Chosen Solution
Got a hand-me-down iPhone 6S, presents no water damage, tried chaning both LCD and battery. No power, no charging, no display at all. iTunes recognizes it under DFU, however, as soon as it unpacks the file, it goes into “Waiting for iPhone” as the device never goes into restore mode. I’m thinking either NAND problem or software problem. Measured both the battery (3 V) and the charging connection, right at the battery port (3.3V - connected to a PC) Also tried disconnecting the battery and the charging cable, pressing the power button for about 20s, then reconnecting it, nothing happened. Any help is greatly appreciated!
No power and no charging with no apparent cause such as liquid damage, dents / bends on iPhone housing when dropping the phone usually point to a power management chip (PMIC), CPU, NAND or Tristar fault. If this was the case of where the phone was working, was charging overnight but refused to turn on or show any signs of power the next day this is almost always the Tristar (U2 Charging IC) chip needing replacement. That phone is definitely going to need at least board-level repair work done to it to get it working again. You’ll need to take it to a reputable repair shop that does board repairs for iPhones and see if they can get it fixed for you.
