Sense stopped tracking heart rate correctly

[Pgperk]

My sense has stopped tracking heart rate correctly during exercise sessions. Used to be very accurate  i can physically feel my heart rate increase dramatically while doing HIIT in gym sessions with instructor.  I had made no changes to wrist placement or band when this started happening out of the blue . I have since tried resetting my sense, moving location on my wrist, cleaning the back of the sense and even changing to a new band.  Still shows below level heart rate for the entire session - sessions usually 45 mins to an hour long.  Heart rate shows at 90 or below when should be 145 - 160 when tracked by other methods

[Guy_]

Hi @Pgperk - this is more or less normal if you look in the forums.

What you can do is give it a clean How do I clean my Fitbit device?  and also sync then do a Shutdown from the watch Settings menu. Wait at least 10 seconds [to allow it to complete] before placing it on charge to restart it and charge it. If it's already charged just push the button for a second and wait.

This should make it work the best it can. Fortunately you should still get good resting heart rate and sleep recognition as the sensor is only adversely affected during certain possibly repetitive movements, like exercises.