How to fix "Fan error" on Lenovo x220 laptop?

Question

My laptop started to show "Fan error" on boot and then shut down.

Answer 1

The cause was that battery was completely empty.

First, I tried to start without a battery with only the AC charger plugged, and it worked.

Then, I put the battery back and plug the AC charger and let the battery fully recharge.

After that, it works without any problem.