Fans full speed, mouse and keyboard stop responding. What part is bad?

The symptoms are when you startup, the fans run full throttle, the trackpad and keyboard work for a little while, you can login. But almost instantly the built-in mouse and keyboard stop responding, and a dialog comes up asking to pair a bluetooth keyboard.

If I connect an external keyboard and mouse they work, but the fans are still running full throttle on the laptop.

Thinking it must be a logic board issue, I purchased a replacement online and just finished installing it using the iFixit repair guide, and to my disappointment the laptop has the exact same symptoms.

And before you say it’s a software issue, if I boot using a macOS installer USB, the symptoms are very similar.

Do you think I should replace the trackpad component next, or what else could be causing these issues considering I’ve already replaced the logic board?