HRM Dual - BT not transmitting HR

This is the same issue for me - no problem until about 6 weeks ago. Connects fine to Garmin 520 via the ANT+ but can no longer use my HRM via Strava or Wahoo apps for running, which is very frustrating. 

I'm not sure if anyone in here have Androids but today Wahoo and other apps on my One Plus Android 11 device stopped working with HRM Dual, just like other tell it connects and sees firmware and the battery status but no HR readings.

If you have this problem on Android please let me know.

And I have a theory...

My setup was following. I had it paired to my Vivosmart 4 watch via ANT+, initially I had Bluetooth there as well but then disconnected it and removed it to conserve the battery.

Afterwards I've connected it to my main phone and secondary older phone(for tracking rides and runs), all works flawlessly so far. But yesterday I accidentally saw a prompt on my watch to add Bluetooth chest strap.

So what we have today. My watch is working via bluetooth or ANT+, my main phone just sees the strap but no data, but surprisingly my older phone is working just fine too, I can measure HR without any issues.

I tried to remove the HRM from my watch and do other manipulation including forgetting the sensor at my main phone without any success, but this make me think that there is some glitch in HRM Dual which make it fail if you try to connect to many devices via Bluetooth and it makes it fail like that.

I had the exact same issue.  Luckily I still had HR data on the apple TV (for zwift).  On the iPhone the HR data was no longer displayed, although the Bluetooth connection was active.  No HR data in health, wahoo app ...

I just removed the battery from the HRM dual for a minute and removed the HRM dual from my Bluetooth connections.  This solved the issue; my heart is beating again on my iPhone :-)

Sounds promising. Thought I had tried that before without success but will try again.

Thanks for sharing that! I'm a bit concerned about opening the sensor as I've seen a lot of feedback that it stop being waterproof or you can easily break it during that operation so I decided to try and debug why it's happening. I've already submitted bug reports to the phone maker  to see if they can find anything abnormal, wrote to Garmin and people who have more knowledge in this field than me, hopefully this would shed some light on the issue.

We recommend replacing the batteries & performing a reverse polarity on your Heart Rate Monitor (HRM) by following the steps below.

Please follow the steps below to perform a Reverse Polarity on your Sensor / HRM – this will reset the module and allow connection with your head unit;

• Take the battery out, and flip it over and insert it back into the  HRM to reverse the polarity
• Hold it in there for 5 - 10 seconds
• This will reset the HRM strap
• Then remove the old battery and install the new battery properly.
• Ensure the module is fixed to the strap correctly. (only applicable to HRM’s with removable module)
• If using a HRM, wet the electrodes on the strap & position it on your body securely as per the user manual instructs.
• Perform 5-10 start jumps to get your heart rate up & commence the pairing process.

You may be required to follow the pairing steps for each sensor after battery replacement as per user manual instruction.

Related Support Link –  Replacing the Battery on the Garmin HRM-Dual

thanks CristoMB, I've seen that support link but also seen feedback of users who got problems with their HRMs after opening them, so I decided to wait until my battery will run out and then replace it and perform the reset as well, especially that I'm not sure that it would cure the issue with the phone, as HRM Dual works perfectly with other two phones or my watch via BT.

The HRM-Dual only supports 2 BLE connections.  Phone connections in particular can be “sticky” and tie up connections.  Try removing/forgetting the HRM-Dual from the other phones (in the Bluetooth Settings) and see if that resolves your issue.

I have same issue. It seem that it has to do it with software - app - system. My HRM DUAL was working fine for months. Last week I was trying new apps for indoor training (Polar beat). After one day of use it stopped working: now it connects but doesn't transmit heart rate. I had similar problem first week after purchase when I was testing different apps, but somehow I managed to get it working back (it seem it was lucky guess which I don't know to repeat).

If I connect it to whife phone it is working fine. When I connect it back to my phone same issue again. I did all from deleting device, clearing chache, changing battery... Nothing works. It seems like one app block HRM on phone system and you can't anything unless factory phone reset which is not option for me.

I believe that maybe HRM Dual is not top end of Garmin products and they have lack of interest, but Garmin should any way try harder to solve a problem which is very frustrating! Not having support in Garmin connect app - well it tells a lot how much they care for their clients! We are forced to use Polar app to use it with phone! OMG