Altimeter and barometer wildly inaccurate

Hey Andy Roc, I tried the same test as you, turning off auto calibration and set sensor mode to barometer only.  It's still no good, the Fenix barometer reading is either well below or well above the local weather station reading.  This then translates to erroneous altitude readings.  While this points to a hardware issue, I agree with you that there's likely a software issue too.  I've found that during activity (e.g. walking) the elevation profile is way off (e.g. -500m to +626m on flat terrain) - if the algorithm is working correctly (and sensor mode is set to auto), it should be pulling altitude from the GPS/DEM not the barometer in this case as the readings are well outside sensor thresholds (e.g. GPS altitude accuracy is +/- 15m to 40m).

Just to be clear, you set the barometer and altimeter to the correct local reading of each before starting your test, correct?

Correct Andy. I manually set the altitude (31 m) and barometer (1014 hPa) to the correct measures at the time (sensor = barometer only). However, for whatever reason, the barometer reading just creeps up and up. Now it's reading 1415 hPa and 1082m. Interestingly, I've gone back to look at the elevation profiles of previous activities around my area (before my watch went haywire) and the altimeter was surprising accurate - it could detect really slight changes in relief (e.g. 7m vertical change over 300m horizontal).  Very good when it works!

This is very unlikely software related. I am afraid you need to ask Garmin for a replacement. 

Are you sure it is a good idea to use soapy water to clean the watch? The soap reduces the surface tension of water and it might get into the sensor holes more easily. Sure it will remove oily residues but it will leave soapy residues which is also not better, if not worse. Could also be BS, but as a scientist, I love making theories :D. 

OK so I think I've got this barometer/Altimeter thing figured out on my watch.

1. Night time auto calibration seems to be working properly once again (not sure why, maybe soft reset, I don't know)

2. I also held the watch under warm running water and exposing ports in order to flush any potential impurities from the sensor. 

3. When I put the Altimeter "Sensor Mode" in "Barometer only" (assumes air pressure changes are all related to barometric pressure), it tracked perfectly with my local airport barometric pressure throughout the day while the watch remained at same location all day. So most importantly I know the pressure sensor works properly.

4. When I put the Altimeter "Sensor Mode" back to auto (assumes changing pressure is related to both barometric pressure and altitude changes) it doesn't do great when the barometric pressure is changing rapidly, but does great on days when pressure is not changing so rapidly. 

5. When I toggle to the Altimeter widget and keep it displaying, I'm actually impressed by its sensitivity.  It changes from when I hold the watch over my head to by my waist (takes about 5- 10 second or so to respond).  

So after all said and done, it seems my Fenix 6 Pro altimeter is working correctly again and I'm not quite sure why. 

My watch did not drift at the same level as you but after 2 weeks it looks like the automode becomes unstable. i shut down the watch and i restart it to verify if it stabilizes the algo. I am wondering if you solved your issue with power  down and power up. 

I'll keep an eye on this and see if drift become a problem again and report back.

For two years I've been reading about lots of problems with the automatic modes for baro/alt, so I've never used them and mine has been working fine all that time. Sounds like some of you are still having issues with the automatic modes to this day, its not that big a deal to avoid using them altogether.

I also decided that the Garmin s/w team was more likely to break things that are already working than they were to improve something I'm actually using, so I froze my s/w at version 9.0 and have never updated it.  It also seems like that decision has paid off for me as well.

Garmin kindly replaced my Fenix 6X Sapphire even though it was out of warranty. The replacement was a ‘refurb’ but it appears brand new from what I can tell. The replacement came promptly and the watch works perfectly, notably the barometer and altimeter. I did some tests, and both features worked on version 19.20 (on arrival) and continued to work on v20.50.  So the issue was definitely hardware related for me.  I’ve been really impressed with the accuracy so far.  Big shout out to Garmin Australia for resolving this.

Matthias, I think you might be right about using soapy water (even though this is the advice for regular watches).  I haven’t been able to find any guidance on cleaning the Fenix watches - has anyone seen any? It’s an issue for me because I do lots of open water swimming, and the water is full of salt and organic particles, not to mention inorganic crap like degraded plastics and oils from road run off (and don't get me started on the high E.coli levels from dog poo).