Garmin Edge having an erratic altitude value

Hello,
I have been using a Garmin Edge 530 for a few months (about 5000km) and so far I was pretty happy with it, but since a few weeks its altitude values way off. Right now the computer is on the table next to me and in the last 3 minutes its altitude value lost 20m after I set it up to my correct altitude.

It means that the altitude, elevation gain and gradient values I get during and after a ride are completely false, which is not cool for a 250€ unit. For example I got this:

After Strava’s correction it looks more like it should:

Funny thing, it also seems that the steepest the gradient is, the worst the displayed gradient is. For example here it shows a -8% gradient while I was actually in a 11% climb (sorry for bad quality):

I looked it up online but I can’t find anything. Is it a known problem? Is there anything I can do?

Maybe the barometric sensor, which helps determine altitude based on air pressure, has failed. Is the unit reporting temperature correctly? Ensure the air port is not covered (see the post below from the Garmin forums). Recommend you contact Garmin support.

https://forums.garmin.com/sports-fitness/cycling/f/edge-520-520-plus/143163/elevation-and-temperature-problem-barometer-issue

Not a solution but did you have breaks @Shanx ? Garmin’s are very poor in handling them and potential massive changes in Barometric pressure whilst stationary.

Also on longer rides I’ve found the Barometric altimeter is heavily affected by weather sometimes. I can remember finishing a ride with my old edge 305 where it had been torrential rain for 105 miles. It said something like 3000ft (circa 1,000m) the corrected elevation was something like 10,000ft (circa 3,500m)

Wow.
Had the same Problem Like the OP several Times. While reading your Post, i realized, i was changing clothes on top of the mountain…

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I don’t think it’s linked to me taking some breaks. It didn’t do that for the first 5000km, and it does that even when I don’t take so many breaks. Sunday I rode 13h for a total of 14h30 elapsed time (so not so many breaks), the elevation gain was off (4300m instead of 5300m).

I’ll try to contact Garmin then…