Issues powering on early-2015 Macbook Pro Retina (A1502)

Try disconnecting the battery from the Logic Board. Be careful though since the battery connector can be a very tight fit. I find I need to pry up on both sides of the connector using at least one non-conductive nylon black stick. Sometimes the edge of the battery connector will actually break because it fits so tight and the plastic is thin & brittle so be careful.

Try powering the laptop on using just the power adapter while the battery is still disconnected. The main thing here is whether it powers on & you get video on the screen. I forget whether the 2015 model will boot at normal speeds with the battery disconnected, or whether it will slow (the fan will be at high speed though since the battery thermal sensor won't be seen). Be careful not to let any metallic or conductive object or surface make contact with the exposed electronics. Placing the laptop on its right side is a good option for testing while the bottom case is removed & laptop is powered on.

Also, if the laptop is powered on when you perform the SMC Reset, then it should immediately power off the laptop. If it does not, then something is wrong.

Melophile wrote:

Now I'm at a complete lost. What could possibly be going on?

Well, a 2015 laptop is 9 years old. That is well past typical end of life -- not that I don't support using computers in good condition even when they are "obsolete," witness my 2010 MacBook Air still working well on all original hardware. But hardware wears out, on/off cycles create thermal cycles which expands/contracts solder joints and eventually those joints or other wiring starts to fail. Laptops are especially vulnerable given the cramped space and greater likelihood of thermal issues.

In your case, I think this laptop is too old to be serviced at an Apple Store, they don't stock parts for computers that old, for instance. But an Authorized Apple Service Provider (AASP), which is a third party store/service center that has Apple certification and access to Apple diagnostic tools, could look at your device and give you a diagnosis and cost estimate to repair. It will likely be quite costly as the parts for those screens are no longer easily found, unless you are willing to roll the dice with something like eBay, which means you are getting someone else's problems. But an AASP will know where new substitute parts might be found, if they exist, and many of them are willing to service older computers that Apple Stores will not.

I think it is likely that a new laptop will be needed here. Apple has a good selection right now, and assuming you have a viable Time Machine backup of the old laptop, migration to a new Mac is very easy and seamless. If you want to go the route of a used computer, I would recommend only reliable sellers who offer a warranty (e.g. Apple itself, or OWC), and make sure the computer is newer than 3 years old. Personally I always buy computers new, never used, and I take care of them as best I can, they usually last a long time. We still have working 2010 and 2013 MacBook Airs, and a 2015 iMac, as well as new models.