Heart rate monitor troubleshooting: Suunto Smart Sensor & Ambit3 Peak

Last November my heart rate monitor was not working properly. Readings were regularly inconsistent and inaccurate during training runs. I use the Suunto Smart Sensor and Suunto Ambit3 Peak (long-term review), which I purchased together in August and used together without incident for the first 150 hours.

Here is an example of its behavior:

Screenshot from Movescount

The readings are sometimes accurate, like between 8 and 25 minutes, when my HR was in the 130’s during a warm-up.

But more often the readings are not correct. My max HR is about 178, so readings in excess of that threshold (like 200+ bpm for the first 8 minutes and again around 55 minutes) are simply wrong. I’m also skeptical my HR was in the 130’s between 30 and 42 minutes, which were the first 12 minutes of a 30-minute tempo at 5:38 minute/mile pace. More likely, it was in the 150’s or 160’s.

Erratic readings are unacceptable. I need my HRM to be always accurate, so that I know my effort:

  1. At the moment, and whether I should change my pace; and,
  2. Over a given distance or time, i.e. average, so that I can compare workouts from different days, weeks, and years.

Troubleshooting: Suunto Sensor

I tried to reestablish correct readings a multitude of ways until one of them worked. For years I have successfully used a HRM, so it was more than simple user error, such as not:

  • Pairing it correctly to the watch (refer to this video),
  • Wearing it correctly around my chest, at the base of my pectoral muscles,
  • Wetting the electrodes thoroughly before use, or
  • Cleaning it regularly with warm soapy water. (I wash mine daily during my post-run shower.)

1. Battery replacement

The Smart Sensor needs one CR20125 coin cell battery. A weak battery may not reliably receive readings from the electrodes or transmit the data to the watch. This didn’t solve my issue.

2. Leave my phone at home

My problem seemed to arise around the same time that I began to regularly listen to music during my runs using my phone. But on runs without my phone, the problem remained.

3. Re-pair

Wet the HRM, put it on, and re-pair the HRM to the watch.

4. Reset the module

This can be done with the Duel Comfort Belt, but not the Smart Sensor.

Remove the battery. Put it back in, inverted, for 10 seconds. Remove the battery again, and put it back correctly.

The module will need to be re-paired with the watch, per #3.

5. Reset the watch

Before attempting this, download all log data to Movescount. If you reset the watch without downloading the data first, it will be gone forever.

To reset the watch, hold all five buttons (for about 12 seconds) until it resets.

The watch can be partially reset in two ways:

1. Enter the Service menu, by holding the two top buttons (Back Lap and Start/Stop) for 12 seconds. Select “Action” and then “Power Off.” To turn the watch back on, hook it to its USB charger.

2. In your Movescount account, go to “Watches.” Select the option, “Restore default values.”

After resetting the watch, it will need to be re-paired with the Smart Sensor.

6. Contact Suunto customer service

On the Suunto website, there are hundreds of Product FAQ’s. If you can’t find information that solves your problem, call them toll-free at 855-258-0900 (USA).

7. Replace the module or belt

Ultimately, I had to replace the belt.

An entirely new Smart Sensor Belt retails for $80. But a replacement belt costs less, about $30. If your current belt’s electrodes or sensor connectors are damaged, this should solve the problem.

Alternatively, you may have to replace the module, which is more expensive on is own. If your Smart Sensor is not pairing to your watch or if the watch regularly loses the signal, it probably is the module, not the belt.

If your Smart Sensor was purchased from an authorized dealer, it is warrantied for a year. Customer Service should take care of you.

Disclosure. This website is supported by affiliate marketing, whereby for referral traffic I receive a small commission from select vendors, at no cost to the reader. This post contains affiliate links. I have no other financial interests in any brands or products.

42 Responses to Heart rate monitor troubleshooting: Suunto Smart Sensor & Ambit3 Peak

  1. Bob S. March 16, 2018 at 5:56 pm #

    I had the same problem with a Garmin Premium HRM reading over 200 bpm. Replacing the battery didn’t work so I called Garmin and complained. To their credit Garmin sent me a replacement HRM and did not ask for the old one back. I’m not sure if I bought a bad replacement battery or the strap was just messed up and needed washing because I sweat more than the average human but over the years I lost track of which monitor goes with which strap. I was able to rule out the problem was not with my bike computer because I had the same issue with my watch.

    Whatever combination of strap and monitor I’m using now seems to work so I’ll just file it under unsolved mysteries of the universe.

  2. Jeff D. March 20, 2018 at 7:55 am #

    I had a very similar problem with the Ambit 3 Peak and Suunto Smart Sensor. Both units were purchased in December 2016 and functioned without issue until November/December 2017. The readings would jump from 70s to 200 plus without cause and it never seemed to settle into a pattern. I replaced the battery then replaced the Smart Sensor without any luck. I even tried heart rate sensor gel since it’s winter and I’ve heard static electricity can be an issue. Finally replaced the sensor with a Polar H10 and haven’t had any issues. I’ve used the Ambit and corresponding heart rate monitor religiously since 2012 and this has been my only complaint. Strange.

