1994 Other Nissan Models EGR Issue during smog test

Approximately 6 months ago, I took my 1994 Nissan Quest into a auto collision shop for damages sustained in a minor front end collision. Components of the Radiator & AC systems, were replaced. I had always heard a low humn, coming from somewhere, but never thought it was anything serious. I live in California and I just took my van in for a smog test in order to register it for 2008.
This is the first year it failed the test. Reason given was EGR. Is there areasonable likelihood that the auto collision repair shop, somehow incorrectly or failed to properly install a component, which would result in a EGR issue during my smog test? Where is the EGR valve located?