Why won't my garage door close?

The three-minute check before you call anyone

Start at the two small units bolted about six inches off the floor on each track. One should show a steady light and the other should show a steady light of a different color, and if either is flashing or dark the pair is not talking to each other. Look along the beam path for the ordinary culprits: a recycling bin, a bike pedal, a coiled hose, a box shoved into the corner. Then wipe both lenses with a dry cloth, because on this coast they collect a fine salt haze and cobwebs behind the bracket that are enough to break the signal.

If the lights look right, walk the bottom of both vertical tracks. A dropped bolt, a bag of compost against the channel or a ridge of blown sand at the base will all stop a door in the same way. Count the flashes on the motor head next and match them to the label under the light cover, which names the fault in the manufacturer's own code. What not to do is hold the wall button down to force the door shut. That override bypasses the reverse, and it is only for a door you can see is clear.

When it is not the sensors

Two settings inside the opener produce a door that almost closes. The travel limit tells the motor where the floor is, and if it is set too far the door presses into the slab, the motor reads that as an obstruction and sends it back up. The force setting decides how much resistance counts as hitting something, and it drifts over the years. A bottom seal that has hardened in the damp adds to the same picture, because the door starts meeting resistance a half inch earlier than the opener expects it to.

The one to take seriously is a door that has gone crooked or has a cable lying loose beside a track. That is a cable off a drum or a broken spring, and the door is no longer being carried by anything designed to carry it. Stop pressing the button, pull the release cord so the opener is out of the picture, and keep cars and people from under it. Sensor brackets rusted through at slab level are worth a look too, since a bracket that has let go drops the eye out of alignment on its own.