    • Uri June 12, 2018 at 8:06 am #

      Thanks for sharing Jeff.
      Just to confirm – is Polar H10 compatible with the Ambit 3?
      Does it pair with the watch without any issue?

      • B July 20, 2018 at 8:38 am #

        Hi Uri,

        Did you test the N10 with the Ambit 3?

        • Jeff D. August 14, 2018 at 11:40 am #

          I have not tested the N10 with the Ambit 3.

      • Jeff D. August 14, 2018 at 11:40 am #

        Sorry for the late reply, but yes the Polar H10 does pair with the Ambit 3 without issue.

        • Uri August 29, 2018 at 1:03 am #

          Thanks. Good to know.

    • Ian December 6, 2018 at 6:02 am #

      I’ve had recently the same issue with Ambit 3 and HR sensor. The belt was replaced earlier this year, and suddenly have seen spikes of up to 223bpm, my max is 180bpm and has rarely gone over this. I 100% feel it when/if it has gone over this. reluctant to change belt then sensor especially at an additional cost of £65 for the 2, in case it doesn’t work.
      I’m going to change the battery first to a known brand to see if this works…..Fingers crossed but it is definitely frustrating not knowing if it’s watch/belt/sensor or external factor like other bluetooth devices messing it up.

  3. John M. March 20, 2018 at 5:57 pm #

    This really is a problem with all “soft straps”. They all will fail, from the pricy name brand versions to the generic versions you can find on Amazon. I gave up myself and bought the old style Garmin hard strap, yea it’s not as comfortable but it should last much longer. (and it’s only $25ish now).

    But even then it still will have the erratic heart rates when you first start unless you really slobber over the contacts or use some gel.

  4. Tom April 10, 2018 at 7:30 am #

    Thanks for the comprehensive information. You rock! My heart rate monitor (smart sensor) Ambit3 was doing the same thing as yours was…worked well for many hours and then was randomly showing blank spaces of no reading for long periods of my workouts and inaccurate readings when it did register. Like you I use it to compare workouts and to dial up or down my pace in real time. My fix was reseting the ambit3 with the 12 second solution. Now I have a proper read and don’t have to spend $80 on a new sensor!


  5. L'toi May 7, 2018 at 7:33 am #

    I have a similar problem that only occurs when I am running (not cycling) and also more frequently when I start the run. The reading is WAY too high, but with (what seems) correct variation – it is like an additional 60 bpm is added to my HR. When I cycle I do not have this problem.
    My initial conclusion was that the HRM does not like bouncing up and down – because that is essentially the difference between running and cycling. I adjusted the strap until it was really tight. No joy!
    Then I read that the polyester that my running shirt is made of might produce static electricity that interferes with the HRM. This makes perfect sense and explains why my readings are mostly off at the beginning of my run (because later on I have sweated a bit and my shirt get damp – no static!) and also why I don’t have any problems with cycling (the shirt does not bounce up and down to produce static)

  6. Grambo July 10, 2018 at 9:48 am #

    Suggested alternatives for a HR belt/sensor that will work with the Ambit3? I’ve replaced my Suunto Smart Belt twice, replaced the battery, re-paired, etc., and the reading is still erratic — works great, and then inexplicably cuts out or shows a reading that’s waaaay too high; and then settles down again. Tired of dealing with this, as I use the HR to dial in my easy days and races.

    • Andrew Skurka July 10, 2018 at 5:01 pm #

      If you find one let me know. Any Bluetooth HRM should work. Unfortunately you’ll have to pony up a fair bit to get out of the Suunto ecosystem. I just keep replacing the belts, and that seems to do the trick (for a while).

  7. Grambo July 11, 2018 at 8:05 am #

    I’m contacting Suunto. It’s almost like they designed this to mess with our heads, “Let’s make it run totally fine on this run/ride…then cut out…then back to normal…then randomly add 40-60bpm for no reason at all.” Will keep you posted. And thanks for the blog piece on this. I have to believe many Ambit3 owners have run into this, and Suunto ought to address it. It’s not like HR monitors or straps are new tech.

    • Grambo July 26, 2018 at 12:41 pm #

      Update: I just spoke with Suunto customer service and they’re sending me a new sensor. Hoping that resolves the issue, although I’m a little skeptical, as my current Smart Belt/sensor DOES picks up my HR just fine about 90% of any given workout. It just continues to jump or stop reading, ranging from a couple seconds to many minutes with no rhyme, reason or pattern. Seems the only way to resolve issues with Suunto is with a phone call to their toll free number. I originally emailed them receipts for my Ambit3 and my new-ish Smart Belt (took a while to find Suunto’s U.S. email) and got a reply back that I had to call. A small hassle, but they were friendly and quick to offer a replacement sensor.

