Volvo: Catalytic converter, oxides of nitrogen, vacuum line

Question
My Volvo 1993 940 turbo failed the Emission test for Oxides of Nitrogen :
State Standards:     2.2000 gpm
Inspection Readings: 3.2854 gpm
Inspection Results:  Fail

a new catalytic converter was installed.  Why did it fail?  What needs to be done to correct the problem and pass the emission test.  Thank you for any expert advise you may able to offer.
camella


Answer
Carmella
The main reason for NOX failure is a bad EGR valve. The valve could be bad or the line clogged, even the vacuum line could be broken causing the valve not to operate. Having it checked out by a shop might be the easiest way to correct the fault.