I've had 2 of them just completely die and quit transmitting an ANT+ signal. Actually I'm pretty certain that the no flash on the wheel magnet is what both of them were doing.

Are you having the 910 actively try and discover it?

When you say 2 of them died, do you mean the magnet or the actual sensor?

Also on my 910 it says cadence/sensor detected but when I start pedaling it says I'm going 1mph

How close is the arm from the magnet? Mine does not need to be super close, but certainly within 1cm.
If your speed reads low, either the wheel diameter is way wrong or it only picks up the pass very rarely (likely due to weak magnet-can get stronger ones or too far distance).

It's super close! I read on some other forums people have had similar issues and have played with taking the battery out, putting in upside down and then putting it in correctly but nothing has worked.

Just wanted to see if anyone on here has any other suggestions!

Or what the potential problem could be

On the sensor settings (on the 910), is the Spd/Cad box checked? (There is also: Spd only, Cad only, Spd and Cad...you don't want any of those.)

That doesn't explain why the green LED isn't lighting up, but who knows...it may be burned out or just not too visible. It is a rather weak light.

The actual sensors died. Magnets seemed fine. If the test button doesn't work, I'd say it's toast. Also call Garmin, they surprise me sometimes with over-the-top good customer service.

When the battery starts to go, the speed detection is lost first. You can also tell the battery is low if the red LED comes on when you press the button but the green one is very dim or not visible at all. But, if the green LED illuminates properly when you press the button but then it doesn't flash when the wheel magnet goes by the arm, then it's probably that the reed switch in the arm is broken.

Note that after you press the button it will only flash the LED for about the first 20 or 30 passes of the magnet(s). After that you're on your own until you press the button again.

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Less is more.