Garmin Fixes the Fenix 8 Heart-Rate Failure From Firmware 23.39 With Update 23.40

Garmin Fenix 8 owners reported that firmware version 23.39 switched off the optical heart-rate sensor on some watches, and Tactix 8 users described the same fault. Garmin has since acknowledged the bug and begun rolling out a fix (Gadgets and Wearables, Notebookcheck).

<blockquote><strong>Update, October 25:</strong> Garmin began a phased rollout of System Software 23.40 on October 13, starting with 20 percent of eligible watches. Its changelog includes the line "Fixes optical heart rate sensor not working," and also fixes a crash when enabling heart rate broadcasting through Garmin Connect Mobile and silent mode failing to mute notifications. Reports say the Fenix E and Fenix 8 Pro remain on 23.39 for now, and that the fix targets watches where heart-rate measurement stopped entirely, not inaccurate or intermittent readings (<a href="https://gadgetsandwearables.com/2026/10/14/garmin-fenix-8-update-23-40-heart-rate-fix/">Gadgets and Wearables</a>, <a href="https://www.notebookcheck.net/Garmin-delivers-new-update-to-flagship-smartwatches-with-heart-rate-sensor-fix-and-other-improvements.1423942.0.html">Notebookcheck</a>).</blockquote>

Date	Event
Before October 3	Firmware 23.39 rolls out to Fenix 8 and Tactix 8
October 3 to 5	Owners report dead optical heart-rate sensors
October 13	Garmin starts phased rollout of 23.40 with the fix

What users saw

The common symptom was that the green LEDs on the back of the watch went dark after the update, and the watch stopped measuring heart rate from the wrist. Because many features depend on that sensor, the loss spread to Pulse Ox readings, overnight heart rate variability, sleep tracking and Body Battery, Garmin's energy estimate. Some owners on the Garmin forums also reported battery issues.

Notebookcheck said users suspected the problem may have started with an earlier version, 23.31. Firmware 23.39 also added an "OK Garmin" voice wake command and workout suggestions, a reminder that a large update can bring new features and new faults at once.

Workarounds before the fix

There was no universal fix. Some owners restored the sensor by powering the watch off for at least 90 seconds before restarting it, others by a factory reset, which wipes settings. One forum suggestion was to roll back to 22.44 to reinstall the older sensor hub software. In some cases the sensor came back only to stop again.

What owners should do now

Check the installed version under Settings, System, About. If it is still 23.39, look for 23.40 under Settings, System, Software Update; if it is not offered yet, the phased rollout probably has not reached the watch. If the sensor is still dead after 23.40, contact Garmin support, and keep a note of the firmware version and the date of the first failure.

Owners who rely on recovery data should remember that readings recorded while the sensor was off will be missing, which can distort long-term trends the watch calculates from past data.

Why this matters

A heart-rate sensor is central to a sports watch, and the Fenix 8 sits at the top of Garmin's range. Most users install firmware automatically, with no simple way back to an older version, which leaves them dependent on the manufacturer for a fix. Garmin's turnaround of about ten days from the first reports is relatively quick, but the number of affected watches was never disclosed.

What to watch next

Watch for 23.40 to reach all Fenix 8 watches, for the Fenix E and Fenix 8 Pro to get the fix, and for any remaining reports of inaccurate readings after the update.

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