Garage door opener stops--even when I take the chain off the gear!
Garage doors have two pairs of settings:


End stops: one at the bottom and one at the top
Force limits: one for heading down, one for heading up


The first pair is obvious: you want the door to stop right when it's properly closed, and right when it's fully up; you don't want the door to stop too early. You've looked at these, and they seem to be fine.

The second pair is less so. If something blocks the door, you want the door to stop moving, rather than squish whatever is in the way. In fact, the doors I've seen, if they hit the force limit on the way down, will immediately turn around and open (so if, e.g., it closed on someone's leg, it wouldn't keep them pinned). There are force settings for both going down and going up, and must be adjusted with the door and lift springs attached.

See page 24 of this Liftmaster 1/3 HP manual (you may have an older model, but the idea's the same). My guess is that your downward force setting is too low, so that the opener thinks something is in the way and reverses.

You may also have a model with a sensor beam across the bottom of the door, to stop the door closing if there's anything in the way BEFORE it hits the object. If that sensor isn't working you might get the same symptoms.