I have an older (circa year 2000) Chamberlain belt-drive opener. Today the trolley has started misbehaving. It opened the door fine this morning, but now will only travel a few inches down from the "door up" position, then reverses itself back to the starting (door up) position.
What I've done:
Disengaged the door from the opener/trolley, to verify it's not a binding or other kind of "sticking" problem. The door travels easily by hand. The trolley exhibits the same behavior with or without the door engaged.
Triple-checked the obstruction sensors. The green indicators are not flashing, but when I place something in front of them, they immediately flash to indicate the obstruction. I also confirmed that the opener recognizes when there is an obstruction: it flashes the lights and doesn't even travel the short distance.
Held the wall button controller pressed. Same behavior.
Adjusted the down travel distance and force. Setting either to the max value has no impact on this odd behavior.
Unplugged the opener for approximately 15 minutes. After plugging it back in, still exhibits the same behavior.
Visually inspected the belt and trolley. There does not seem to be anything wrong with them, at least visually.
I'm willing to accept that a nearly 20-year-old unit may have met its end, but I'd like to exhaust all DIY troubleshooting before either buying a new one or paying for a professional service call (which seems likely to result in a replacement anyway).