There are a couple things that can cause this. One is your door sensors are not connected to the opener and the other is:
Make sure you put the black cup back on the end of the motor shaft and that it is pressed all the way on. Make sure the RPM sensor is plugged in. If neither of these are the issue, then you probably did not have the motor shaft pushed all the way back when you tightened the coupler after replacing the worm gear. Loosen that coupler up and from the front of the motor (opposite end in which the coupler is on) push the shaft back as far as it will go. Hold it there while you tighten the coupler back up and then try it.
Answers & Comments
WITHOUT THE SAFETY EYES CONNECTED IT WILL GO ABOUT 6" AND THEN REVERSE.
TRY CONSTANT CONTACT ON THE PUSH BUTTOB OR IF YOU ARE SHORTING AT THE SCREWS.
IT SHOULD WORK WITH CONSTANT CONTACT.
BE SURE TO LOOK AT THE INSTRUCTIONS TO BE SURE YOUR CHAIN / BELT IS IN THE CORRECT POSITION.
HOPE THIS HELPS
There are a couple things that can cause this. One is your door sensors are not connected to the opener and the other is:
Make sure you put the black cup back on the end of the motor shaft and that it is pressed all the way on. Make sure the RPM sensor is plugged in. If neither of these are the issue, then you probably did not have the motor shaft pushed all the way back when you tightened the coupler after replacing the worm gear. Loosen that coupler up and from the front of the motor (opposite end in which the coupler is on) push the shaft back as far as it will go. Hold it there while you tighten the coupler back up and then try it.
Should help you