4 Flashes
Chamberlain Garage Door Opener
Severity: ModerateWhat Does This Error Mean?
Chamberlain LiftMaster diagnostic LED flashing 4 times means the safety sensor wires are shorted or have been wired backwards.
The opener will not close the door from the wall button or remote with this fault — only the constant-pressure close-down works.
Cause is usually a staple driven through the wire during install, a chewed wire, or a sensor connected with reversed polarity.
Affected Models
- Chamberlain B970
- Chamberlain B1381
- Chamberlain B6713T
- LiftMaster 8500W
- LiftMaster 8550WLB
- LiftMaster Elite 8500
Common Causes
- Staple driven through the sensor wire during installation
- Mouse or rodent damage to wire insulation
- Sensor wires reversed at the terminal
- Wires touching at a bare splice
- Failed sensor with internal short
- Damaged wiring after a hit on the sensor mount
How to Fix It
-
Disconnect power to the opener.
Unplug the opener from the ceiling outlet.
Working on the sensor wiring with the opener powered risks accidental door movement. -
Walk the wires.
Trace the white and white-with-black-stripe wires from each sensor up to the opener head.
Look for staples, bent nails, or rodent damage.
Photograph anything suspicious before pulling on it. -
Verify polarity at sensors.
Each sensor has a striped wire and a solid wire.
Both sensors must connect with matching polarity to the opener — striped to striped, solid to solid.
Reversed polarity at one sensor causes 4 flashes. -
Verify polarity at opener.
At the opener head, two terminals (typically labelled 2 and 3 or white and white-stripe) accept the sensor wires.
Confirm the sensor wires match the LiftMaster manual diagram for your model. -
Repair the damaged wire.
If you found a staple or chew through, cut the wire cleanly past the damage, strip both ends, twist together, and use a wire nut or solder + heat shrink.
Tape alone is not enough — sensor circuits run for years and bare splices corrode. -
Test sensor LEDs.
After repair, restore power.
The sending sensor LED should be steady amber/yellow.
The receiving sensor LED should be steady green when nothing is in the beam.
One or both off = wiring still wrong; one off only when the beam is broken = working correctly. -
Replace failed sensor.
If wiring is correct but 4 flashes persists with one sensor LED dark, that sensor has failed.
Genuine LiftMaster sensor pairs are 30-70 USD.
Replace as a pair to keep the alignment consistent.
Frequently Asked Questions
Why won't the door close even with the sensors aligned?
4 flashes is a wiring fault, not an alignment fault.
Even perfectly aligned sensors cannot communicate with reversed polarity or a shorted wire.
Walk the wiring before adjusting sensors.