iPhone 6 turns off moments after locked

I've already changed the battery, the motherboard and update the software , apparently everything is working perfectly.

The phone works normal but when I lock the screen (or wait to lock by himself) moments later it turns off, and only turns on with hard reset.

i don't know what to do anymore

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Maybe a bad power button / flex assembly? Maybe the phone is still on but you can't unlock it? Can you unlock it with the home button?

crwdns2934271:0crwdnd2934271:0crwdne2934271:0

it turns off completely, the power button its ok.... and yes i can unlock with the home button..... when it is locked and goes for 5 seconds without unlocking it, it turns off

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