Sensors are the most common single part we replace, and the most commonly misdiagnosed. The symptoms — a door that will not close, a door that reverses, a door that closes with the lights flashing — can all come from a sensor, but they can also come from a wiring run, a logic board, or a door that is not travelling straight. Sensor alignment is the first thing tested, because it is the most common cause and the cheapest to check.
How the safety sensors work
Almost every modern garage door has two photo-eyes mounted low on the tracks, roughly waist height, aimed at each other across the opening. One sends an infrared beam, the other receives it.
When the door is closing, the opener requires that beam to be unbroken. If something interrupts it, the door stops or reverses. That is the whole safety mechanism, and it depends on the beam being clear and the two units actually talking to each other.
They also work as a courtesy light. Most openers flash the courtesy light when the safety path is blocked or misaligned, which is a useful diagnostic you can check without any tools.
The important limitation: the sensors are a presence detector at one height and one plane. They are genuinely effective, but they are not a substitute for keeping the door visible while it moves, or for a clear path to the door when the weather is bad.
What the symptom is telling you
| Symptom | Most likely cause | Also check |
|---|---|---|
| Door will not close at all, lights flashing | Sensor beam blocked, or sensors out of alignment | Wiring run, a loose bracket, a dirty lens |
| Door reverses a few inches then closes | Misalignment, or the door is not travelling straight | Track plumb, rollers |
| Door closes on the first try sometimes | Intermittent alignment or a loose mounting bracket | Damp in the garage, a bumped bracket |
| Opener works, sensors dead | Failed sensor, or a cut wire | Low-voltage wiring run |
| Door reverses with nothing in the way | Force setting too low, or genuinely misaligned sensors | Force limit, a binding door |
The order matters: the cheapest and most common test comes first. Sensors get knocked by a bike, a broom, a car in the driveway, and by the door itself as it passes over them.
What you can check before calling
These are safe, quick, and they fix a surprising share of sensor problems:
- Look for an obstruction. A broom, a leaf pile, a dog, a bicycle, a cardboard box. Move everything out of the path of the beam.
- Clean the lenses. Both of them, gently. Dust, spider webs, and road film are enough to make the beam unreliable.
- Check the alignment. The two units should be level with each other and aimed directly at each other, roughly level with the sun if you can get it to shine on one. Most mounting brackets have slots for this.
- Look at the brackets. If a bracket is bent or the unit is hanging loose, alignment cannot hold. This is what a car in the driveway does.
- Follow the wire. Look for a cut, pinched, or disconnected low-voltage wire between the sensors and the opener. This is the part DIY attempts often break.
- Test with the door open. With the door fully open, wave a hand through the beam. The courtesy light should flicker.
If the sensors are aligned, clean, and secure, and the door still will not close, the cause is further along the circuit. Call at that point rather than taking the opener apart.
Align, repair, or replace
Most sensor work is adjustment: brackets slotted, units re-aimed, and the door re-checked. That covers the majority of calls.
Beyond alignment, the other possibilities are:
- Broken brackets. Replaced or re-formed, then the alignment is set properly rather than forced.
- A damaged low-voltage run. A cut, crushed, or corroded wire between the opener and the sensors. This is common where a wire passes through the jamb, and it is a contained repair.
- Failed sensor units. Both sides are replaced as a pair, because a new sensor paired with an old one is a coin flip on when the next one fails.
- A failing control board. If the wiring is sound and the sensors are good but the opener still will not close, the board is the remaining suspect. We will confirm that before anyone talks about replacing the opener.
Worth knowing: alignment can drift again, particularly in a garage where a car regularly clips a bracket. In that case a firmer, non-slip mounting is the actual fix, not another nudge to the alignment.
Related services & problems
These come up often alongside the work described above.