Most likely sensors. If they aren't aligned, door will stop and you'll have to hold down the button. If it's happening infrequently, could be they they aren't screwed in tight enough to brackets, and the shaking/rattling of the door knocks them out of alignment, and sometimes not.
Or they need to be cleaned/replaced. Condensation or something inside them is loose etc. But sensors cannot be bypassed unless you come up with a low tech solution such as taping them togerther.
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