How do I test the safety reversal system?
Place a 1-1/2 inch board (or 2x4 laid flat) on the floor centered under the garage door when it is fully open, then operate the door in the down direction. The door MUST reverse on striking the obstruction. If it does not reverse, increase the down travel and repeat the test. (Page 29)
From the official Chamberlain Chamberlain Garage Door Opener manual, page 29
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