The three usual causes, in order: the safety sensors near the floor are misaligned, blocked, or blinded by direct sun on the receiving eye; something is actually in the sensor path, including a cobweb or a leaf; or the door's travel and force settings no longer match a door that has grown heavier or rougher with age, so the opener reads normal resistance as an obstruction. Sensor alignment is a five minute check. If the door is also loud, crooked, or heavy by hand, the reversal is a symptom, and the door, not the opener, needs the visit.
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