Hi @ettore26292
If your diffuser is model number HAD-001-GLW-A0 check that there is 24V DC on the output plug of the power adapter that is used with the diffuser.
If the adapter is OK here's a teardown [https://www.youtube.com/watch?v=LfFJl1TOpaQ|video] that shows how to disassemble the diffuser so that access to the motherboard is possible.
Once you have access to the board and if nothing obvious is wrong, post some close up images back here so that others may see.
Here's how to add images to your ''question'' in ifixit [guide|21499]