The EGR system is what controls the backflow of exhaust gases into the engine. By doing that, it lowers the combustion temperature and results in your engine emitting less pollutants. That's what the California smog tech told me when my EGR valve started to leak. It was a small leak, and eventually it stopped leaking when he tested it, but I had to go have it checked out anyway.

If it's just a solenoid... and you're handy with electrical stuff, you could probably just buy a similar solenoid from an auto parts shop and put it in. Then again, what do I know? (not much!)