Edge 840 cadence sensor not detected

Has it worked in the past or is this a new sensor?  A new sensor needs to be paired via the Sensors menu.  For that to work, the sensor needs to be awake.  In the case of a cadence sensor, the crank should be rotated to wake up the sensor.  Here's what Garmin says:  www8.garmin.com/.../GUID-E2E2DC23-7B94-43B4-A30D-5FF32270BEC4.html

try it with an another head unit or try an another cadence sensor with your head unit. because there is a high chance that the cadence sensor is dead. (1st of all, try to close the 2 battery pins with a metal for 20-30secs, or insert the battery upside down, and try the pairing process)

Thank you for the link. It seems  that later returning to the problem and replacing the battery once again and ensuring the body is flush before twisting to lock, and then rotating the crank, enabled pairing successfully. I went out for a ride today and all data collected. Many thanks. 

Thank you for taking the time to help. I have a solution.  See my response above.