Test the Android system for possible errors on the mobile phone

Welcome all happy!

The warranty on my Sony Xperia 5 II is about to expire so if I have to send my phone in for service I have to do it pretty quickly.
The last time you did this, the repair shop sent the phone back with the phrase: “Your product will be returned due to the fact that the described error cannot be reproduced.”

Now I don’t want them to do it again, because with that you are ready. Is there software built into the phone so that you can run a test to see if it works perfectly or is it something crazy?
Does Elgiganten have test software so they can tell if something is wrong with the phone?

How is it possible that when you send your mobile phone abroad to fix the error, you get a reply that “The described error cannot be reproduced”? Are they just ignorant technicians working in such places or what is the problem? Perhaps they were ordered from the highest place not to find such bugs that it can be expensive to fix?

I can pay if there is a company in Sundsvall that can test my mobile phone if there is something weird with it. Finding out what’s wrong is quite complicated, but in short, the mobile phone has behaved very strangely many times.
It just freezes and then won’t start at all after that. But the next day it shows a weak sign that it needs charging and after that it works again for a while without problems but then it usually shows itself again.

So there must be something wrong with that, but what? What’s the point of a warranty if they can’t find the fault? Do I still have rights after the warranty has expired, if a mobile phone malfunction can be proven from the start?
I’m tired of these difficulties and I don’t think my next mobile will be called Sony.