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Laptop is insanely sluggish—essentially unusable. When I am able to finally check the activity monitor, the CPU usage exceeds 100%. Did a fresh install of the OS hoping it would fix things, but nothing. Fans run non-stop at maximum speed. Not quite sure how to fix or diagnose the issue.
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Well the good news is you don’t have a thermal sensor issue! The bad news is you have a voltage sensor issue which the diagnostics would have clued us in on.
Time to grab the schematics and boardview drawings and start probing the power rails. Unless you’ve done this before and have the needed tools it might be wiser too find someone with deeper mac diagnostics skills and has the needed micro-soldering skills to fix your logic board.
Otherwise you may want to look at getting a replacement logic board Late 2012 13” MacBook Pro Retina Logic Boards
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Thank you, Dan! At least now I know what the problem is. I'll look to see if it's cost effective to get the logic board repaired or replacing it myself.
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Let's run the onboard diagnostics, restart your system and press the D key yo enter. Let us know what the errors are.
Reference: Mac startup key combinations
by Dan
@danj
Holding the D key while restarting the system results in a black screen. MBP runs (as evident by the fans revving to max) but it’s just a black screen if I hold down the D key. Doing Option + D results in an error: “Cannot load ‘EFI/Drivers/TestSupport.efi’
by James
Well, that sucks, Thanks Apple! When Apple updated the firmware in High Sierra they messed up the onboard diags and still haven't fix it!
On to plan-B At this point you'll need to install TG Pro so you can get a better idea on what your sensors are doing. Post a snapshot of the main window making sure you expand it so all of the sensors are visible (the right slider needs to disappear) Adding images to an existing question
Let's see if they can tell us what we need to tackle.
by Dan
@danj Just posted the snapshot
by James