My S9 had been working just fine for a long time.
One day out of the blue the phone started randomly restarting and shutting down. Mostly restarting.
After doing some research online, it seems like other S9 owners have this issue.
1. Clearing the phone cache data in recovery mode
2. Starting in Safe Mode and diagnosing the issue
3. Factory resetting the phone
None of these seem to fix the issue. After doing some reading online it seems that leaving the fingerprint scanner on causes this issue. Turning it off seems to remedy the issue.
I use the fingerprint scanner all the time and find this issue and resolution to be unacceptable. Samsung support is there anything you can do?
Unfortunately it sounds like an issue with the hardware part which is fingerprint sensor or others. Due to a lot of places shut down at the moment the fastest way in getting it repaired will be contacting 1-800-samsung and sending it off to them to be fixed. Or You may visit to local near Samsung service center. Hope this helps and I’m sorry to hear about your situation.
You will need to contact the authorized repair center directly for pricing information. https://www.ubreakifix.com/samsung
One other thing you can do, is install a newer android version. I had a tablet that also would just restart, and I upgraded the firmware from andoid 6.01 to 8.01, and now it is rock solid. Worth a try. You can find firmware for your device here (search for your model):
Its a pain to download, but this worked for me.