Brymen BM257s DC volts + Temperature malfunction

[pazuwu]

I've got a BM257s which started acting up recently.

More exactly, when measuring temperature it displays a whacky value at room temperature, e.g. -149C. The temperature probe works ok with another multimeter.

The second symptom I noticed a while after is filmed here: https://www.youtube.com/shorts/501oxSP0ymo.

When selecting the DC volts range, without probes inserted, the display toggles between -3.000V and 0.000V. When connecting the probes, the multimeter keeps seeking a value ad infinitum, without actually displaying anything. And it doesn't show anything when connected to a battery either.
Auto LoZ seems to have the same issue. 
Other functions seem unaffected.

Normally, I'd send it in for repairs, but I haven't been able to locate someone who can do this in my country. I'll open the unit up and check everything I can think of, but since I'm pretty noob with electronics, I don't expect to figure out the problem myself.

So I could use some hints about what could be wrong, maybe someone has a service manual too...?

Thanks!

Edit: added some hi-res photos of the board https://drive.google.com/drive/folders/1a2uad6Ypdre2sfcD8KpZAJaUS_igxlNU?usp=sharing

Edit 2: came across a video of Dave's  with another Brymen multimeter with a somewhat similar problem. Guess it's not trivial to fix such a thing.



Edit 3: seems mine works on the DC volts if i manually force the range. Auto range seems to be the issue.

[aduecho]

it seems not the HW issue, but the SW, the calibration data in the EEPROM seems lost or corrupt...