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Take the pcb cover off and see to start with. The only other way is to take it apart and see, or plug it in in the garden and see what happens (film this bit though)
Is the moon made of cheese? depends if it's damaged or not.
DC25 motors dont fit DC15s. From what you have said, I think you'll struggle with this. DC15s are among the hardest machines to get right. Do you have a multimeter?