Chain-Drive Opener Repair: before you call

The door opens about a third of the way and stops. Turning the limit screw on the head has made no difference, and the trolley seems to sit in the wrong place along the rail when the door is fully closed.

Worth answering before the call

What make and model is on the opener's label, and roughly how old is it? Does the motor run when you press the button, and does the door move at all? Is the trolley stuck at the very end of the rail, near the motor or near the door?

Should I use the slide lock on the door with a chain drive?

No. The manual slide lock on the door is disabled or removed once an operator is connected, and the technician checks that during the repair. A motor pulling against a locked door strains the chain, strips the drive gear and can bend the top section of the door. If you want the door secured against remotes, the lock button on the wall control switches them off, and a track lock designed to work with an operator does the rest.

What is the trolley, and why would it get stuck?

The trolley is the carriage that rides along the rail, pulled by the chain, with the door arm hooked to it. It gets stuck when the travel limits creep and it is driven onto the stop bolt at the head end or the stop tab at the door end, where the motor hums but nothing moves. The technician slackens the chain, runs the trolley off the stop under power, and pulls the limit back so it stops short in future.