Computer boots up ONLY when chilled to <60° F.
This computer was in storage for months because it will only sporadically start up at room temperature when pressing the power button. The issue is definitely temperature-related. It went to the Genius Bar a couple of years ago for the exact same issue. They "fixed" it by replacing the motherboard, costing us >$300. The genius explained to us that the model was too old and they would not fix it for us again. That fix lasted a couple-three months before the issue returned.
The issue: Computer off. Press power button. Power LED comes on, but no chime and the screen doesn't light up. This is an intermittent failure, on rare occasions, the computer actually did start up, but most often it did not. It was completely unreliable.
Attempted fixes: Reset NVRAM, failed. Reset SMC, failed.
Once, when it did start for me, I took the opportunity to upgrade its system software from Snow Leopard (10.6) to the latest it could run, El Capitan (10.11). When attempting to restart to install the new system, of course the computer again failed to chime and the display failed to light up. After repeated tries, it went into storage.
Getting close to the holidays now, I decided to take it into a local Mac shop to see what they could do to it. I put it into the car parked in our unheated garage so I would not forget to take it with me when I left. It was ~55°F in the garage, and MBP sat in the car for a couple hours before I left.
When I got to the repair shop, after checking the battery state, the tech immediately pressed the power button… CHIME!, display came on and the computer booted right up. He looked at me, "I guess I just have the magic touch" he said.
I left it on and took it back home. As soon as I arrived home, many other issues became apparent: no keyboard backlight. Both fans running at top speed (6,000 RPM) constantly without loading the processor. Apps taking forever to open. All signs of NVRAM needing a reset. By that time, the computer had warmed up to our ambient temperature, a little over 72°F. Intending to do another NVRAM reset, I clicked "Restart", the computer shut down and bingo, right back to the original issues.
But now I had something new to try— putting it back out in the garage. I force-shut it down, and took it out to the garage and left it there. Couple of hours later, I went out and pressed the power button. Chime and the computer started right up again. Finally, after repeated tries, I managed to reset both the NVRAM and the SMC, so everything runs fine (keyboard backlight works, fans running at or near the target rate ~1,900 RPM).
Except when I try to start the machine inside our house. Then, it goes right back to the original issue: power light, no chime, no display. But all I have to do to restart it is leave it in the garage. This is easily and totally replicable, and the only only dramatically different change between the garage and our living room is the ambient temperature (well, and there is no car parked in our living room).
I haven't tried refrigerating the Mac… yet. Thanksgiving leftovers, y'know. But I bet it would work if I did.
Here are some of the causes I have been cogitating on (in no particular order):
1. The hard drive (it's the stock OEM HDD, 200 GB, probably 5400 RPM). I know sometimes HDDs can be brought back to life by putting them in the freezer, but I can't see hard drive issues adversely affecting the display.
2. Maybe a poor connection between the motherboard and the display? But why would that keep the computer from finding a system and chiming?
3. One or more of the internal temperature sensors (although it seems a malfunction there would adversely affect the machine AFTER it's started up).
At this point, I can't even tell if it's running a POST when I turn it on.
If you managed to read this far, holy crap— you're amazing. (And parenthetically, it might be time to get a life. You know, just sayin'.) And if you have a similar issue, absolutely try cooling it off very thoroughly to about 55°F and see if it works for you. And anyone who wants to chime in (see what I did there?) with suggestions about the cause, know that I am genuflecting to you not just because of your awesome knowledge, but also the fact that you would share it with someone of my humble stature.
Thank you SO much!