Chosen Solution
I fixed the sensor flex PCB and after that, it was fine. But after I put the unit back, I realized I forgot to put the rubber holder for the ambient light sensor on, so, I put it on and after that, the phone will not turn on anymore. On that same day, I was able to detect the device on my pc and charge(?), but after a few days, it will not turn on at all and pc won’t be able to detect it. I checked the voltage on the battery, it is 2.95V. Any suggestion of how to fix it? I am only fixing phones for friends and I don’t have a ton of equipment. This is actually the first time encountering such problem. The other phone I fixed along with this one was fine. Maybe the battery’s problem? I’m not sure. The screen and touch and touch ID are fine when I placed the whole assembly on another working phone.
I do believe 2.95v from the battery is to low to power on the phone. It would require closer to the area of 3.4v to power on the phone. So it could possibly be a failing battery. It may be a number of things but it could not hurt to start by trying another battery first as the one you have will not power on the phone due to not enough voltage being supplied to the motherboard. May also be the charge port is damaged or needs cleaned and your battery has discharged below the required voltage to power on the phone due to a damaged charge port.
Can you explain what you mean by sensor flex PCB? Is that the front camera ribbon cable flex assembly that goes behind the screen?
