how to troubleshoot fan

Does your ac work?If so turn the ac on and see if you have power and ground to the fan motor.Most likely its the cooling fan motor thats bad.

the late question replaced the cooling fan, it worked for a few days. now it is not turn on when the temperature at the fan-on level, replaced the sensor, still not on.

Hello,

It sounds like the cooling fan relay is not working. but to be sure here is a guide and the cooling fan wiring diagrams so you can see how the system works with the relay location as well:

https://www.2carpros.com/articles/how-to-check-an-electrical-relay-and-wiring-control-circuit

Check out the diagrams (below). Please let us know what you find. We are interested to see what it is.