A1398 Common issues
The MacBook Pro may appear to turn off. The fans will still spin and caps lock will still work. The cause is that the GPU is losing power due to flexing and brittle solder around U8900 (GPU Buck converter). Solder U8900 with an iron and leaded solder. Triple check all joints under a scope.
L2013-M2014 (820-3662, 820-3787)
Screen off with computer on, or crashing: Sounds like the FDMF6808 chips are failing and causing black screening. You can try fixing this by replacing all 3 of them with NEW FDMF6708 or FDMF3030 chips from a reputable source like Mouser or Digikey. Possible software fix linked here
If you have dedicated graphics on your board then the above fix has a pretty low success rate.
If you have internal graphics, aka no graphics chip on your board then the above fix success rate is very good.
Additional notes : Symptoms are very unlikely to be a screen or screen cable issue. If you’re not getting image when this happens, then it’s not related to backlight either. Very very small chance it’s a GPU/CPU issue unless the fail is major. The FDMF chips issue arises from the board flexing. The chips tend to fail and cause the device to not wake from sleep/randomly shut down due to CPU VCore dropping out. Machine will stay on but basically never wake from sleep at times. If you’ve never messed with microsoldering before, I would recommend sending it to someone who has experience fixing this issue. If you have done board repair, then this repair is a little tedious so you want to make sure you get GOOD chips as the ones claimed to be new in China often end up being sanded down garbage to look new.
2015 dedicated graphics (820-00163, 820-00426)
These boards absolutely require a good charged battery to fully boot into macOS. As usual it's not uncommon to get 2 or 3 bad batteries in a row so make sure you are using a known good one, one that for sure works with others 15" 2015 DG boards.
With a bad battery or no battery, the machine will reboot during or at the end of macOS boot. In this case disconnecting the touchpad often allows it to boot fully since it causes throttling and lowers the overall power consumption of the machine.
Additional insight from Tim @TCRS Circuit regarding dedicated GPU L2013-2015 and also U8900
The question was : Do you still have good success rate with mosfets on dedicated GPUs A1398?