Disconnect the trolley threaded shaft from the chain by removing the master link clip on spring master link cap and master link bar.
How ro replace garage door chain.
Remove the chain and cable assembly.
This should give you room to replace the chain drive module.
Pull out the idler pulley bolt and slide the idler pulley out of the rail window.
The top two only need to be loosened when you take out the bottom screw the cover will slide down.
To do this put some lubricant on a piece of rag this use the rag to wipe the chain.
Pull the chain cable out of the rail window.
Position a work table near the area under your motor unit to hold the garage door opener.
Press the wall control button or remote so that the opener completes its close.
If the chain on your garage door is rusty you need to apply lubricant into it to loosen the corrosion.
Documenting how i repaired the garage door.
Lower the motor unit down.
I am not a professional.
You can just let the cover hang by the wires plugged into it.
Remove the chain and cable assembly.
If your chain is badly corroded you may need to remove the chain from the carriage assembly and soak it in lubricant for a few hours.
There are three phillips screws holding on the garage door opener back cover.
The chain got loose and popped off the gear.