How to replace cooling fans

It is normal for the fans to turn on and off with the modern electric fans used in today's cars. They were originally driven by the fan belt, meaning they ran continually, but with the addition of electric motors and engine computers, they get turned on and off as needed. When the coolant temperature rises above a certain level they'll get kicked on, and when they're not needed they get turned off. So they won't turn on immediately when the car is first started when it's cold, and may even run for a while after the car is turned off.

If that's the reason you're considering replacing them, then you probably don't have anything to worry about. Generally the reason you do so is because they aren't coming on at all, so if they turn on at any time they're working correctly.

Hopefully that answers your question, if you still have something you're wondering about feel free to ask.