Impossible to get cadence working

I have a duotrap on a Trek Madone and it works as a combined speed/cadence sensor reading both speed and cadence.  I strongly recommend connecting via ANT+ and not BT. That might solve your problem if connected via BT.

You might try disconnecting it all three places (speed, cadence, and speed/cadence) then connecting again as speed/cadence.  Another thing to do might be to reset the duotrap.  You have to remove its battery, put it in flipped over for a few seconds (this shorts the battery terminals in the sensor) then reinstall it in the correct direction.  Verify with its lights that its picking up both magnets.

U rock sir! Trying it first thing tomorrow morning. Thank you 

Here’s something crazy. I tried everything you recommended and it didn’t work. I called Garmin support. They said I don’t need the DuoTrap speed sensor because the GPS will calculate super accurate speed on its own. So I have the Edge 820 paired to the Garmin candence sensor and I’m getting the speed from the GPS unit. Any downside to this? 

Other than the fact that it's not true, no.

I have been using both Garmin GPS and wheel-rotation-based multiple measurements for over a decade.

GPS-based distance measures are biased downward - if you turn a corner, unless you are sampling every point, you will cut out part of the true path and use a slightly shorter straight-line approximation.  This produces noticeable differences which are dependent on the route - I live near an urban/rural boundary, and the difference is much more noticeable on winding urban/suburban streets than on long rural stretches.

You can calculate the "distance" traveled by connecting the dots of the locations recorded in the .fit file and the derived .gpx file. You can compare this to the distance traveled and speed values saved from the sensors in the .fit file, and there will be a systematic difference.

Also, GPS signals disappear in tunnels and even really thick leaf coverage, so no speed or distance there.

Finally, just take a look at how close GPS gets your route to the actual road under difficult situations with buildings or canyons and weak signals.  I was wheeling my bike out through the lobby of an hotel in Maui and after a bizarre initial misreading, it took a couple of minutes for my Edge 800 to be convinced I wasn't in Ohio (the unit has never been east of the Rocky Mountains).

Yes.  A wheel sensor provides significantly better distance and instantaneous speed than GPS.  That is in fact why edge units have  the capability and Garmin sells wheel sensors.  If you use auto pause, it functions much more reliably with a wheel sensor.

IDK why you can't get yours to work.  My duotrap has worked properly from the start.  I also use GSC10 combo wheel/cadence senors and they work as well.  If I can think of anything else to try, I'll let you know.

Among the devices I have used is an indirect power meter.  It relies on speed and cadence signals supplied by sensors in addition to its own windspeed and inertial sensors.  The manufacturer includes the GSC-10 as an iffy sensor which is known to become unreliable over time in that occasionally the speed sensor silently and intermittently fails and the head unit switches over to GPS.  Didn't seem particularly likely to me until I ran all my old fit files through code to identify the sensors, and found that I apparently have owned twice as many GSC-10s as are sitting in my parts drawer, which suggests a high failure rate.  The non-magnet garmin sensors take a few revs to start up, but they are much less hassle.  The newer Garmin speed sensors store ride info, and if you set it up they will upload to Garmin Connect, which is convenient but can lead to duplicated rides if you happen to also use your GPS unit with it.

Thank you! I’m going to keep repeating the full reset process until it pairs then. You also mentioned pairing via Ant+ instead of Bluetooth. I can’t seem to find any information on how to do that. But I find Ant+ usually works better

When you first a pair a sensor and have the Edge search for new sensors, it will offer both BT and ANT+ connections if the device has both.  Select the ANT+ connection, not the BT connection.