/D28 · Lic. 1138150 · Serving the Sacramento Valley since 2018.