Garmin Fenix 8 Update 23.40 Fixes the Dead Heart Rate Sensor
BiohackingGarmin’s software 23.40 for the Fenix 8 fixes an optical heart rate sensor that, on some watches, went completely dark after the previous update, but at the time of writing the rollout had reached only 20% of users. Gadgets & Wearables reports that the changelog says plainly that 23.40 fixes the optical sensor not working. For Fenix 8 owners who lost wrist heart rate, and with it overnight HRV, sleep and Body Battery data, the update means a dead sensor was a software bug, not failed hardware.
What Happened
The problem surfaced after software 23.39. Gadgets & Wearables first reported on 3 October that owners were seeing the green and red sensor LEDs go dark and heart rate readings disappear, including during recorded activities. Measurements that depend on the optical sensor went with it: Pulse Ox and overnight HRV, with some users also reporting missing sleep and Body Battery data. Tactix 8 owners were affected too, and similar complaints had appeared with software 23.31 in September.
The workarounds were unreliable. Some owners revived the sensor by switching the watch off for at least 90 seconds; others needed a factory reset or a firmware reinstall, and in some cases the sensor came back only to stop again. Garmin support told one owner on its forum that the watch might need replacing if reinstalling the software failed.
Version 23.40 makes three other changes. It fixes a crash when active heart rate broadcasting is enabled through Garmin Connect Mobile, and a bug that let notifications make noise in silent mode. A final entry covers unspecified minor issues. Gadgets & Wearables describes it as a compact hotfix, not a feature release.
Why It Matters
Recent Fenix software has been untidy. Garmin issued 22.44 after display complaints in August and an earlier Fenix 8 Pro update aimed at battery drain, and 23.40 arrives only a few weeks after 23.39, a substantial release that added the “OK Garmin” voice wake phrase, indoor XC skiing and several Garmin Coach changes.
The cost of a silent sensor is bigger than a missing workout graph. Gadgets & Wearables says the failure turned an expensive multisport watch into a far less useful device until the sensor came back. Our inference: because recovery-style scores like Body Battery and overnight HRV are read as trends, nights with no optical data also leave gaps in the history those trends are built from. Wearable metrics aren’t diagnostic either way, but a trend with missing nights is harder to read.
What This Means for Owners
Gadgets & Wearables advises affected users to install 23.40 before treating the sensor as failed hardware. With the release staged, many owners won’t see it yet, so a watch that still shows no heart rate may simply not have received the update. Garmin’s own data policies are also in the news this week, as Stack Archive covered in why Intervals Pro shut down under Garmin’s new API terms.
The So What
Fenix 8 and Tactix 8 owners whose wrist heart rate went dark after 23.39 should install 23.40 as soon as it reaches their watch, before booking a warranty replacement, checking again while the staged rollout expands. Owners who rely on HRV or sleep trends should note the dates the sensor was dark, so the gap isn’t mistaken for a real change once readings return.
Source: Gadgets & Wearables
Content created with AI assistance and reviewed for accuracy.
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