A firmware update is supposed to fix things. For a growing group of Garmin Fenix 8 owners, version 23.39 did the opposite. The optical heart-rate sensor went dark and stayed dark.
The symptom is consistent across the reports. The green and red LEDs on the back of the watch stop lighting up entirely. No wrist heart rate. No Pulse Ox. No overnight HRV. And because those features feed other systems, sleep tracking and Body Battery go missing too.
If that happened to your watch, the important thing to know up front is that it looks like a software fault, not a dead sensor, and there is a recovery path that has worked.
What Is Actually Happening
The clearest account comes from a Garmin Forums thread started by a Fenix 8 51mm AMOLED owner. After installing 23.39, their wrist heart rate was set to Auto, Battery Saver was off, and the sensor still produced nothing.
"The green optical HR LEDs do not turn on at all. No heart rate value is displayed. Starting Broadcast Heart Rate does not activate the sensor and no HR value is detected."
They tried the usual fixes. Forced shutdown, watch left off for more than 90 seconds, then a full reinstall of 23.39. The LEDs stayed off across all of it. Their watch was running Sensor Hub 40.08 and BMX 25.09 alongside system software 23.39.
A second owner in the same thread, going by jaxer, described the same failure on a Fenix 8 51mm AMOLED Sapphire bought in October 2024:
"Right after 23.39 installed (around 26 September) the optical sensor went completely dark: no green or red LEDs, no wrist detection, no HR, no Pulse Ox, no overnight HRV."
That thread alone carries at least three 51mm units with the dead sensor, plus a separate French-language thread on the same problem.
It Is Not Just the Fenix 8
Garmin pushed 23.39 to more than the Fenix 8 line. The same rollout reached the Tactix 8, the Enduro 3, the Fenix E and the Fenix 8 Pro and non-Pro models, each with its own forum announcement. Tactix 8 owners are reporting the same dead sensor, and at least one Fenix 9 Pro owner saw the identical symptom after an update, though a reinstall fixed it there.
It is also not purely a 23.39 problem. Owners say the trouble started earlier with version 23.31 in September, and that the 23.35 and 23.39 betas did not resolve it. Garmin had acknowledged reports tied to 23.31 and pointed people toward 23.39 as the newer build. For some of them, installing it changed nothing.
The failure is not universal, which is part of what makes it frustrating to pin down. One owner with a Fenix 8 47mm Solar on 23.39 reported the sensor and battery both working normally. So the same software is behaving differently on different units.
What 23.39 Was Supposed to Do
Software 23.39 was a real release, not a tiny patch. It added the "OK Garmin" voice wake phrase, a suggested running workout, indoor XC skiing, and several Garmin Coach changes. It also fixed pace and speed dropouts, strength activity resets, Spotify problems and a list of smaller bugs.
It follows a run of corrective Fenix releases this year, including 22.44 after display complaints and an earlier Fenix 8 Pro update aimed at battery drain. We have tracked how Garmin has been shipping its Fenix 9 features back to older watches in our backport coverage.
One owner also flagged a battery side effect worth knowing about. With Always-On Display off, their watch estimated 7 to 9 days of battery after the update. Turning off the new "OK Garmin" always-listening feature pushed that back up to about 13 days at 48 percent charge. That is a single user's observation rather than a confirmed power bug, but if your battery estimate dropped after 23.39, the new voice feature is the first thing to test.
The Fix That Has Worked
The most useful reply in the thread came from a user named Martin, who suggested rolling the watch back to 22.44. That reinstall brings back the older sensor hub firmware along with the older system software. If heart rate returns, you can then update to 23.39 again.
jaxer ran it and reported back: "i just did and that helped indeed."
So the working order of operations, based on what owners have reported, is this:
- Full power-off for at least 90 seconds, then restart. This has revived some units on its own.
- If that fails, reinstall 23.39. It has not helped everyone.
- If it still fails, roll back to 22.44 to restore the old sensor hub, confirm the LEDs are back, then reinstall 23.39.
- A factory reset has worked for a few owners, but it is the last resort, not the first.
If your heart rate vanished after this update, treat it as software before you assume the optical sensor has physically died. And if none of the above brings the LEDs back, open a case with Garmin, because the forum thread shows this is showing up across multiple 51mm units and the older Fenix 8 thread where Garmin ended up replacing the watch.
If you are weighing a hardware reset against just living without wrist HR for a while, a chest strap like the Garmin HRM will keep your training data flowing in the meantime, and it sidesteps the optical sensor entirely.
Bottom Line
Garmin shipped a substantial update to its flagship line and a subset of watches came out the other side with no wrist heart rate. The evidence points at the sensor hub firmware that 23.39 installs, and the rollback that swaps the older hub back in has restored the sensor for at least one owner.
Garmin completed the rollout to 100 percent, and plenty of watches are running 23.39 without issue, so there is no reason yet to tell everyone to avoid it. But if your Fenix 8 or Tactix 8 went quiet on heart rate after the update, you are not alone, and you do not have to accept it as a dead sensor. Try the extended shutdown, then the 22.44 rollback, and report it to Garmin if neither works. A hotfix from Garmin would be the cleanest ending here, and given how many units are in the thread, one is worth waiting for.
If you want to compare notes on the rest of Garmin's recent software record, we have covered the Fenix 9 bug backlog and the Fenix 9 Bluetooth fixes.
Sources: the Gadgets & Wearables report dated October 3, 2026 on the 23.39 heart-rate failures for the symptom summary, the affected models, the framing of 23.39 as a substantial release and the note that Garmin had been recommending 23.39 over 23.31; the Garmin Forums thread "Garmin Fenix 8 51mm AMOLED after updating to System Software 23.39" (thread 445048) for the original report, the Sensor Hub 40.08 and BMX 25.09 versions, the user reports from jaxer and other owners, the 47mm Solar counter-example, the battery observation tied to the OK Garmin feature, and Martin's 22.44 rollback advice with jaxer's confirmation; Garmin Forums announcement posts for the Fenix 8, Tactix 8, Enduro 3, Fenix E and Fenix 8 Pro/MicroLED 23.39 rollouts; Garmin Rumors for the Fenix 8 Series stable version 23.39 release notes and the 22.44 display-fix history; and Garmin's own support article on a watch no longer displaying heart rate.
