Forerunner 235 Optical HR Sensor Problems


Arghh, heart rate monitors! They’re temperamental beasts that never seem to work correctly. The Garmin Forerunner 235 has a built-in optical heart rate sensor at the wrist, which promises to make HR measurements more convenient and comfortable than with the old-style chest strap. Unfortunately my testing has found the FR235’s wrist-based HR measurements to be wrong by 10-30 bpm often enough that it’s of limited use for tracking my running.

An optical HR sensor works by measuring tiny changes in the reflected light from your wrist veins with each heartbeat. It’s not quite as accurate as the electrical measurements from a chest strap, and it can sometimes be thrown off by other sources of changing light, but ideally its convenience will make up for those shortcomings. Before purchasing the FR235, I read several reviews that focused on its optical HR sensor, especially DC Rainmaker’s excellent review. The consensus was that the FR235’s optical measurements were generally within a few beats per minute of a chest strap’s measurements, especially for runs at a fairly constant effort level, which describes most of my running. The only place where the reviews identified a significant problem was when doing hard interval repeats, where the FR235 would sometimes take too long to recognize when HR had gone back down at the end of an interval.



After a few weeks of running with the FR235, I can only wish it worked half as well for me as it did for DC Rainmaker. When running intervals, it doesn’t just lag with the HR decline at the end, but typically misses the interval completely. Here’s a workout I did recently, where I inserted some short sprints at 5:30/mile pace into an easy run at 9:00/mile pace:


Five sprints are easily visible as sharp spikes in the blue pace graph. For the first and third sprints, the HR measurement from the wrist-based optical sensor barely changed at all. Looking solely at the HR data, you would never have guessed an increase in effort had occurred there. The fifth sprint showed a more gradual increase in HR accompanying a gradual increase in pace, but no HR response from the sprint itself. Only the second and fourth sprints showed a HR response more like you would expect, with a sudden increase in HR, but even there the HR didn’t spike very high. Contrast this graph with a similar workout one month earlier, using a chest strap for HR data:


With the chest strap, each of the six sprints is accompanied by an obvious spike in HR that reaches 160-170 bpm. The HR response to the sprints looks completely different than in the data from the FR235’s wrist sensor.


Cadence Lock

I knew intervals would be a problem for the FR235’s optical sensor, so I can probably excuse this. But what about easy runs at a constant effort? A HR sensor can be useful for ensuring easy runs stay truly easy, if you have a tendency of pushing them too hard. Unfortunately the FR235 frequently fails there too. At least once or twice a run, I’ll be moving at a very easy pace that I know should put my HR in the mid-130s, but I’ll see a HR number in the mid-150s. So I’ll slow down and take tiny little steps, only to see the HR number increase even further into the 160s. Finally I’ll slow to a walk, but the HR number will stay in the 160s before abruptly dropping to the 120s. What seems to be happening is that my cadence (steps per minute) is close enough to my HR (beats per minute) that the HR sensor somehow locks onto cadence instead. Here’s a recent example:


At about 28 minutes I walked for a moment, and immediately afterwards the HR measurement shot up by 20 bpm. It stayed at this artificially elevated level for 4 minutes, until I walked again. Here’s another graph from the same run, showing HR vs cadence. Garmin normally displays the two on different vertical scales, so I’ve used Photoshop to align the cadence and HR scales for a zoomed-in graph:


From 28:20 to 31:40 it’s obvious that the HR number reported by the FR235’s optical sensor is actually my cadence, not my HR. It didn’t correct itself until I walked at 32:00 (orange dots way down near 100). This happens fairly often, and the effect can last for many minutes. Grrrr.


