Looking for previous 26.xx firmware for my Fenix 6 due to altimeter issue with FW27.00

I am afraid that the sensor is broken. It is a unit with a pressure and temperature sensor in it. Impossible values indicate it is malfunctioning. That is why Garmin Support told you it needed replacement. Everything else still works normally, so if you don't need altitude you can use the watch with this defect. Software downgrade does not solve this. Sorry for the bad news. 

Hum... Very strange... It comes with fw update.

In fact got the right widget for temp now... Value is not existing.

Do you think I can try to flash with an old firmware?

Hello.

My watch is working again...

Suddenly temperature value comes to the screen. I did an altimeter calibration et voilà, it is good now

This comes after the last update of the Garmin app update (I know, nothing to do). After the sync it comes good !

That is miraculous! Enjoy your revived watch! Very odd that it seems related to an update to the mobile app, but well, these things sometimes work in mysterious ways.

This morning, altitude is back to a bad value while temperature sensor still ok...

Issue is elsewhere

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Maybe too hot. IIRC there was an issue were the watch was OK after putting it in the fridge, but don't remember what it was

Try diagnostic mode to see if the sensor is broken. See instructions here: www.hardreset.info/.../

Scroll through the screens with the Back/Lap button. Active tests using the Start/Stop button.

Did some...

What happens if you calibrate the altimeter manually?

The temperature looks OK. I'd think that the baro/temp unit is OK. I am also curious what happens when you manually calibrate the altimeter.