You will have to take the cover off the motor unit to check the gear.
Garage door opener why the door slide back down.
It can also be caused by dirt on the sensor eyes loose or corroded wire connections at the sensors or the motor head or just that the sensors have gone bad and need to be replaced.
Slipping down a few inches after opening posted by nashbamafan on 5 10 16 at 12 35 pm to sicboy probably have 2 springs above door and one of them is broken.
Garage door opener problem.
If you garage door is lowering then reversing back to the open position there could be something obstructing the sensors.
Check to be sure these two sensors on either side of your garage door.
Door sliding back down once the door is up all the way it s resting on the horizontal parts of the track and the resistance of the motor to turning because it is low geared should prevent it coming back down.
We recommend unplugging the garage door opener and then replugging it back in after any repairs or adjustments to reset the system.
The one that is not is not strong enough to hold door up on its own but strong enough to still open the door.