Having weird issues with the fan

After too much investigation, multiple resets, PRAM reset, CMD R, S, CMD S, even replaced the CPU thermal paste twice with high quality Arctic Silver 5, I've narrowed down the issue I'm having to something related to the fan.

A bit of history, this is my Daughter's computer. She is on the final year of her Psychology major, so imagine how terrified is she with her computer failing on her. We don't live in the US, so visiting a genius bar is out of the question.

The symptoms are like this:

The computer have been running issues free since over two years, but a few months ago she had some issues with the trackpad. Like it was stuck and didn't click. I thought about replacing it, but a few taps cleared it out, and continued to work normally.

Then a couple weeks ago, she had some issues with the volume control muting, or bringing the volume down without pressing any key. And couldn't get the volume up with the keys, only with the screen slider. It also self corrected a couple days later.

But last Thursday, she called me all stressed out, that her computer was not turning on. It reached the login screen, and shut down.

I checked it out, and though that it was the thermal paste, so having a bit of Arctic Silver from a recent PS4 repair, last Sunday I replaced the paste.

When booting, it behaved exactly the same. So I started to freak out. I posted the question on the Apple support forum, and lots of people told me that it was the SATA cable, and that the SSD could have been corrupted. I put the SSD on an external enclosure and backed up all her files in my computer, so thankfully the data is safe. She also has a recen TM backup. I erased the SSD, and was planning on installing macOS from scratch. But every time I tried to do it, it didn't even finished loading. I don't really remember hearing the fan spinning up.

Yesterday, out of desperation, I took her apart and replaced the thermal paste again, but when reassembling, I forgot to connect the trackpad ribbon cable, and the keyboard cable was also a bit loose. So, I turned her on, to try to reinstall the OS, but this time something happened. I could hear the fan roaring. I started the installation, but the USB image was corrupted, and it failed. So I turned the computer off to change the USB installer. This time I reconnected the keyboard and the trackpad. Turned it back on, and the fan was silent. And shut down. I started to think that it's something about the fan, or the trackpad, so I disconnected the trackpad, and tried again. Same behavior.

This morning, I thought about the keyboard issues she was having, so I disconnected the keyboard, and tried again. Guess what, the fan roared again, and I'm in the middle of the new installation.

So, up to this moment, the keyboard is shorting something that keeps the fan from running at full speed. As crazy as it sounds.

Thanks for reading this far...

Any ideas?

Well, it was a long boot. It took almost 12 hours to boot and run the App. Remember that I disconnected the keyboard and its backlight, and the trackpad.

All I can see is that says that the Hard Drive sensor is not working. Is that the one on the trackpad?

So, if I replace the trackpad, will it fix the issue?

Thanks!

Well, I just took the trackpad apart, and brushed it off with an old toothbrush and rubbing alcohol, making sure to reach every crevice where I can find dust and grit on, and put it back in. Booted, and it behaved normally. I was able to run TG Pro now, and it shows all sensors working 100%.

Here is the screenshot:

I know is not a definitive solution, but is a proof that the computer is not damaged, is only a sensor issue. So I'll order both the trackpad and the SATA cable, as you mention, also doing the tape thing to protect it from rubbing on the metal case.

Thanks a TON for your help, Dan. You saved us a lot of grief, and my Daughter will be so happy to learn that she'll be able to finish her thesis.

Have a great day