Heart Rate Way Wrong

Same issue here when hiking in the mountains. 5S was fine for 2 years but only recently HR reading suddenly drops. I have climbed the same mountain about 10 times during the last 4 weeks and this only started happening the last couple of times. When I expect my HR reading to be at 140-160 on a steep ascent it drops to about 100 and stays there. I've just tried cleaning the sensor to see if that helps at all but not confident it will.

Same issue here. I come from 3HR where there wasn't such a problem. But my Fenix 5+ shows HR way below the actual one when on an activity. Daily HR data is fine, the problem is only when I start an activity. So I believe it's a software issue. Very frustrating. 

the only problem is, garmin does not acknowledge the software problem. instead, they are exchanging watches, as in my case, which was completely useless as the new device had the same behaviour as the old one.......but I gave up :( 

The same as with my recent bluetooth reconnection problems I reported to garmin support. The correspondence was a joke, they had no technical understanding whatsoever, completely useless. 

I have recently discovered the same thing. I always use my HRM.run, but recently I had an injury. Now I am just starting to do some walks without HRM run. I found out that daily hart rate is just fine, but when I start walking activity my hart rate stay below 60 most of the time. I am walking uphil with quite fast pace, so no way my hart rate is below 60. it should be higer than 100.

I did a 13K walk today. OHR completely useless:

Same here, I bought a watch to keep track of walking/every day heart rate. When I start walking instead of 55-60 bpm the watch records over 100. I will have to find out what the local service is going to do about it. I'm not sure even if they replace it that will change anything.

Just went for a walk with my 5X. As the heart rate dropped back to the 60's after peaking around 90-100 during the walk, it briefly (for a minute) jumped to double (120's) then back to 60's as I was sitting...

I'll add to the 'same issue here' crowd.  Seeing that calories burned it tabulated from the HR, that is unreliable too.  I bought a Venu 2 with the new HR sensor.  That one is great, but now I know that I want the maps that the fenix offers, so I'm back to the fenix 5.  I'm hoping that when the fenix 7 comes out it has the new Venu 2 HR sensor on it and all the issues will be resolved.  I know this time to test it out before the warranty expires.  I hate buying extended warranties but these watches are by no means cheap, Maybe the new fenix will be here for Thanksgiving?

The issue was solved for me with a firmware update - version 13 if I remember it correctly. No problems since then.