Failure to Recognize Elevated Heart Rate

The final problem I’ve observed with the FR235 optical HR sensor is more subtle. Sometimes I’ll increase my effort level significantly, such as for some tempo-pace miles in the middle of an easy run, but the FR235 doesn’t seem to notice that my HR has increased. Here’s an example from today:


Initially I was plodding along at a 9:15/mile pace, with a HR around 145. At 27 minutes, I increased the pace to 7:55/mile, and held it there for the next four miles. This was on a pancake-flat bike path, and the pace was nice and even, so my HR should have been roughly constant. Yet the FR235 showed an initial HR jump to the mid 150s, followed by a return to the 145 level, where it remained for a long time. At about 50 minutes it showed an abrupt HR jump of 20 bpm up to 165, even though my pace and effort hadn’t changed. The HR numbers then remained around 165 until I slowed back to an easy pace, at 58 minutes.

Based on knowledge of my HR zones from past runs, the numbers around 165 are likely correct, and the measurements from the 23 minutes before that are bogus. 23 consecutive minutes of faulty HR data, during an even-paced, even-effort run on level ground. You could fit an entire 5K race into that window. Grrrr.


Improvements and Conclusions

Maybe I’m wearing the FR235 wrong, or using it incorrectly, leading to poor HR measurements? In the interest of science, I shaved my wrist in the area under the watch, to ensure the sensor had a clear patch of skin for measurement. Per the instructions, I wear the watch slightly further up my arm than I normally would, so it’s well away from my wrist bone and can sit flat against my skin. The strap is adjusted pretty tightly, to guarantee ambient light won’t sneak in under the watch. It’s tight enough that the watch-shaped dent in my skin is still visible 30 minutes after the watch is removed. Any tighter would be unacceptably uncomfortable.

Some reviewers recommend trying the watch on your other arm, or turning it around so it’s on the inside of your wrist. I’ve not tried these solutions, since they defeat the easy and convenient appeal of a wrist-based HR sensor. Who wants to wear their watch on the inside of their wrist? I would sooner just wear a conventional HR strap.

Fortunately it’s possible to do exactly that. The FR235 can be used with an external Garmin HR strap, like the strap from a FR230 or FR220. If present, the FR235 will use the HR data from the chest strap instead of its own optical HR sensor. This makes it possible to use a chest strap for important workouts and races where accurate HR data is crucial, while getting sort-of-accurate HR data from the wrist sensor on other runs. It’s not great, but it’s still better than wearing a chest strap every day, or not getting any HR data at all for those other runs.

Outside of running, the FR235 also measures all-day heart rate. I’ve found this data to be surprisingly interesting and useful, especially the measurements of resting heart rate. By watching how RHR changes from day to day, I can identify periods where I might be overworked and need to take an easy day to avoid overtraining. I’d never get that sort of data from a chest strap.

For now I’m living with the iffy HR data from the FR235 on most runs, even though I know it’s sometimes off by 20 or more beats per minute. For workouts where I care about getting more accurate HR data, I wear the chest strap. Meanwhile, I’m looking forward to what sensor improvements Garmin and others may bring in the future.

25 thoughts on “Forerunner 235 Optical HR Sensor Problems

  1. Yep. great piece. I am having the same problems. It’s very frustrating that the HR spikes or as in an interval workout today, remained at the highest levels throughout the entire workout despite rest periods. I am very disappointed that Garmin has not addressed these issues. Will resort to a chest strap for more accurate readings and seek a non-Garmin watch the next time around.

    • I share the same frustration, I use my 235 for cycling and gym workouts. I have had endless conversations with Garmin and their answer is “refund”. The only answer is it is not a medical device and how you are wearing it. With that answer, you might has well purchase a $100 watch, , or even a Puls Oximeter for $35 if HR is what you are interested in. I am impressed they offered the refund after 3 months of use, but I do like the watch when it does work. Final answer, do I except the times it spikes and ignore or get a refund.

  2. Same problems here. The HRM worked for 6 weeks, after that, whenever I run I have only spikes of higher HR; the average is around 88… I am 55 years old, and my HR is around 135-145 for a 5 K. Don’t what to do at this point.

  3. At last found some confirmation of my suspicions. Am a 70yo runner and have had some odd medical events so a bit “sensitive” to HR issues. Bought a Garmin (a) as a wristwatch (b) backup GPS for recording orienteering routes but hey its got HR so what’s my heart doing. Yikes 160, 170 on easy runs! Have I got heart flutter? See my sports doctor. Waste the country’s medical budget. Probably all the time its my cadence. Thinking about taking it back, this country has consumer legislation requiring “fit for purpose”.

