With a Subdrive you are saddled with a three phase motor and probably very small wire. This locks you into replacing the Subdrive as there are few other ways to convert your normal single phase house power to three phase to run that motor. With "faults #1 through #whatever, you can see there are many things that can cause a VFD to not function. A bad cooling fan or dirt on the circuit boards is just one of many possible problems. Most people replace the Subdrive two or three times before they realize they would be better off to bite the bullet, pull the pump, replace the three phase motor with a normal single phase motor, so they have something dependable and long lasting that doesn't have a multitude of faults and things that can go wrong.