failed smog test, check engine light.

This problem can be due to carbon build-up in the EGR system. It's important that a diagnosis is performed to determine the fault, merely replacing parts in the hope that it will fix a problem is a waste of money, as you are finding out. Check out this article about this fault, and pass it on the the mechanic if they are having problems diagnosing this: http://repairpal.com/OBD-Code-P0401