Hoping to save some people trouble with this.
was running along happily with my Garmin 510 with the doutrap speed/cadence sensor then I got a Stages Powermeter. For those unfamiliar it is a left crank arm power meter, easy to install just take off old crank and put on the new one.
However afterwards the speed cadence sensor stopped working. I assumed some kind of conflict with the new power meter. I tried so many things including a full reset of the 510.
Turns out the doutrap *only* initializes based on the cadence sensor. I had planned on using the cadence sensor in the power meter so I never bothered to move the cadence magnet on the the new power sensor crank.
Once I reinstalled the cadence magnet on the power sensor crank, the 510 picked up the speed/cadence sensor right away.