Heart rate sensor issues (too high)

Downgrade back to 3.10 and see if that fixes the behavior. I made an assumption that the Hr stopped behaving correctly after 3.77 when respiration was introduced. You will need to download a beta file that has the downgrade file inside. If you decide to do it and your watch behave correctly, then it will prove my point. Just got my 245 last week. it came with 4.2. i downgraded to 3.10 and have done many test. The HR matches my chest strap in 24/7. No activities started.

I'm experiencing exactly the same problem! Have already posted a lot about this on the forum (started with an update around oktober-november). I've had my watch checked in a selling point (because I thought it could be a hardware issue), but they stated that there was no hardware flaw. Still waiting on Garmin for an improvement... A downgrade, as already suggested, could help. 

Thank you Pierotsc.

on my watch it says in "about" software version 4.00 (accb293). how do I downgrade that to 3.10? i tried with the description in this link: https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-245-series/209240/how-to-downgrade-from-software-3-7-to-3-10/982394#982394
but after following their walkthrough my watch prompts to update, then it restarts but the version remains 4.00 (accb293).

i would like to try your downgrade suggestion, so it would be great if you could help how to get it done. thank you again!

Yes, this is the thread i followed to downgrade my watch from 4.2 when it arrived. do you have an APAC version ? if you rename the update file and put in the root of the garmin folder, when you disconnect, it should ask you to update. be sure you are using the 310 file that you have to rename.

my watch is an APAC version... does it mean that I might need another downgrade file? because I tried the walkthrough of that link twice now and the version remains 4.00

then, yes i assume so. if it worked for me and not for you. i guess you need to find a beta file that was made for apac version. to tell you the truth, i am just guessing here.

Same issue here. I thought it was a ploy to get us to buy the chest strap... which worked.

Same issue-- happening when exercising but seems to be fine when at rest.

I have similar issues. The readings can be very erratic. I did a bike ride yesterday and the heart rate was reading over 230 and stayed at 195 or more. This all happened after about 15 minutes. 

Exact same problem. Just sitting here with my Forerunner 55 enjoying a relaxing coffee. HR jumps to 156 bpm. Warning beeps on Garmin that HR is abnormally high. Felt my pulse and HR is near 60. What is wrong with Garmin software? Version is 7.08 - the latest version.