Has my plenum crashed?

[Jeff_S]

I have a plenum set up in a separate sump. Water goes from the main sump thru a protein skimmer to the plenum and back to the main sump. This setup has been working well for about 2 years now but in the last 2 days it has started producing a foul sulfur odor. Is this an indication that the plenum has crashed? Should I shut it down or continue to let it run? This is a fish only tank and all the fish appear fine, the clowns even spawned again a few days ago.

Thanks
Jeff