iMac is not booting (black screen, no chime)

Hello Mac Experts (@mayer, @danj , @reecee and others)

I have an iMac 27", Late 2012 that is not booting properly. It is usually just a black screen with no chime although the fan spins and the first two diagnostic LED's light up. Occasionally, the boot process does happen and I get a white/gray screen then nothing else happens. On rare occasions it will start the progress bar but it has never completed the boot process.

There is also a vertical line, pretty much in the centre, of off-colour pixels (~ 5-10 pixels wide). I'm assuming it is LCD related as the connector looks fine.

The customer originally thought it was the power supply and had ordered one to install himself. Time constraints were such that he dropped it off at the shop so that I could install it.

So here is what I have tried:

• SMC and NVRAM reset (just to be sure)
• Checked diagnostic LED's, first 2 are on when I press the power button (note- when it did occasionally start the boot process, all 4 would light up)
• Installed new power supply, no difference
• Removed motherboard, re-seated RAM modules, removed CR2032 battery and re-installed.
• Visual inspection of motherboard, no anomalies found
• Removed SSD (as part of Fusion Drive) and installed known good HDD with bootable MacOS in place of existing HDD

At that point, the computer started the boot process, chimed and I pressed D to run the Hardware Test. It went into test mode and started the test but froze about 1/3 of the way through and just hung. After another re-start, just a black screen once again with no chime but spinning fan.

At this point, I have to assume it is motherboard issue. I'll start probing but wanted to see if anyone had any ideas.