We have two trane Heat pump units A larger and smaller unit The fan on the larger unit I can see running outside The fan on the smaller unit I have not seen runnning. We have tried turning it off and on and the heat up and down Should it come on and run. I believe the air out of this unit is cooler. We replaced a capacitor last year on this unit Would that be the problem again?

Yes,sure it should,, possible a capicitor,, its cheaper than replacing the motor and not needing it,, but first make sure there is power being sent to the non-working unit
Don`t just look out the window walk out and see if its making any noise/trying to run