Ever since Roger's has released the android 5.1.1 build 23.4.A.1.200 for Xperia Z3 (D6603) I have been getting random soft reboots or stealthy reboots. This started about a month ago. The first time I noticed was when I woke up one morning and went to wake my phone (double tap) and it wasn't responding. Then a second or two later it vibrated two or three times and the boot animation screen came on for a couple seconds and then brought me to my home screen. I thought it was really odd because it has never happened before so I just thought it was a small glitch or something. Later on that day I was looking at my battery usage in power management and saw a pretty big gap in the graph where it basically looked like the phone was off for most of the night and then came back on. This really started to concern me but then I was thinking about it a bit more and thought "if my phone was off for that amount of time, why did my alarms go off earlier in the morning way before the reboot happened?" so I tried looking up this issue on the net but couldn't really find anything that was exactly like my problem and just left it thinking it was just a one time glitch.
The next morning my phone did the exact same thing except this time it did the soft reboot right after I turned my alarm off, and then again later on when I went to wake my phone up to check a text message and in the battery usage graph there was another big gap in the graph. I decided that I was going to repair my phone using PC companion and then reformat my SD card, double tripple check the sim card was not loose and everything was properly seated. After about 2-3 days of no issues the same thing happened again. This time it happened at work when I went to wake the phone up to make a call. So long story short it has been doing these random stealthy reboots ever since and I can't think of anything that has changed with my device except for that update Roger's released.
I had no 3rd party apps on my phone after I repaired via pcc, I made sure all apps were updated, phone is not rooted nor is bootloader unlocked, have never dropped my phone or damaged it in any way. The battery life is still really good and everything works...except for extremely poor LTE signal, but I never use LTE unless I'm in a n area where it's good, otherwise I only use GSM because the signal is solid 24/7 no matter where I am. I use wifi alot but always turn it off when I'm not using it, mobile data I only use when I need to and then turn it off when I don't need it, GPS and auto sync are always off. I never ever put my phone on airplane mode, I don't mess with settings in developer options, I have never got my phone wet or did anything stupid with it....but I need help because this is very inconvenient and I'm scared that these reboots could be causing damage to the phone.
One more thing I noticed. When I went to diagnostics the reboots come up as "system incidents" and shows a time but does not give any details about what the system incidents actually are.
Please someone with some knowledge can you help me out? I refuse to believe it's hardware related, it has to be software.
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Sounds like you've eliminated a number of potential causes by the troubleshooting you have already done.
I would suggest trying a factory reset on your device.
Let me know how that goes, looking forward to your reply.