Erratic Wrist based Heart rate : BUG + Root cause

I totally agree. My wrist HR monitor became unusable during activities. For example, my real heart rate is 150+, and the watch shows values between 120 and 140. At times, it's even lower, and because of that, my VO2 max and training status are incorrect. That's why I started turning the monitor off before each activity. When I'm not exercising, my HR is totally fine. I compared it with the values I get from a pulse oximeter, and it's correct.

I opened a thread yesterday not knowing the "can I trust you Garmin" thread is about the same thing: https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-255-series/324185/not-picking-up-heart-rate-without-a-reboot

Temperatures well over 10 degrees here in my area

The heart rate doesn't seem to be exactly half of the actual value, maybe sometimes, but I have my heart rate stuck at just over 100, so probably not 

I actually don't think the issue has been around for that long, at least I didn't encounter it. It usually picks up the heart rate in about a minute, which is totally fine. I started to worry when it spent half an hour at around 70bpm. I rode uphill, and I know my cardio stuff sucks. The actual heart rate should be over 180, and it doesn't register AT ALL. My usual bike rides stay over 170 for the majority of the time (see my screenshots), 70bpm is definitely really wrong.

Rebooting is kinda fine, at least the progress isn't lost... but the problem is that rebooting doesn't always fix the problem, also I have to stop and wait for it to reboot so the GPS tracks are normal. (also after reboot, it goes directly to the workout screen, but pressing start doesn't work. I need to do Resume later > Workout > Resume. Latest firmware seems to have fixed it, haven't tested it.)

If it's still broken, I might want to get a replacement (if I could...?) Even though this is clearly a software issue (?). Good opportunity to fix the terrible buttons (got un-clicky after a week or two. And the 5 buttons feel different right out of the box too), although I'm going to wait for another 3 weeks before HRV does it's thing right... Garmin...

The algorithms used to measure heart rate differs between the activity types, so they will behave differently. 
I am currently using firmware 14.15, and heart rate readings seem to be pretty good for indoor running and outdoor bicycling in my activities.

Hi, my experience is different. When I perform a running activity heart rate is fine (compared various times with HRM dual connected to another watch or compared with Venu on the other wrist). It's also fine when I'm at home (sit down working at the table or moving inside the house), but if I start to walk outside , after 5 minutes, heart rate become crazy (even 160 for a normal walking). I did many test and compared to Venu on the wrist which despite the fact that has an older generation of HR sensor it is not reproducing the same bug. Those wrong bpm rate when walking are generating extra and wrong weekly Intensity minutes even without an activity. I bought the watch in november and firmware version was 12.27 . New firmware installed didn't change the bug. My personal feeling is that algo developed for normal HR monitoring developed for 255 and new sensor has a bug . Even if I start a running activity and instead I simply walk, the heart rate goes up after 5 minutes like crazy. My question to Garmin is: " why an old generation of smartwatch is performing with no bug ? I expect that , spending money for a new watch, to have more accuracy, not more bug"- I seriously think to use Venu as daily watch and 255 as sport watch only, but I loose HRV measuring at night...... crazy.....

HRV is supposed to be part of the new revised Physio True Up I believe so that one can use multiple devices. There seem to have been quite few chnages in this regard recently as can be seen on the GC app and GC web by the devices. Unfortunately I only have one device so I cannot test this theory....

Thank you for putting together the related articles.

I still have this problem and am slowly losing faith in Garmin. The way they keep quiet. What I'm asking is not that they fix this right away. But I need to know that Garmin is listening to user feedback and has a plan to fix it?

If in a while (maybe 3 weeks or a month) nothing changes, I will probably sell the 255 and buy another sports watch that isn't from Garmin.

I fully agree with you. My impression is that Garmin is not considering the various complaints for the erratic heartrate because even in a full working  conditions  sometimes HR on the wrist is not ok due to personal situations, like transparency of the skin, the way in which  is weared, external temperature and  so on. But what Garmin is underestimating in the case of 255  is the fact that there is a new sensor (and I suppose a new algo)  which is clearly performing worst than  previous Garmin smart watches, As said in my case I compared with a Venu but today I compared with an old Vivoactove HR , even an older generation of sensor: no problem at all, I can walk with a reliable HR monitoring. As you said the impression is that simply Garmin doesn't care, thinking that those that are writing on the forum is a small % of dissatisfied customers that they always have with HR sensor on the wrist.

Maybe Garmin is right about thinking like that, and that the watches are working better than before for most customers? After all, we are individuals. When they make a small change in the sensor or an algorithm, that could work better for one  person, and worse for another. My old Vivoactive HR was certainly significantly less reliable as a wrist heart rate monitor for me than the Forerunner 255.

I doubt it is the problem with the sensor

It worked reliably a few updates ago (at least for me)

A reboot solving it probably indicates a software issue, maybe it doesn't believe that my heart rate suddenly jumps to 185 bpm a minute or two into the workout, when the hr hasn't locked on?