When I plug the equipment into an electrical source, its green light turns on, but when I push the start button, the microwave does not work. I think there is an electrical problem , which prevents its motor from working properly.
When I wrote "electrical problem", the sentence became meaningful and complete, and the second clause in bold is a detailed description for the electrical problem. Thus, I guess it is non-defining clause and, therefore, comma is required here. A native English teacher disagrees, so what do you think?