  4. I´ve had FR235 for a about six months now, and it has been worked quite well (despite intervall training, where HR is chancing fast). Yesterday I descovered, that watch didn’t show HR properly, I was running my long, 60-90 min workout (normally HR about 145-160 during the training) and watch showed that HR was constantly only 99?!? It didn’t show HR above that: I tried to stop training and started again, changed wrist etc.: no help? And same thing today; watch dont measure HR higher than 99? Anybody has similar problems?

  5. Hey Steve and team,
    I’m kind a new to all this tracked running. I recently bought 235 and most of the time is good but if I want to run within HR zone it’s more or less useless.
    Can you guys recommend some good hr strap that will work with 235 so I can see my results while running?

  6. Couldn’t agree more with your very detailed comments. This watch (235) in no way reflects it’s marketing promises. Extremely disappointing as they’re not cheap. Looks like I will now have to buy a chest strap to get anywhere close to accurate information.

  7. My problem with the Garmin forerunner 235 hear rate sensor is that when running or driving downhill it shows crazy (high) heart rates (around 160) which I NEVER have when I measure HR with a strap. When the slope downhill is finished it lasts about 1 min until HR normalizes again.
    Thus, it is frustrating to read “fake” heart rates- this produces erroneus calorie expenditures and recovery time. Wirst monitoring is in fact “fake heart rate monitoring” which is frustrating
    Has anybody a solution to this problem?

  8. exactly the same story, so I use HR strap for workouts, as inbuilt HR monitor is as good as nothing (or even worse, as it’s inaccuracy is irritating). Inbuilt then is good only for RHR.

  9. I’ve encountered all these problems too. Extracting and comparing heart rate data from a heart rate chest strap (recorded on my phone) and the 235’s optical HR data shows clearly that the 235 data is regularly off by 10 to 20 beats per minute. This is so inaccurate & misleading it’s worse than not having it at all.
    I’ve ended up doing the same as you – using a chest strap when I need accuracy, and in other situations using the convenient but semi-accurate optical data with great caution.
    I’ve also have an additional problem, where the 235 fails to find my pulse accurately even when I’m not moving. Before running my pulse is often around low at around 50 bpm and the 235 tells me it’s found my HR at around 75 bpm.

  10. Many thanks for your report. Today I did a set of 8 hard 30 second runs with 1:30 very slow intervals for the first time in over a year after injury. With my old 405 with chest strap this would have shown very obvious HR spikes correlating with pace. With the 235 my results look very similar to yours with actual dips in HR as the pace increases. I really like my new watch but for interval training I will follow your lead and use the chest strap.

  11. Thank you for the excellent write up. I purchased the watched based on the DC Rainmaker review but I am having the same problems you described.

  12. Wish I’d read this very detailed piece before buying my Forerunner 235
    My observation is I will look at the HR on the watch whilst checking HR on a phone app.
    Interestingly the value displayed on the watch will always start well below that displayed on the phone and within a few seconds the watch value will begin to rise until
    It matches the phone value. I’m usually seated and resting when doing this.

  13. I just completed a 6 mile steady state run this morning. Course was an out and back with a gradual incline, so I expected an elevated HR the first 3 miles running uphill while a more relaxed HR on the return. Boy did I get a shock! HR was constantly measured 190-205 the entire run and from feel, I’m guessing it was more 145-160. For the record, I am 54, been running for about 8 years and weigh in around 185.

  14. Struggling to find a solution for the very same problem after less than 2 months: unreliable readings, many times stuck to a fixed vale (eg 128) while changing pace.
    TRied on other arm, tried on my wife’s arm (just to avoid shaving, you know…), same problem.
    Funny thing that after the first 3-4 times when it worked smoothly, it now happens randomly, very often (60-70% of activities would say) but randomly. Goodbye Garmin (I bought it as I wanted to get rid of the chest strap)?

