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Garage Door Will Not Close: Reading the Safety Sensors First
A door that opens fine but refuses to stay closed is one of the most common calls in this trade, and it is also the one most likely to be fixable in ten minutes without a technician. The cause is usually the safety sensors, and they announce the problem clearly if you know what you are looking at.
What the sensors are
Two small units sit near the floor on either side of the opening, roughly six inches up. One transmits an infrared beam, the other receives it. If the beam is broken while the door is travelling down, the opener reverses immediately. This is a legal requirement on residential openers and it is the reason a closing door will not trap a child or a pet.
Because the system fails safe, anything that stops the receiver seeing the beam produces the same behaviour as a genuine obstruction: the door goes down a short way, stops, and goes back up.
The signal to read first
Both sensors have indicator lights. The exact colours vary by manufacturer, but the logic is universal. One light shows power. The other shows alignment, meaning the receiver can see the transmitter.
A steady alignment light means the beam is good. A flickering, dim or dead alignment light means it is not, and that is your fault found. Look at both units before touching anything, because which light is misbehaving tells you where the problem is.
It is worth knowing which unit is which before you start adjusting. The transmitter usually shows a single steady light whatever the receiver is doing, because it has no way of knowing whether anyone is listening. The receiver is the one whose light changes. Moving the transmitter when the receiver is the misaligned unit is a common way to spend twenty minutes making the problem worse.
The five causes, in the order they occur
The first is something in the beam. A bin, a bike wheel, a coiled hose, a stack of boxes that has slumped sideways. This sounds too obvious to check and it is the single most common cause. The beam sits low and is invisible.
The second is a dirty lens. Dust, cobwebs and salt film build up on the lens face and eventually attenuate the beam below the receiver's threshold. Wipe both lenses with a soft dry cloth. Do not use solvent, which can craze the plastic.
The third is alignment. The brackets holding the sensors are thin and bend easily. A knock from a car door, a bike or a lawnmower is enough to move one a few degrees, which is enough to lose the beam. Loosen the wing nut, move the sensor slowly until the alignment light goes steady, and retighten without nudging it.
The fourth is sunlight. Low direct sun falling straight into the receiver lens can swamp the infrared signal, which produces a door that will not close in the late afternoon and behaves perfectly at every other time of day. If the fault has a time of day, this is almost certainly it. A small shield or a sun hood solves it.
The fifth is wiring. The sensor wires run along the wall and are usually stapled in place. A staple driven too tight, a wire pinched behind a shelf, or corrosion in the terminal will produce an intermittent fault that comes and goes with temperature or vibration. Corrosion is more common near the coast than most people expect.
When it is not the sensors
If both alignment lights are steady and the door still reverses, the sensors are working and something else is stopping the door.
The most likely candidate is the close force setting on the opener. The unit measures resistance on the way down and reverses if it exceeds a threshold. Rising friction anywhere in the system, worn rollers, a dry hinge, a track slightly out of line, raises that resistance until it crosses the limit. The opener is correctly reporting a mechanical problem.
The tell is repeatability. A sensor fault reverses the door at varying points depending on what is in the beam. A friction or force problem reverses it at the same point every time, because that is where the resistance rises.
The second candidate is the down limit. The opener needs to know where the floor is. If the limit is set too low, the door reaches the floor, keeps pushing, reads the resistance as an obstruction, and backs off. This one often appears after a door has been serviced or an opener replaced.
The third is the door itself. If a section is bent, a roller has jumped its track, or a cable has come off its drum, the door will bind on the way down. That is not an adjustment, it is a repair, and cycling the door repeatedly to test it will make it worse.
What to check, in order
Look for an obstruction in the beam. Wipe both lenses. Check both sets of indicator lights. Align the sensor whose light is unhappy. If the fault only happens at a particular time of day, shield the receiver from direct sun. If the lights are steady throughout, stop adjusting sensors, because the problem is elsewhere.
At that point the useful next step is a proper look at the door's mechanical condition rather than more opener settings. If you would rather not work through force limits and travel adjustments yourself, an experienced technician deals with this specific fault constantly and can usually identify it from a short description.
What not to do
Do not disable the sensors. They can be bypassed, and every so often somebody suggests it as a workaround for a door that will not close. It removes the one system that stops a heavy door closing on a person, and it is illegal on residential installations for exactly that reason.
Do not keep cycling a door that is binding. If the door is catching mechanically, each attempt adds wear and can turn a track adjustment into a bent section.
And do not assume a reversing door means a failing opener. In this specific fault the opener is usually the only part behaving correctly.