I took my duo valve apart....the smaller "tin cup housing" has a magnet in it and an impeller/fan blade type. I sanded it down...had metal build up/ridges on it... smooth them out, sanded the inside of the magnet housing and reinstalled. Next, removed the duel housing...took out the small brass "nipples," they were corroded...sanded them, cleaned up the inside where the "nipples" are housed and the rubber seals..reinstalled...added coolant, fired up the engine...a few minutes later...heat! I used 1000 grit paper.
I experiment on another duo valve first...got it at the bone yard. Now I have a back up for the future.
If yours has and electrical issue....this won't help you.