Radiator fan not coming on when overheating? Is it Fuse or relay?

Honda Civic 2001

My car has been overheating lately. There are no leaks and I noticed the fans don't turn on when the car overheats which I found to be the problem. The fans do work however when I turn on the AC, hot or cold, which lead me to believe my fans were perfectly fine.

What can be wrong I sure hope it's not the fan itself. A mechanic told me to buy the sensor and that it's the problem, but I want some other opinions. I heard it can also be the Fuse or relay. What does it sound like to you guys?

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