Xperia Z no boot

Hello all,

I have 2 Sony Xperia with weird problem. The phone got volume/powerflex change and after will boot to Sony then turn off. Change power flex and same problem. Try other motherboard and same problem. Before it did not have this problem.

Hook it up to power supply and at 3.7v it shows Sony then turn off. At 4.3v it will boot fine everytime. Can anyone give me ideas on what can be the problem?

I have tried new power flex, new battery, new motherboard and even on power supply.

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