I have been looking online for a while on how to fix a bug with my Garmin Edge 510 Cadence/Speed sensor but no luck, and now I'm indoors training it's more important that it works (so I can get "speed" data to approximate power). It often drops out as in this ride and this ride for example. It seems I'm not the first to encounter this problem but I'm struggling to find a solution. Is there a fix?
I also have an Edge 510 (since ~3 months) with the same sensor, but I have not experienced this. I have recently updated to the latest firmware, which often solves similar issues with power readings.
Since you were riding at quite low temperatures, the capacity of the battery does decrease, but since the speed reading is fine I would not suspect the battery if the sensor/battery is not old (?).
There is a line/groove on both the sensor and the magnet -- check that they still line up properly.
Check also that the sensor is installed in the correct orientation: the battery cover faces the wheel in my case.
You should also verify if the sensor also acts this way when used indoors at room temperature.
Try re-pairing the 510 with the speed/cadence sensor. It seems like it's not connected and the speed data is coming from the GPS rather than the sensor. As suggested by others, the alignment of the speed/cadence sensor is important, but it must be close since you seem to get a little cadence data before it drops out. Perhaps it's too close and your crank or spoke magnet has actually bumped the sensor and pushed it away?
I did have a similar thing happen with my Edge 500, and re-pairing the sensor seemed to fix it.
I had to switch bikes for a major climb and transferred the ANT and sensors to my other bike. After that nothing worked. The 510 found no sensors to pair with. I tried searching for other compatible devices and it also found nothing. When searching for speed/cadence sensor it prompted me for sensor number for which I did not have and never entered before. The LED was flashing on ANT when pickups were in play on bike stand...I put a fresh battery in ANT and no improvement.
Fix: downloaded firmware update from Garmin to 510. Not sure why this worked but it did.
Now it found the ANT but wouldn't log cadence. The physical characteristics of this Fuji compared to the Specialized that the ANT was transferred from were just enough so that the ANT had to be angled outboard to "see" the cadence pickup clearly. After that it worked great but sucked that it was so marginal in sensitivity. But I guess you need that in a peloton with a lot of magnetics in close proximity...