La412 RPM reversal error

[winter5470]

I have a liftmaster LA412 single arm gate opener. It is about 5 years old. Recently I got the #5 blink error code. When I use the learn limit mode the gate will open a few feet then stop. Re push the button and it moves a few more feet. The gate swings freely. With the motor dis-engaged it moves a little stiff but I don't think enough to be an issue. I lubed the pivot points, arm etc. I replaced the batteries and still get the RPM error. Any thoughts??
I am thinking I need to replace the arm (RPM sensor) but hate to spend the money if it is the control board.
Any advise would be appreciated.
Thanks
Kurt