  15. I do 6km consistently paced runs and my Garmin forerunner 235 periodically shows a heart rate approximately 20bpm over/under the actual.

  16. I had my Forerunner 235 for a year and the HRM worked fine. A couple of weeks ago the HRM said my rate was 193. I’m a 74 yr old runner so naturally i was concerned. My next few runs
    the HRM had a rate of 133 which is a normal rate for my age while exercising.
    Then my runs were showing a HR of 180- 190. I went to a cardiologists. He gave me a monitor to wear while running. No problem, my HR was a high of 135.
    This cost me ( and my insurance) quite a bit of money. You cannot trust the HRM on this Garmin.

  17. Thanks for your thoughts on the Garmin FR235. I have the exact same problems here with my watch.
    Just returned home from an easy run that suddenly spiked to 165bpm at a decent pace. I used my Samsung S7 to measure my heart rate and it showed me 133bpm. The heart rate of my watch stayed above 150bpm so I restarted the watch and afterwards it was fine for about 15 minutes. Then the same problem reoccurred.
    It really p***es me off that this watch can’t be used for heart rate zone training. I will try to use a strap but that can’t be the solution for a watch that is made for running without a strap.

    Have you mailed Garmin about the problem with a link to your article, that describes it perfectly? Would be curious about their answer since you are not alone with that problem.

    Regards from Germany

  18. I have the same issue.
    Interesting to read about correlation with cadence and HRM. Do you think that shadows cast across the device (which would match arm swing cadence) could be causing the false reading?
    I get

  19. I have the same problem. Sometimes it works well during a run, but often it seems to take the maximum HR. Today, the first half was problematic, only showing the (maximum??) HR, the second half of the run it suddenly worked. Wearing it during a walk seems OK. So, it might be an issue with the cadence. Hope that Garmin is going to do something about it, as I bought the Forerunner 235 just for the anticipated easy HR sensor. Looks as if I need to go back to using a strap. Could have stayed with my Forerunner 910XT.

  20. Excellent analysis, exactly the same problem for me to. I’m sure it used to work when new, I have now had he watch for just over a year. Also tried shaving my wrist.
    I have now just ignore HR on intervals. It seems mostly fine on long slow runs. Is this just 235 issue or all Garmin wrists based HRM I wounder.

  21. Yes, exactly the same issues here with mine. I’ve had it for 2 years and over the last 6 months it’s really got worse. The most common issue I see is what you refer to as Cadence Lock above. I run at 165-170 spm regardless of pace and it will sometimes show 20 bpm higher than what I know it should be or 20 bpm lower. Confirmed via a good old 2 fingers to the neck for 20 seconds.

    Now, it seems to stick with this incorrect HR for the entirety of my 45-50 minute run and doesn’t correct itself even if I slow down or speed up. Great for impressing your friends with your low HR for fast paces on Strava 🙂

    When I suspect it is doing it I have taken the watch off my wrist altogether and held it in my hand and the HR stays about the same. It even moves up or down a few bpm without beign attached to anything and it certainly doesn’t go to 0 as one would expect!

    Thankfully I still have a very old HRM1 strap from many years ago and after sticking in a new CR2032 battery I just paired it with the 235 and it seems to work fine.

    I was almost going to buy the 935 and the HRM strap to solve the problem but my jerry-rigged 235-HRM1 will do for a while …

  22. Thanks, this was helpful! My 235HRM worked well for a while and in the past 6 months, sometimes it works when I run and sometimes it doesn’t… Yesterday I ran for 1 hr in the mountains and the HRM said I was between 70 and 90 the entire time. Haha. When that happens, sometimes turning the HRM off and back on will reset it, and sometimes not. It is frustrating because I’m training under a certain heart rate – and want to slow down when I’m it’s too high.
    Based on your findings, and all that I’ve read, I imagine buying a chest strap will be more helpful! Thanks again.

  23. You are spot on. Having a Garmin forerunner 235 for the past 2 years. As the time goes by, the problems are becoming worse, with Optical HR. Quite a frustrating experience. Thinking of replacing it. Which are the reliable Optical HR watches available now ?

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