Some Garmin Fenix 8 owners say software 23.39 has switched off the optical heart-rate sensor on their watches, as Gadgets & Wearables reported. The green and red sensor LEDs go dark after the update and the heart-rate readings disappear with them. Pulse Ox and overnight HRV stop too, and a few owners also report missing sleep and Body Battery data. Tactix 8 owners are affected too.
Some owners got the sensors back by powering the watch off for at least 90 seconds before restarting it, and a factory reset has worked in a few cases. There's still no clean fix, though, because others say reinstalling the firmware or the long shutdown changed nothing. The trouble may predate 23.39, since similar complaints appeared with 23.31 in September and Garmin has acknowledged receiving reports on that version. Garmin has finished rolling out 23.39 and plenty of watches run it normally, so anyone who loses wrist heart rate should treat it as a software problem before assuming the sensor has failed.