Mid 2012 MacBook Pro running hot, but CPU usage very low and Memory in Activity Monitor also low

Hello edraagyn.

Welcome to Apple Support Communities. I understand that your MacBook Pro is still running hot after you have cleaned the fans and the dust off of them. I am happy to help.

Please use the steps here for keeping your Mac within acceptable operating temperatures: Keep your Mac notebook within acceptable operating temperatures - Apple Support

Take care.

You don't have to worry about the 32bit apps unless you are going to be upgrading to macOS 10.15 Catalina. If you upgrade to macOS 10.15 Catalina, then those 32bit apps just won't work. Most common and still supported apps should have 64bit versions available. If updating the apps to 64bit doesn't remove the 32bit versions, then follow the developer's instructions for uninstalling the old versions of the apps before upgrading to Catalina. If the apps don't have a new 64bit version, then either find another app to replace it or stay with Mojave.

Try an SMC reset and a PRAM reset (hold the PRAM reset for at least three chimes).

Run the Apple Diagnostics to check the cooling system.

Did you select "Show all Processes" in Activity Monitor?

Have you always monitored the fan speed and temps before checking them now to know they are not normal?

The heatsink thermal compound may be dried out causing the idle CPU temps to rise 10°C. Replacing the thermal compound will lower the temps a bit.

Hello edraagyn.

Welcome back to Apple Support Communities. I understand you’re getting an error when running Apple Diagnostics. I’m happy to help.

For Macs 2012 or earlier, please use the steps in the section titled "Use Apple Hardware Test" here to run Apple Hardware Test: Diagnose problems with your Mac - Apple Support

Take care.

When I was inside my 2012 MBP changing out the hard drive, I decided that was a good time to check the fan for dirt (my 13-inch has oly one). The fan was not too dirty but I almost missed what was dirty--the "radiator" fins in the heatsink the fan services. Five years of use had left a deposit like felt cloth on the fins, blocking half of them:

Did you check that area as well?

Hello edraagyn.

Thanks for following up. I recommend using the links at the bottom of the previously provided link to contact Apple Support or visit an Apple Store or Apple Authorized Service Provider with a locator link provided.

Take care.

Adware:

Safari Extension: Amazon Shopping Assistant

Reason: Adware name match

you are running Pharos Notify and popup is that deliberate?

Flip4Mac is obsolete and the formats it supports are very rarely seen any more.

Top Processes Snapshot by Energy Use:

Process (count) Energy (0-100) (Source - Location)

WindowServer 6 (Apple)

mediaanalysisd 2 (?)

cloudd 2 (Apple)

knowledge-agent 1 (Apple)

Activity Monitor 1 (Apple)

what is medianalysisd and where is it come from ?

what about Amazon shopping assistant, that Etrecheck says name matches ADWARE?

32-bit Apps will not run on Catalina. If you are not ready to run Catalina, you don't care about 32-bit Apps -- they run just fine today under Mojave or earlier.

The error message you posted above is not a Diagnostic error message. You are not yet running Apple diagnostics, which failed to load, and that message is from Internet recovery, telling you it Apple diagnostics failed to load.

This is because, as cp from OH said, Macs before 2013 do not support Apple diagnostics, and can not be diagnosed from Internet Recovery.