Forerunner 255 dynamic source switching off, lactate threshold values fixed, LT guided Test restored

Can you upload a working firmware please?  Have you another, earlier fw - somewhat like 16.xx where OHRS has worked properly? Have you music or a plain fr255?

DOWNGRADE INSTRUCTIONS

Have the needed files from older firmware:
You can find these under C:\ProgramData\Garmin\CoreService\Downloads\Firmware (only if you used garmin express)
Shared 18.21 17.21 11.12 for 255 non music (not sure if these will work for 255 music model)
drive.google.com/.../1-AnnqFzK_5ZfEhq-OFubFSpKlkXC79vz

Downgraded to 18.21 - which gave me GPS 9.03 CIQ 5.09 BMX 15.5.0 WHR 15.05.00 ANT/BLE 0.06 Sendor Hub 28.11

0. Have a Backup of the your settings before continuing check the actual fw version + GPS CIQ BMX WHR ANT/BLE Garmin Hub version

1. Download version for your model from the link and unzip the files

2. Connect your watch to the computer, open the explorer and copy both the gupdate.gsp and force.tmp files to the GARMIN folder of your watch.

3. Disconnect the watch from the computer. After some seconds it will detect the files and propose to perform the "update"

4. will take several minutes on "Loading FIT files" + restarts

5. Disable automatic updates on the watch. Important: Also disable this on Garmin Express in the settings (update device: never !)

6. Re-pair the watch with the Garmin connect App
If new firmware supports this: Backup Up & Restore -> Device Backups -> select the backup and click "Restore from Backup"

7. Resync: this will bring back a lot off settings, workouts etc …

8. All done

You will have to check all the settings, glances etc ...