Liftmaster RSL12V showing 5 blink error

[rwood359]

I have a Liftmaster RSL12v that stops every few feet. It does not backup, just stops and shows a 5 blink code.
The manual says probably board or RPM sensor. I replace the board. No help. I put a meter across the White and Green wires (Signal from RPM sensor). When I rotate the motor slowly, the voltage toggles between about 3.8 volts and 1.8 volts. Is 1.8 too high to be recognized as a low? I believe I need to replace the RPM sensor. I found the instructions for replacing it. It says "remove the chain from the socket". Do I need to use a chain break and open the chain, or is there enough slack that that I can get the gearbox by turning it?
Thanks

[cr-garagedoor]

The voltage fluctuation you're observing (between 3.8V and 1.8V) seems significant. While the exact threshold for recognizing a low signal can vary depending on the design of the RPM sensor and the control board, it's possible that 1.8V might be on the borderline of what's considered a low signal.