Low heart rate reading for garmin 265 music during running

I hope its not a HW or sensor issue.

I have this problem when doing floors climb activity.

Its like the watch has forgotten to switch on the optical HR sensor to measure.

I have the exact same issue for the last ~2 months. I have the watch for half a year and it wasn't happening before. When I start running the heart rate just doesn't go above zone 2. For me pressing "back" and then resuming the activity fixes the issue, although this is still very annoying.

Just to update. My running and floors climbing HR are working now.

I tried to tighten my garmin watch by 1 hole. Also updated my firmware to 18.26.

I have been having very similar issues where HR is too low for first 2-10 mins of a run and then spuriously jumps up to what it usually should be for the pace I'm doing. I have also not had the watch very long (since Jan 24) but the watch & HR sensor have been working fine up until  the last month or so. To me it seems one of the recent FW updates that seemed to cause it (there have been quite a few recently but I'd say I noticed the issue around the 18.25 mark). I've raised this issue with Garmin support and they said they are aware of 'the case' and are working to resolve it but were not able to offer any more advice or timeframes for a fix. I share your frustration

My HR seems to fix after firmware 18.26. Also I tried tightening the watch by 1 hole.

So I am not sure which one is the fix for me.

I'm still dubious of my latest readings when I ran the day before yesterday, it seemed higher compared to before however. I have been mainly using my 245. 

Plan to do a long run over the next few days. I will wear both my 245 and 265 to see if they are similar or vastly different. Annoying that I have to go to these lengths to understand my readings given the purpose of the watch. Very much doubt I will get another garmin given the cost of them these days and these seemingly ongoing issues.

For the price I should be able to to just wear the watch and forget about it, not having to constantly trouble shoot and try different things to get it to work.

Went on a 17 k run yesterday. To see check the reliability of the HR readings I wore both the 265 and 245 (ridiculous that I need to do this). Wore the 265 tightly, 245 tightly too.

Vastly different readings and given my effort levels I definitely think the 245 was the more accurate. Garmin you need to wake up and stop wasting peoples money

how do you know 245 is more accurate ?

i can confirm its the watch tightness is affecting my HR.

1. Here is my floors climbing activity I just did.

I tried a hole loosened and climbed lap #1.

HR was very low throughout lap #1 until I tighten up. HR started to pick up after lap #1.

I did 5x laps of 19 floors each.

I don't know for certain but highly suspect it is the case.

*given historic data from 245 over the years for similar runs at similar effort levels

*also given that when I purchased the 265 in May 2023 the data was very similar to the data I had been getting from by 245 in previous years. Only since Jan 2024 the heart rate data for my 265 has seemed very off.

Also the data you are showing is from floor climbing. From other threads seems like this issue for some reason is mainly affecting running, possibly cycling too. 

If I tighten the 265 anymore might stop blood circulation to my hand

245 is using the 2 generations older sensor. You could be getting less accurate HR with this watch.

Just make sure the watch isn't sliding on the wrist. Don't really need to be so tight.

I notice if I run with the watch sliding or moving, the HR is less accurate.