  8. Betsy July 18, 2018 at 8:20 am #

    I was glad to read these. Thought I was going to have to visit a cardio doctor !
    I used it for a long time with no problem while rowing paired with CrewNerd app. Changing battery helped some but still some erratic spikes.

  9. Terry August 6, 2018 at 4:14 pm #

    Seems like a common problem. I have the same thing.

  10. Betsy August 6, 2018 at 5:28 pm #

    BlueTooth HR monitor recommended by app. Ordered and retired the Suunto monitor.

  11. Terry August 6, 2018 at 5:43 pm #

    Betsy What did you actually buy?

  12. Betsy August 6, 2018 at 6:09 pm #

    Arriving tomorrow, via Amazon : a generic BlueTooth HR belt. Should pair up better with my CrewNerd rowing app. Will know better on Wed after morning row. For now I have a Fit Bit which does not quite know about rowing nor give the instant feedback. However, it does not show indications that HR is at 210 bpm while at dock before even getting in boat !

  13. Betsy August 7, 2018 at 3:37 am #

    Onlyesh CooSpo H6 Dual Mode Bluetooth & ANT+ Heart Rate Monitor/Bluetooth V4.0 Wireless Sport Heart Rate Sensor Chest Belt Monitor for Real-time Reciever

  14. Christopher Robinson August 12, 2018 at 12:20 pm #

    I recently bought a used Ambit3 and have decided I need a HRM. Confused that the standard strap is medium and even Suunto doesn’t seem to have a larger strap. My chest is in the 40s. Any links to a larger one?

    Betsy, I would love to know how your CooSpo is working out.

    • Betsy Mitchell August 13, 2018 at 8:37 am #

      I have rowed a few times since new HR monitor arrive, replacing the Suunto belt that was erratic. Once I was able to pair with app and phone ( takes patience to be “discovered”), my CrewNerd app worked great. I compared it to the Fitbit while doing a piece and found that over 90bpm, the Fitbit was less dependable than the (Amazon) CooSpo HRM.
      Once paired , it automatically displayed info with the app.
      Happy for now.

    • Bob S. August 13, 2018 at 2:02 pm #

      Christopher,I have the same problem with my Garmin chest strap; If I gain 2 more pounds I’ll need a bra extender. Personally, I think chest straps are uncomfortable so I set the optical sensor on my Garmin watch to broadcast heartrate data to my bicycle computer. Maybe it’s not the most accurate way to monitor heartrate for a professional athlete but it works well enough for my needs.

      I don’t think I have ever seen Andrew weigh in on dedicated optical forearm band HRMs vs chest straps so if anyone has tried one I’d be curious to know how well they work.

  15. Jeff D. August 14, 2018 at 11:45 am #

    After having the same issue with both the Suunto Smart Sensor and the Polar H10 (which pairs to the Ambit 3), I came across the article below on thoroughly washing the strap. Apparently, hand washing the strap wasn’t removing all of the body oils because it’s worked flawlessly after a comprehensive wash. It’s free, so worth trying before spending the money.

  16. stephanrunning August 14, 2018 at 1:41 pm #

    Thank you very much for this blog post and the very helpful comments. I had exactly the same issue of completely implausible readings. Inspired byJeff D, I started using the Polar N10 in May. This has worked almost perfectly (only a very few temporary drops in the heart rate mostly at the beginning of some workouts).
    After reviewing my data, Suunto Support advised to buy a new belt. Since they had already sent me a new one only a few months before (in January) I decided to switch to the Polar. By the way, I don’t find the N10 less comfortable than the Suunto smart belt which is a little lighter. Hopefully this solution lasts for a while.

  17. Terry August 15, 2018 at 11:38 pm #

    Unlikely as it sounded, I tried the comprehensive wash described above and my HRM has been functioning perfectly ever since. So it’s worth a go.

  18. marc August 23, 2018 at 1:08 am #


    Thanks for sharing… had exactly the same issues… finally hearbeat was not detected anymore and showing faulty on watch alternate… not accurate at all.
    Resetted the watch 12sec and problem solved.

  19. Andrea September 6, 2018 at 1:21 am #

    Thanks for helpful infos, I have same problema, but I’m a little bit skeptical about solutions proposed.

    First: mu Suunto Smart Sensor works perfectly recording the activity with cell and Suunto app: no peak no hole… the heartbeat of a man not an humming-bird!

    Second: also the barometric reading are fuzzy: storm fake-alarms and altitude graph with jumps of hundreds of meters

    I’m going to try with a different bluetooth belt…. Hope never die!

