Garage door won't close: the six causes, in order of likelihood

1. Safety sensors out of alignment

By far the most common cause. The two photo-eyes sit six inches off the floor on each track. If they can't see each other the opener refuses to close, and most units flash the overhead light or blink a sensor LED to tell you so.

Look for a steady light on both. Blinking or dark means misaligned, blocked or unwired. Loosen the bracket, aim one at the other, tighten when both go steady.

2. Something in the beam you haven't noticed

A bike wheel, a bin lid, a hose coil, a leaning broom, or a snow drift blown in under the door. Crouch to sensor height and look along the beam line rather than scanning the floor from standing.

3. Dirt, sun or spiders on the lenses

A film of dust on the lens is enough. Wipe both with a dry cloth. Low winter sun shining directly into a sensor also swamps it — if the fault only happens at a certain time of day, that's your answer.

4. Close-force or down-limit set wrong

If the door lands then immediately lifts again, the opener thinks it hit an obstruction because it's still pushing after the door reached the floor. That's the down-limit setting, not the sensors.

One careful adjustment is reasonable. If it doesn't resolve it, stop — over-cranking close force disables the auto-reverse safety.

5. Track obstruction or bent track

Run your eye up both vertical tracks for dents, a lifted bracket or a roller sitting proud. A door that stops at the same point every time is being stopped by something physical at that point.

6. A spring or cable problem

If the door is also crooked, heavy by hand, or you heard a bang recently, the balance is gone and the opener can't control the descent. Stop using it and call. This is the one cause on the list where continuing to try makes things worse.

Securing the house tonight

If it's late and the door won't close, you can usually slide the manual lock bar across, or clamp a pair of locking pliers onto the track just above a roller to stop the door lifting. That holds the house secure until a technician gets there.