Opener Runs but the Garage Door Does Not Move
If the opener runs but the door does not move, the most likely causes are a stripped gear and sprocket kit, a disengaged or broken trolley, or a snapped chain or belt. All three are repairs rather than replacements, typically $100 to $350, provided the opener is not already past its useful life.
What is actually happening
The motor and the door are connected through a chain of parts: motor, gear and sprocket, chain or belt, trolley, arm, door. If the motor runs and the door does not move, one of the links in that chain has broken.
Which one is usually obvious from watching it.
The rail does not move at all. The failure is between the motor and the chain: a stripped gear and sprocket kit. This is the most common opener failure there is.
The rail moves but the door does not. The trolley has disengaged from the carriage, or the arm connecting it to the door has broken.
The chain or belt is hanging loose. It has snapped or come off the sprocket.
Stripped gear and sprocket
Inside the motor housing there is a plastic gear driving a metal worm. The plastic is the sacrificial part by design: it is cheaper to replace a gear than a motor.
It strips because of age, because the door has been out of balance and the opener has been working too hard, or because somebody kept pressing the button on a jammed door.
A gear kit replacement is typically $100 to $250 including labor and it is a routine repair on LiftMaster, Chamberlain and Genie units.
Important: if the door was out of balance and that is what killed the gear, fitting a new gear without fixing the springs just buys you another year.
Disengaged trolley
The manual release cord disconnects the trolley from the carriage so the door can be moved by hand. If it has been pulled and not re-engaged, the opener runs the carriage up and down the rail with nothing attached.
Re-engaging is usually a matter of pulling the door closed by hand and then running the opener until the carriage clicks back into the trolley. Some models need the release lever pushed back towards the door first.
If it will not re-engage, the trolley itself may have broken, which is a $75 to $200 part.
Snapped chain or belt
Less common, and usually the result of long neglect or a door that jammed hard. Replacing the chain or belt runs $100 to $250.
A belt that has snapped on a unit under ten years old is unusual and worth asking about, because the underlying cause is often that the door has been heavy for a long time.
The question behind all three
Every one of these failures is made much more likely by a door that is out of balance. An opener is designed to guide a door that the springs are already holding, not to lift a heavy one.
Before spending money on the opener, pull the manual release and lift the door halfway by hand. If it does not stay where you leave it, the springs are the real problem, and the opener is the thing that has been paying for it.
Related questions
What does a stripped gear sound like?
A grinding or rattling from inside the motor housing while the rail stays still. It is a distinctive sound and once you have heard it you will recognise it. The plastic gear inside has lost its teeth.
Is a gear kit worth fitting on an old opener?
On a unit under about fifteen years old with no other problems, yes. Past that, or if the opener predates 1993 and has no safety sensors, put the money towards a replacement instead.
The door moves an inch and stops. Is that the same thing?
No, that is more likely a broken spring or an obstruction. A gear or trolley failure means the door does not move at all while the motor runs.