This is a problem that many have brought up, and there have been many replies, but nobody from Garmin seems to have a solid answer.
Background - bought a used 500 with GSC-10. The seller had the unit pairing just fine. When I put it on my bike, it did not pair, so I researched online and began going through the various suggested fixes. After replacing the battery with a new one, and resetting the Garmin to factory settings, it sensed the meter and I got a cadence reading. Then, one week later, the cadence field just shows (--). So I did the following:
Took battery out and reversed it to reset the meter, and left it out for 10 minutes to be sure. Nothing.
Pushed the reset button. It flashed red and green. Nothing.
Span the wheels to make sure magnets are aligned and sensed. Light flashes red and green as each magnet goes by (Green for speed, Red for cadence). Nothing.
Went into settings and turned off the sensor. Turned off the Edge 500. Turned it back on again, turned on sensor, pushed reset button on sensor, selected search on Edge 500. Crank symbol flashes, but still not picking up sensor.
Checked sensor information. The Speed/Cadence sensor box is checked. Selected just speed and retried search. Nothing. Did same for cadence only. Nothing.
Sensor Status = Not Connected
Sensor ID = 0
No other Ant+ meter in the house.
Am I missing anything? Has anyone else resolved this using any other method than those I have already done?