Fenix 6X heart rate consistently low. 2853730 over 3 years ago. I bought a Fenix 6X pro last month, and used it for a few weeks and everything seemed fine. I had to get it replaced since there was something funky with the watch since the back of the watch says "Sapphire" though I paid for a Fenix 6x Pro only and customer service confirmed that Fenix 6x Sapphire heart rate inaccurate. John99_1 over 3 years ago. I got my Fenix 6x Sapphire a couple of weeks ago. I liked the idea of having a bigger screen (I've owned a Fenix 3 for a couple of years) and the latest heart rate wrist sensor. I've taken the watch on three different activities and the WHR has been off by at least 30%. Garmin. Garmin has introduced pulse ox sensors into a large part of its range, with the Fenix 7, Fenix 6X, Fenix 5X, Vivoactive 4 and Forerunner 245/645/945 all packing an SpO2 sensor. In this To set your Garmin Vivoactive 4 or Garmin Venu to monitor oxygen levels all night: 1. Press button B. 2. Scroll to settings ( gear icon). 3. Enter the settings menu and scroll to "wrist heart rate". 4. Scroll to "Pulse Ox" and set to "All day tracking". 5. UGSRM.