How to check u2 before replace

When I started in mother board repair, we would wonder a lot about this exact question.

- OK so everything before seems to be working properly.

- We have the right symptoms.

- Yet we are not confident enough in our micro-soldering and micro-BGA skills, so we really want to be sure before attempting to replace it.

- Here's the spoiler: there is no real fool proof method to rule it out until you.. replace it. This is the single most efficient way to trouble shoot it. If you are good at it, it should take 5 minutes to remove, clean pads, install new one. 20-25 minutes from lifting screen to reassembling last screw.

- Bonus point: Tristar chips (which you call U2, but should really call Tristar if you want to sound professional) are sort of consumable parts of an iPhone board. They "deteriorate" as you use them specially from power spikes, fake chargers, fake cables, poor quality car chargers, lightning accessories. They are cheap too. So if you suspect them, it is always good to replace them. You can only do good for the device and the trouble shooting process.

- Common Tristar symptoms: not recognized by computer; not charging; charging slowly; booting from charged battery but unable to charge a flat battery.

There are components near U2 IC that tells the voltage they have when phone is powered on, meaning test them with battery and phone powered up. You can see in the schematics the value, 5v and 1.8. if the result value is lower than that then its time to change the U2.

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-Mike