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.

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9 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?

  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.

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