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.

Thanks for the tip Don. Just to be clear, the shaft shoud be pushed toward the end that has the black cap or away from it.

Push the shaft of the motor toward the black cap.

Hello
Hope you can help. I too replaced plastic drive gear in unit and now have a unit that travels about 4 to 6 inches and immediately reverses to original position. Of interest is that sensors are not lit. When I took sending sensor and moved close to opener the green LED would light when wired to Terminal 1 and 2, yet it was wired to 2 and 3 when I took the wires off the opener (I shouldn't have in hindsight....) I cannot get the sensor to light up when wired correctly, though the LED is working when wired to 1 and 2.......
Can't see anything wrong with RPM sensor, very frustrated and mad at myself for not being able to figure it out. Door is well balanced and easy to move up and down manually. Help would be very appreciated