Heart Rate Accuracy Issues with Fenix 8

I see most of the responses here are over 7 months ago yet I have a current issue. Since approximately August 2025 my Fenix 8 has been slow to react to heart rate changes and is consistently reading low in high exertion exercises such as indoor bike spin classes. This is obviously creating an issue with my load focus and training plan.

I see these posts are a few months ago however I’m having heart rate accuracy issues with my fenix 8 which appeared to start in August 2025. Specifically in bike spin classes the watch responds slowly to heart rate changes and consistently has a lower heart rate under extreme exertion to the max level so I now almost never hit my anaerobic max targets so my load focus and general heart rate reports are little use.

Hi Simon, I suspect this is another issue than the one described in this thread. Have you done all the updates? Have you compared similar exercises with a chest strap and noted a difference? I that's the case, you should turn in your watch for repair or exchange.

Hi Patten, thanks for the message. The next step is to try my chest monitor. I’ll report back next week after spin with an update. ATB Simon

Whilst there's clearly an issue, (as it's not isolated going by this thread an others) it doesn't seem consistent.

I don't personally struggle with my wrist HR accuracy with my unit and it seems to be pretty accurate and where I expect it to be during my runs. Maybe there is a faulty batch in circulation? To others who mention HIIT etc, this is common. Wrist based OHR sensors are never accurate for these types of activities, never. It's pretty common knowledge that chest straps are best for these types of workouts or anything that involves a bent wrist.  

Hi Simon,

I am 59 YO and consistently have the same issue of high heart rate and heart rates during the first mile of a run that can hit 180+…Impossible.  I see a lot of reply’s saying use a chest strap, but unfortunately this watch should perform better.   I purchased the F8 over a year ago for the bigger screen and flashlight and recently (3+months) the heart rate has been inaccurate during the first 5 minutes of a run or when pushing high heart rate intervals.  I have run now every day for 130+ days and wear my Epix on the other wrist to observe the common F8 inaccuracy.  Sure, you can pause your run and let the watch settle in and within a few seconds and then you go from 158 to 118.  You can also try to spin the watch 180 degrees on your wrist.    I also have paired the F8 with a OHR arm band and Wahoo Ticker-x and no issues.  For wrist OHR - My Epix or 945 does NOT have the issue that the F8 has.  If I am needing more accurate HR for a race I will opt for a chest strap or OHR arm band. (plus also wear the Epix)…. But for daily usage it should perform better.  I hope this gets better with a firmware update.

No optical sensor can perform as reliably as a chest or electric impulse sensor. Optical sensors have to overcome more variables.  If your watch starts at 180, that sounds like cadence lock. Your epix 945 may have fit on your wrist slightly different.

If that's the case then it would be consistent over the last 9 years and several models of watches that I've had. But it's not. Garmin improves and sometimes reintroduced bugs in the heart rate monitoring during workouts. Fenix 8 had improved significantly last year but it turned bad about 2 months ago again.

Those seeing recent issue with wrist HR might want to consider the colder weather, depending on your locale.

This makes a HUGE difference for me (I have very deep veins, basically I can't bruise, for whatever reason) in the fall/winter.  I find that most of my wrist HR is inaccurate, at least until I'm significantly warmed up, so for low-aerobic stuff not at all in most cases in the cold weather.  For activities I really care about tracking accurate HR in the colder months I always wear an arm strap, or occasionally a chest strap (upper arm optical is very close in most, and more comfortable IME/IMO).