Altimeter problems

I had exactly that on my out-the-box-new FR970. I did any and all testing I could, down to a factory reset, but no avail, had to conclude it was a faulty sensor. I contacted garmin support and they opened up an exchange, my replacement has just shipped so I'm eagerly awaiting it's arrival!

Are your issues like mine?:

• Altitude all over the place - can do a manual calibration at the start of a gps-activity but then it quickly goes ALL over the shop (I once soared above the moon and then went deep underground - on a route I know to have a total elevation gain and loss of about 285m each!)
• Temperature all over the place (watch told me I was fluctuating between -275 and +199C (I wouldn't be alive if this were true)
• Power data all over - comparing a 5K race I did last year (FR955) and this year (FR970), at similar effort, near identical conditions, identical course, my avg power last year was 390W with max just shy of 500W, but this year I went from 0W to 3995W up and down over and over about 10x in the 5K
• Anytime I start a GPS activity (not even press start, just enter the activity and let the GPS start searching for satellites) the sensors go haywire
• Exit out of activity, go to the glance/app for altitude (make sure I'm outside with GPS signal) and watch is slowly come down to where it should be, then exit out of elevation and go into temperature and that's totally normal and stable too
• When things go haywire, temperature history stays on my glance/metric, but elevation history gets cleared (graph is empty and just starting up from scratch). If I've not done an activity (GPS) for a few hours, elevation graph retains data, but clears again when the sensors go bananas with my next activity

So the only conclusion I could draw (and garmin support seemed to agree) was that the sensors were faulty (I suspect as simple as a dodgy connection, someone will open it up, take a soldering iron and re-fix the sensor, and it'll be good as new for someone else to enjoy soon)

A colleague has the 970 also, and I've checked some of his runs and rides, and the elevation and power data all seem totally fine. So I have high hopes for my new 970 when it arrives, hopefully in a few days

FYI here's my original post on my elevation issues: forums.garmin.com/.../elevation-issues

Got the exact same issues with my 970 straight out of the box.

Try some basic troubleshooting (reboot, soft reset, turn barometer and altimeter on/off, do a calibration, hard reset - but back up first, makes restore easier just won't restore any watch face customisations from ConnectIQ so take screenshots!)

But mine was out the box too, hardware fault. If troubleshooting and resetting does not work, it might be hardware in your case too. Contact Garmin Support, they're really awesome, they might ask for some troubleshooting steps too (so detail anything you've already done), but ultimately if it is hardware, they'll offer a warranty replacement (whether you bought direct from Garmin, or from a 3rd party like a sports store or Amazon) or refund (only if you bought directly from Garmin)

Do you have a screenshot of the flat elevation you can share?

I had the same trouble, and had the issue with my 965, too (sold and am currently using the 970). For both, what worked for me without fail was changing the altimeter to manual calibration before every activity. It’s an extra step when starting my runs, but it fixed the issue for me. 

That's brilliant, so your altimeter is okay just a software glitch. In my case I was calibrating before every run, but still sent me to the moon and then underground throughout my runs - for hardware faults this won't work but it's a good troubleshooting step either way

Garmin altimeters have big sensitivity problems and if you don't clean the holes often and thoroughly they have problems, on the old 945 it was a disaster, now I have a 955 solar from November 2023 and it works very well, I wash it twice a day (morning and evening) with soap and warm water and I use manual calibration before each activity

This is very true (my old 935 was perfect until it wasn't, but that was years). My 955 altimeter is perfect but it's less than 3 years old and also washed after every activity (mostly water but at least once a week a bit of soap and a thorough rinse)

I do think though that the 970's seem to have at least one batch with faulty sensors, given how many people have reported the issues. In my case, my watch, brand new out the box, the altimeter was banjaxxed from my first ever run. Watch less than 6 hours old (bought late morning, charged, set up, then went for a run). Even after cleaning, still same. All points to hardware fault. It happens, Garmin could've received a faulty batch of that sensor, and thus a number of watches affected