A garage door that reverses before contacting the floor is not misbehaving — it is working exactly as designed. UL 325, the safety standard all residential openers must meet, requires that the door reverse automatically if the entrapment protection system detects an obstruction or fault. The problem is the standard does not distinguish between "there is a bicycle in the door's path" and "one sensor bracket was bumped by a trash can." To the opener logic board, both look identical: an interrupted circuit.
The photo-eye pair sits roughly six inches off the floor on both sides of the door frame. One side transmits a continuous infrared beam; the other receives it. Break the beam — intentionally or by misalignment — and the opener halts the door and reverses. That is the design working. Your job (or ours) is to figure out why the beam is broken when nothing visible is in the way.
The diagnostic order we follow
First, check the indicator lights on both sensors. Most LiftMaster and Chamberlain openers put a solid green light on the receiver and an amber or green light on the transmitter when the beam is clear. A blinking or off light on either sensor points directly to the problem sensor. If both look normal but the door still reverses, the issue is upstream — force calibration, not the sensors.
Second, physically inspect the mounting brackets. Sensor brackets are flimsy by design (they are meant to bend rather than break if kicked), and a bump from a lawn mower, a trash can, or a bike tire is enough to rotate one sensor a fraction of a degree off axis. The fix is ten seconds with two fingers.
Third, wipe the lenses. Garage environments are dusty, and a dirt film on the lens does the same thing as a misaligned beam. We carry lens wipes on every truck and this takes thirty seconds.
When it is not the sensors
If the sensors check out — aligned, clean, lights solid — the issue is almost always the opener's down-force setting. Every opener has an adjustment that controls how much resistance triggers a reverse. If it is set too sensitively, the weight of the door itself at the bottom of travel trips the safety reverse. This is a screwdriver adjustment under the opener housing; we document the correct setting for your door's weight in the service record.
If none of these fix it, the logic board is failing — common on openers over twelve years old. A new logic board runs $130–$200; a new opener runs $180–$450. At that age and cost, we usually recommend the opener, because you also get updated safety features and the ability to connect to the MyQ app.
The sensor reversal is one of the most common calls we take, and it resolves in under an hour on almost every visit. But it should never be bypassed or disabled — that reverse function has prevented real injuries.