My speed sensor was flashing (green?) so I replaced the battery.
Now I cannot pair with 520 head unit, and speed sensor appears not to function.
Any ideas?
Take the battery out, put it in the wrong way round for a couple of seconds and then replace it the right way round.
To clarify ... this will do a hardware reset on the device. Luckily nothing is stored on the device, other than (maybe) devices it's paired with, so you loose nothing.