Most automatic gate openers have U/L mandated safety mechanisms. These can take several forms and can possibly be a combination of more than one type.
The primary safety mechanism in use today is a photo safety beam as close to the gate opening as possible. This type can be a reflector type of a send and receive type. These photo safety devices usually have some type of light indication that they are “OK” or “Blocked”. If the photo safety device is working, you should see a change of status when you block and unblock the photo beam. Sometimes you will be able to hear a clicking sound when testing. If no change is observed when testing, the most common cause is something blocking the path of the photo beam. Vegetation is usually the cause. If the photo beam is not blocked but still not working, the reflector may be covered with dew, of perhaps the units have been knocked out of alignment.
If the closing of the gate is intermittent, it is possible that the reflector and send/receive device are just slightly out of alignment. The procedure that we use to check this is to remove the reflector and move it both vertically and horizontally to locate the real center. If the center is found to be different than the original mounting location, you can adjust the send/receive device or remount the reflector to the new center location.
Most current LiftMaster gate openers that have battery operation or battery backup have power saving mode that turns off the photo safety device when the gate is idle (open or closed). To test these units you must “wake-up” the opener. To do this simply use your radio control device to try to operate the gate opener.