Test garage door safety sensors two ways, monthly. First, the physical reversal test: with the door open, lay a solid object such as a 2x4 flat on the floor in the door's path and press close — the door must stop and reverse when it contacts the object. Second, the beam test: press close with the doorway clear, then wave an object (or your leg, from the side) through the invisible beam near the floor — the door must stop and reverse without touching anything. Before either test, read the sensor LEDs: a Chamberlain-family pair should show one steady amber and one steady green, a Genie Safe-T-Beam pair one solid red and one solid green. If a test fails or the green LED will not hold steady, Chamberlain publishes a wiring-isolation method — bringing the sensors to the opener on their wires — to tell a wiring fault from a failed sensor. Never disable, bypass or tape over a sensor to make the door close.
What you are likely to notice
- You want to confirm the safety sensors still work
- The door recently reversed for no visible reason and you want to verify the eyes
- You just realigned or replaced a sensor and need to prove it protects the opening
- You are checking a rental, a newly bought home or an opener that sat unused
- You are not sure the door reverses on contact the way it should
Check these in order
1. Read both sensor LEDs first
On a Chamberlain, LiftMaster or Craftsman opener the sending eye shows a steady amber LED and the receiving eye a steady green LED; a green LED that is off or flickering means misalignment, obstruction or wiring. On a Genie Safe-T-Beam pair the sender is red and the receiver is green and both must be solid. Fix an unstable LED before running the movement tests.
2. Run the physical reversal test
With the door fully open, place a solid object at least 1.5 inches tall (a board laid flat is the common example) on the floor centered under the door, then press the wall button to close. When the door bottom contacts the object it must reverse and return to open. If it does not reverse, or it keeps pushing down, stop using the opener and correct the down-travel and force settings for that exact model.
3. Run the beam-interruption test
Remove the object and press close with the doorway clear. While the door is moving down, pass an object through the space a few inches above the floor between the two eyes — a broom handle works, or step a foot through from the side, never your body under the door. The door must stop and reverse without anything touching it. This is the test that proves the photo eyes themselves are doing their job.
4. If a test fails, separate wiring from sensor failure
Chamberlain publishes a method that brings both sensors to the opener on their existing wires to check whether the LEDs behave correctly away from the field wiring. If the LEDs read normally at the opener but not on the brackets, the run of low-voltage wire is the suspect. Only use the exact procedure documented for your sensor generation, and stop if it requires access or wiring work you are not equipped for.
5. Check alignment height and aim
Both eyes should sit no more than about six inches above the floor and point directly at each other. Mounting them higher leaves a gap a child or pet can cross below the beam. Loosen one bracket, aim until the green receiving LED is steady, and retighten without pulling the wire.
6. Never bypass a sensor
Twisting the two sensor wires together, taping a sensor to face a wall, or facing the eyes at each other on the floor to force the door closed defeats the entrapment protection required on openers built since 1993. If the door will only close with the button held down, that is the opener telling you the sensor circuit is not satisfied — diagnose it, do not defeat it.
Two tests that check two different things
The physical reversal test checks the opener's contact-reversal behaviour: a door that meets an object on the way down must back off. The beam-interruption test checks the photo eyes themselves: the door must stop before it touches anything when something crosses the invisible beam near the floor. A door can pass one and fail the other, which is why both are worth running. Manufacturers recommend testing monthly, and after any sensor alignment, replacement or opener service.
Read the sensor LEDs before you start. On a Chamberlain-family opener you want one steady amber and one steady green; on a Genie Safe-T-Beam pair, one solid red and one solid green. An unstable green receiving LED will make the movement tests unreliable, so stabilise it first with beam, alignment and wiring checks.
Isolating a wiring fault from a failed sensor
If a test fails or the green LED will not settle, Chamberlain publishes a method that brings both sensors to the opener on their existing wires so you can see whether the LEDs behave correctly away from the field wiring. Normal LEDs at the opener but not on the brackets points at the run of low-voltage wire — staples, pinches, a nick where the cable turns a corner. Abnormal LEDs even at the opener points at a sensor.
Use only the exact procedure documented for your sensor generation, keep the door stopped and the doorway clear while you work, and stop at documentation if the job needs wiring access or tools you do not have.
Do not defeat the sensor to get the door closed
Twisting the two sensor leads together, aiming the eyes at a wall, or setting them face to face on the floor will make the door close, and it also removes the entrapment protection that has been required on residential openers since 1993. A door that only closes with the wall button held down is reporting an unsatisfied sensor circuit — that is the thing to diagnose, not to bypass.
If you cannot restore normal sensor operation, a door can be operated safely in the short term only by an aware adult holding the wall button in constant contact while watching the full opening, with the remote and keypad kept out of use until the sensors are fixed.
Stop DIY troubleshooting when…
- The door does not reverse on contact during the reversal test
- The door does not stop when the beam is interrupted
- The green receiving LED will not hold steady after alignment and wiring checks
- Sensor wiring is damaged inside a wall or beyond an accessible splice
- The door is unbalanced, binding or has damaged spring or cable hardware
Garage door springs, cables and a falling/unbalanced door can store or release dangerous force. Those are door-system problems, not just opener settings.
Common questions
How do I test my garage door safety sensors?
Run two tests monthly. Reversal test: lay a board flat under the door and press close — the door must reverse when it touches the board. Beam test: press close, then pass an object through the beam near the floor — the door must stop and reverse without contact.
What should the sensor lights look like during the test?
One steady amber and one steady green on Chamberlain, LiftMaster or Craftsman; one solid red and one solid green on Genie Safe-T-Beam. Fix an off or flickering green LED before testing.
The door won't reverse when it hits the board. What does that mean?
Stop using the opener. The down-travel or down-force setting for that model is wrong, or there is a travel fault. Follow the model's travel and force procedure before relying on the door again.
How can I tell if it's the sensor or the wiring?
Chamberlain's method brings the sensors to the opener on their wires. Correct LEDs at the opener but not on the brackets means the field wiring; wrong LEDs even at the opener means a sensor.
Can I just bypass the sensors if they keep failing?
No. Bypassing them removes required entrapment protection. Diagnose the sensor circuit; only operate the door with the wall button held down, in view, until it is fixed.
Manufacturer sources used
These links are the factual baseline for the diagnostic claims above. Model-specific manuals can supersede general guidance.