Lot of bugs on F945 since 3.90 ...

Apparently the downgrade also resets all your settings to factory default. Please check and redo your personal settings accordingly.

Yes, you're right, the downgrade erase all your personal settings. I also did a hard reset, after downgrading to 3.30.

Apparently, all is working well as before, EXCEPT the HR monitor.

I'm unable to do it work ... no measurement, no light on it ...

Any idea to solve that issue ?

The only thing I can think of is the wrist heart rate and activity tracking setting in the watch. Check both settings and make sure they are enabled. You can check the settings in the watch itself or in Garmin Connect.

When you say "hard reset", are you referring to holding the power button until the unit goes dark as GARMIN-Blake suggested?  Or the "Reset Default Settings" through the menu's?  The terminology is a bit confusing.  My understand is the "hard reset" is mostly just a power cycle.  The reset through the menu, resets things closer to factory settings, such as what happened when you installed 3.3 and lost your user settings.

It almost seems like your sensor hub didn't get updated properly when you updated to 3.7 and 4.0.

Regardless, I suspect the Garmin people, like GARMIN-Blake aren't in over the weekend so you'll have to likely wait until at least Monday, if not the first of the year, to get more definitive feedback.

When I said hard rest, it was through the menu, I did a total reset (including the lost of my data).

I also read on the web some operating procedures to do a "master reset". Does someone know what it is ?

Eric, both wrist heart rate and activity tracking settings are enabled. I don't know where is the issue with the HR monitor.

Concerning the battery consumption, it's far better than with 3.90 or 4.00, but it's still consum more than before (in one day, I lost about 25/30% of battery (without any activity recording), which is not normal. And again, I think that the watch is less fast than before when I press on buttons.

Have you added any CIQ stuff?

Not be sure to well understand your question, sorry ...

For the moment, I just added some watch faces, and the application "Drew's CPR Assistant".

It's hard to imagine any software update can brick the WHR monitor. But it sounds like your sensor is really dead, and you have tried everything including the doomsday factory reset already. I only hope you will get a prompt response from Garmin support once they are back online and you can get an exchange. I am out of ideas now sorry.

Can you confirm what version of whr, ant/ble/bt and sensor hub software are now on your watch after the 3.30 downgrade?