What the sensors actually do

Every UL 325 compliant opener has two photo-eyes — a sending unit and a receiving unit — mounted four to six inches above the floor on either side of the door. When the beam between them is broken while the door is closing, the opener instantly reverses. That is the only non-contact safety device on the door. Everything else (force limits, reversal logic) is contact-based.

The diagnostic order we use on every call

1. Look at the LEDs first

On almost every brand — LiftMaster, Chamberlain, Genie, Linear — one sensor LED stays solid when the system is healthy. If either LED is off or blinking, you have your answer before you touch anything.

  • Both LEDs solid — sensors are fine, look at opener or wiring
  • One LED off — power or wire problem on that side
  • Receiver LED blinking — alignment problem

2. Clear the opening and the lenses

Cobwebs are the number-one culprit in Richardson and Plano garages with seasonal pest activity. Wipe each lens with a dry microfiber. Move any stored items — bikes, recycling bins, extension cords — out of the beam path on the floor.

3. Realign the sensors

Loosen the wingnut on each bracket, point both lenses straight at each other, and tighten while watching the receiver LED for a steady light. The brackets bend easily — a bumped car door is the most common reason a sensor falls out of alignment.

4. Inspect the bell wire

The thin two-conductor wire from the opener to each sensor is run along the ceiling and down the track. It is staple-shot to the framing and gets nicked surprisingly often during garage organization projects. Look for crushed insulation, a staple driven through the conductor, or a cleanly cut wire near a recently mounted bike hook.

5. Swap sensors left-to-right

If you suspect one sensor is dead, swap them physically. If the problem follows the sensor, that one is bad. If it stays on the same side, the wire or the opener terminal is the problem.

6. Reseat the opener terminals

The two safety-sensor terminals on the opener (usually labeled white and white/black) sometimes loosen from heat cycling. Cut a fresh half-inch of conductor, strip clean, and re-clamp.

When it is not the sensors

If both LEDs are healthy, the beam is clear, and the door still reverses on close, the next suspects are the close-force adjustment on the opener, a binding spring that is making the opener think it has hit an obstruction, or a worn opener logic board. Those are diagnostic calls, not DIY fixes. If you have followed the steps above and the door still won't close, see our won't-close diagnostic page or schedule service.

North Dallas note

West-facing garages in Plano and Frisco see the worst summer-evening glare problems — the sun lines up with the sensor lens between roughly 6:30 and 8:00 p.m. If the door only refuses to close in that window, sliding the receiver an inch toward the door (so the lens shades itself) almost always fixes it.