Last summer I was getting ready to climb Donner Pass Road in the Sierras. Another cyclist was a few feet away. I fired up the Edge 500 and it locked on to the other cyclist's HR strap. I couldn't get it to pair with my HR strap until I rebooted it.
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DMC, have you tried clearing the sensor data from your receiver and re-trying it (i.e., turn off cadence sensor, re-start the unit, then turn it back on again)?
Tried synching outside of the class today, but I was only 6 feet outside of the door, so it was still giving me 'multiple sensor units'. I found one teammate with a garmin, and he's never had trouble synching. We guessed that it was because his has been synched before, so it was easier to lock onto his sensor.
OakLeaf: Definitely worth a try. I left the bike in the team bike room, so I'll have to try that the next time I'm up there.