Early 2015 13" MacBook Pro Retina Won't Power On It worked fine prior to vacation, never used it while away, schlepped it in computer bag, only removed from bag at airport security on the way there, and again on the way back.
I know the bag didn't get too hot because my candy bars didn't melt :-).
Plugged it back in on its stand the next day and nothing happened when I pressed the power button.
What, I've done so far:

Opened the back, unplugged battery, can boot with battery disconnected while plugged into power, but runs slow due to no battery.
Thought maybe battery went bad even though it only had 3xx cycles, so bought iFixit kit and replaced the battery.

Still would not power up. Displayed the battery as made in 1979 at that time.

Watched Louis Rossmann video and cleaned out JTAG connector, booted normally, ONCE, after that, with full speed and good readings on CoconutBattery, (battery made in March 2020, 1 cycle).


The motherboard is not dirty at all, it lives on my desk on a stand, except when on vacation, but thought I saw small bits of solder between legs on JTAG under microscope, used tweezers to clean.  Would like to remove the connector, but don't really have the equipment to do so, it is way smaller than the boards I used to rework for Seagate back in the early 90s.
Issues before and after battery replacement have been somewhat intermittent in terms of power connector LED and CoconutBattery readings.
I have removed and reseated the part the MagSafe connector, connects to (power module?).  It's not dirty.
I would hate to replace the logic board, but sending it for repair would be a similar cost to replacing it.
What can I do to fix this problem?
Have you tried the Apple Diagnostics?  This source has instructions for the Apple Diagnostics which are for machines after June 2013 so it should work on your computer.
If this doesn't help you and if you live near an Apple Store, take your computer to the apple store and just ask them to check it for you. Explain your problems and see what they say. Maybe they'll find the problem. Tell them you ran Apple Diagnostics but if offered no help.