So this repair is mildly interesting.  The user’s late 2012 MacBook Pro just wouldn’t start up one morning.  It would get as far as a “missing boot disk” icon and shortly thereafter the fans started running full blast.

Apple Diagnostics recognized that an SSD was installed but was unable to determine any of its attributes like capacity, etc.

The computer would boot from an external USB drive but it ran very, very slowly and mouse cursor movement was erratic.

So the first task at hand was to attempt to recover the user’s data before trying anything else.  Some quick research on ifixit.com revealed that Apple really doesn’t want its customers repairing or upgrading this laptop.  I thought I had every conceivable security bit in my kit, but nope, I had to part with $8  for a P5 Pentalobe driver kit.

Adding insult to injury, for a very short time in 2012-2013, Apple used a proprietary SSD in the Pro and Air laptops.  Just for giggles, it uses the same connector as an industry standard mobile SATA drive but has a different pinout.  There are only a couple of options for replacement/upgrade SSDs and they are really expensive.  But at least you won’t spend a lot on RAM for this machine because it is not user replaceable.

So there went another $16 for a SSD HDD to SATA 22Pin adapter that may never get used again.

Some good news!  I hooked up the SSD to another Mac and it was 100% fine.  I backed up the user’s files to flash drive so he could get back to work while I investigated further.  The initial diagnosis was that the logic board had gone bad and we agreed to a maximum repair budget of $300.

Upon initial inspection, there was no evidence of physical or liquid damage anywhere inside the notebook.

Halfway through the disassembly process, I found one possible symptom source.  When I removed the heat sink from the logic board, I noticed a small spot of rust on the surface of the CPU.  Could this be interfering with good thermal transfer between the CPU and the heat sink, causing the CPU to overheat?

Next problem:  when I removed the I/O board data cable, I noticed some white corrosion on the cable connector and socket opposite the logic board. (See step 12 of the logic board replacement guide on iFixit.)

It took a bit of time under a large magnifier with some sharp dental picks, but I was able to clean all the corrosion off the cable and I/O board connectors.

It appeared that a very small amount of liquid (probably plain water given no evidence of stickiness) had been sucked in by the left fan and blown across the I/O cable and between the CPU and heat sink.  The user never noticed how or when this happened.  Perhaps the laptop was being used on a surface where something had spilled or was briefly used in the rain.

I carefully reinspected the MacBook and found no further signs of liquid or corrosion so I decided to just clean up the CPU and heat sink with some solvent and gave both a light polish with a new dollar bill to remove the rust.  I put a dab of Arctic Silver thermal paste on the CPU, reinstalled the heat sink, and reassembled the MacBook.

Bingo!  Everything works normally now.  So for $24 for tools and $0 for parts, the laptop was revived.