  20. ove September 21, 2018 at 4:24 am #

    Hade the same problem yesterday when running – seemed like adding 60 beats to my HR. Reading on Ambit3 watch read 200-215 beats.
    When I downloaded the run and looked on movescount it seemed to correct the numbers to what looked like normal HR. Anyone know what is up with that? Does movescount have an algorithm that corrects HR?

  21. Dexter October 11, 2018 at 9:56 pm #

    My readings were getting erratic over the last 3 or 4 uses. It would either be low (50-60’s) during exercise or “–“. It paired fine, and I tried all of the other remedies short of purchasing a new strap. I washed it and it works again. Brilliant!

    • Dexter October 18, 2018 at 9:24 am #

      Update to my post on Oct 11th. The cleaning did not correct my problem. It did initially find the HR monitor better but the readings did not improve in consistency. After a little more inspection of my HR monitor I noticed that the battery was not secured properly inside. It wiggled around inside of the monitor and therefore would lose connection which was causing my problem. I removed the battery and bent the 4 connections that hold the battery in place in a little bit which firmly secured the battery again. Vioala! HR monitor connected immediately and recorded perfectly this morning.

      • Dexter December 6, 2018 at 1:14 pm #

        Update on 12/6/18: Despite the cleaning and resecuring the battery clips, I still continue to have issues with the HR monitor. I find that it regularly will not connect as it should.To remedy the situation, I have to remove the battery from the monitor, reinsert it, re-pair it to the watch then it will work as it should. The problem is that it loses its pairing almost after every day that I use it. Not sure what the root problem is. Wish it could be fixed. Not really wanting to buy another one.

  22. Betsy M October 18, 2018 at 9:51 am #

    Good to know. Thanks

  23. Grambo December 6, 2018 at 12:11 pm #

    Update: A new sensor resolved the issue. Phew!
    It took a while to get through to customer service and to have them send a new sensor, but it was worth the hassle. My watch’s HR readings have worked flawlessly since.
    The few times it dropped the signal were almost surely because my skin was too dry starting out on a run or ride.
    Hope that helps. Good luck, everyone, and enjoy a great, outdoorsy winter.

  24. Hussein December 28, 2018 at 8:02 am #

    happy to read I am not the only one having those trouble with HR & ambit 3.
    Suunto is sending me a new sensor/belt for the second time …
    I am still owning my ambit2 (I am still using it) where I had troubles too but only the 10mn of my activity (running only). Thanks for your posts.

  25. Urig January 2, 2019 at 2:30 am #

    Sadly I was also provided with a new sensor and strap, and they worked fine for about two months. Now this set is completely off – showing bad readings throughout long runs (beyond 90 minutes).
    Cleaning, battery replacement etc. do not help.
    Sorry – this is a sub-par product.
    I have decided to purchase a 3rd party HR sensor as I don’t believe Suunto has a real solution.

    • Andrew Skurka January 2, 2019 at 2:18 pm #

      Suunto’s “solution” has been to send replacement units. But that’s a band-aid, not a fix.

      I’m going to ping my Suunto media contact to see if you knows of anything in the works.

    • Urig January 12, 2019 at 5:05 am #

      Purchased a polar H10 sensor. Works perfectly with Ambit3 sport.
      Problem solved.

    • Grambo January 15, 2019 at 8:09 am #

      Yup, exact same situation here. My Suunto replacement sensor/strap worked beautifully for a couple months. Now, it’s as irratic as the one it replaced. Frustrating. Guessing it’s just a bad design that wasn’t thoroughly tested in the real world before Suunto put it on the market. Too bad, because, for me, the failure of this one accessory hurts the reputation of an otherwise great watch (mine is the Ambit Peak3) and great company.
      Not sure what I’ll do now. Probably buy a Polar H7 or H10.

  26. John Payne January 2, 2019 at 6:15 pm #

    Same problem, but I’ve been using the Suunto sensor with the MoveSense app on an iPhone that I carry in my hand. Mine has also been erratic — almost always swings wildly at the beginning of a run, even if I get the strap wet. It seems to stabilize once my skin is a little bit sweaty, but it has also stopped working in the middle of a run, then worked again afterwards. The app has lost my data several times, too. It’s not the battery. I think it’s a combination of imperfect connections between the device, the strap, the athlete’s skin, and bad software and/or hardware that doesn’t react well to those connection problems.

  27. Grambo January 15, 2019 at 8:10 am #

    Yup, exact same situation here. My Suunto replacement sensor/strap worked beautifully for a couple months. Now, it’s as irratic as the one it replaced. Frustrating. Guessing it’s just a bad design that wasn’t thoroughly tested in the real world before Suunto put it on the market. Too bad, because, for me, the failure of this one accessory hurts the reputation of an otherwise great watch (mine is the Ambit Peak3) and great company.
    Not sure what I’ll do now. Probably buy a Polar H7 or H10.

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