In my journey to improve my 1999 Disco series I I've noticed that the supplemental cooling fans do not run when the AC is turned on. How common is it that both fans fail? What would be the procedure to test the fans before I invest in replacing them?

Thanks for the help!

 

First check and make sure they turn freely, then check the fuses, then check with a test probe for power to the fans.
You could have one of the 2 sensors, a/c or temp, that are not working.