Could be your RPM sensor but here are a few things to check.

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 none 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 the try it.

I had a similar problem with a Craftsman Garage Door Opener. I called the manufacturer, which I found on a web search, and learned that the opener was set to OPEN, but I installed it on a closed door. So, I had to remove the chain, run it through a cycle, and replace the chain.
It worked!

I helped my son replace the gear assemble on his garage door opener last week and we have the same problem. Did you get your opener to work--was it the RPM sensor or something else? I removed the chain and it runs a couple of seconds stops and reverses for a couple of seconds and then stops.