iPhone 6 Plus no power/not booting

Hello,

I've got an iPhone 6 Plus board that is not powering on. It just suddenly stopped working one day and I'm not sure why. Initially I was getting 0.4V at the battery connector. Did some research and it was often recommended to replace the Tristar chip. I managed to desolder the chip, but then just left it and didn't get round to replacing it. I just put in storage for several years.

Recently I had the urge to try replacing the Tristar chip on this board just to see what happens and if I could successfully do it. So that's what I did. I was getting signs of some life. The Apple logo flickered, but didn't stay on long. With no battery and a USB cable plugged in, the battery connector reads between around 3.7-4V. It's not stable though and bounces all over the place. I do not get an Apple logo in this state.

I plugged in the battery with the USB cable connected to my PC and also connected my USB amp meter to see what that tells me. From what I could see, it started jumping from 0 amps to 0.100mA and occasionally would hit 0.477mA (which is the limit of the port it's connected to) like it was trying to charge. It'd go like 0 > 0.100 > 0 > 0.477 > 0 > 0.100 > 0.94 > 0. It's like it was trying to do something, but couldn't.

Thought maybe the battery was bad as it was also sitting in storage for a while. I checked the voltage and it was at 3.3V. After some fiddling about with the phone, I got some stable amp draw on the USB amp meter. It was staying around 0.470. Battery voltage started climbing up.

At about 3.75V, it seemed to stop charging (amp draw was still stable at around 0.477). Couldn't get it to do anything further on my PC, so I plugged it into a wall charger. Again, the same behaviour was observed (with it dropping to 0 and then drawing some amps) only this time the amp draw was higher. After some more disconnecting/reconnecting, the amp draw stablished and the battery seemed to charge again. It was charging at about 0.800A.

At about 4.1V, I disconnected the charger and connected it back to my PC. It was doing the same thing again (going from 0 to 0.100-0.400). I would also occasionally get "USB not recognised" prompts on my PC.

Thought maybe it's a bad battery/cable, so I got my DC bench power supply, set that to 4.2V and connected it to the battery connector - battery, USB cable and screen were removed. With no prompt to boot, the amp draw remains at 0.000 which from what I've watched/read seems to be the expected behaviour.

When I press the power button, it jumps to around 0.094A and then goes back to 0. It does not draw any more current after that unless I try to press the power button again. This is all it does with the power supply.

Based on the DC power supply test, this rules out a faulty battery and USB cable as the culprit, right? It should be able to boot with the DC power supply based on what I've read and watched.

Any idea what might be wrong here? I have checked for shorts in the usual places (VCC_MAIN, PP_BATT, PP_CPU, PP_GPU, NAND) but there's no shorts. One thing I did notice is there are 3 missing components around Tristar (these are C1739-1, C1754-2 and C1700-2). Could this be the issue or is something else wrong?

Appreciate any response. While my main goal with this phone was just to replace Tristar and see if I could get the phone to do something different (which it did), I'd still like to try and see if the board could be fixed completely. It's a fun little goal I've set for myself.