Latest Firmware update to Garmin 255 has rendered Coros arm band HR Monitor useless. I deleted the device, re - paried to my Android phone . Device shows as connected to my 255 when I start a run , but no heart data is communicated to the watch. Separate issue, but using wrist based HR monitor on the 255 seems more inaccurate than ever. , showing insanely high heart rates at very low intensity, I have tried shutting the watch down, re-sterting, heart rates still way too high for the effort exerted.