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Near the floor on each side of your garage opening sit two small eyes firing an invisible infrared beam at each other. Their one job is proving the doorway is empty before the opener lets a 100kg+ door descend. When that proof fails — for any reason at all — the door refuses to close, reverses back up, and the opener light blinks at you. Which means "my door won't close" is usually not a door problem: it's a seeing problem, and most seeing problems cost nothing to fix.
How the sensors actually work
One unit transmits, the other receives. Break the beam mid-close — a bin, a bike wheel, a dog — and the opener stops and reverses instantly. The design is deliberately fail-safe: if the receiver can't see the transmitter for any reason (dirt, misalignment, dead unit, cut wire), the opener treats the doorway as blocked and refuses to close under power. That fail-safe bias is why sensor faults produce a door stuck open rather than a door that closes blind — annoying by design, and much better than the alternative.
The five-minute checks, in order
Run these before you call anyone
- Clear the beam path — bins, brooms, hoses and box overflow are the number-one cause
- Wipe both lenses with a dry cloth — dust, cobwebs and salt film fog them over months
- Read the LEDs: both steady = healthy; one blinking = misalignment; dark = power or wiring
- Square up a knocked sensor by hand until its LED goes from blinking to solid, then re-test
- Check the brackets are tight — a loose bracket re-drifts within days of being straightened
Need the car out right now?
Most openers will close in constant-pressure mode: hold the wall button down the whole way and the door descends while you watch the doorway yourself. That's the legitimate workaround while a sensor fault waits for repair — taping the sensors together is not.
The afternoon-sun fault
Here's the diagnosis that confuses everyone: a door that closes perfectly all morning and refuses after about 4pm. That's low afternoon sun, sometimes bounced off a light-coloured surface, flooding the receiving sensor until it can't distinguish its partner's infrared beam from the glare. West-facing garages with an open outlook get this constantly. The fix is a small glare shield over the receiving eye, repositioning, or swapping the transmit and receive units so the receiver faces away from the sun. It's a minutes-long job on a sensor repair visit, and no amount of lens cleaning will cure it.
Wiring: the silent killer
The sensors talk to the opener through thin bell wire stapled along the wall at skirting height — where it gets nicked by trailers, chewed by rodents, perished by sun, or pinched by the very staple holding it. A dark LED with the other sensor lit usually means broken wire rather than a dead unit. Wiring faults are also behind the maddening intermittent version: sensors that work for days, then drop out when a breeze moves the loose strand. If your fault comes and goes, suspect the wire before the eyes.
When the sensors are actually done
Sensors last around 8–12 years — less in humid, salty air. They're done when: cleaning and realignment fix it only for days at a time, an LED stays dark with confirmed power, the units are visibly sun-bleached and brittle, or the opener throws sensor codes with the beam demonstrably clear. Replacement in Perth is quoted for the pair, fitted, wired and aligned, and always finishes with a real auto-reverse test — a block on the floor, door closed onto it under power, reverse confirmed. Sensors must match the opener brand's protocol — check your model in B&D's help and support library or Merlin's support centre before buying — which is why random online pairs so often pair badly or not at all.
Why you never bypass them
There are videos showing how to tape the sensors together facing each other so the opener always sees a clear beam. Understand what that does: it permanently blinds the only system watching for a child, a pet or a car boot under a closing door that weighs more than a fridge. Sensor faults are among the cheapest genuine garage door repairs there are — there is no version of the maths where defeating the safety system wins. Product Safety Australia is where recalls and safety notices for openers and their sensors are published, if you want to check the unit you own.