I know there is no notifications of malfunctions in-game. Usually, it is at a very inconvienient time that you discover one.
Like crash diving and realizing your engines stop instead of going to flank. Or you're 3/4 of the way to America and your notice your fuel supply has dropped considerably and you have barely enough to get home from where you are. You start looking for milk cows or friendly supply ships in neutral ports real fast.
My latest was a fogged attack scope and a task force with a Southampton cruiser and 3 destroyers coming my way and me in a perfect positioon. I got him though through judicious use of the observation scope and the blurry attack scope.
At least I think it was amalfunction and not a graphics issue. During the subsequent light depth charging, my attack scope was damaged. After surfacing later and repairing it, the scope was back to normal.