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Cel with p0421 and 431...how can i troubleshoot and determine the root cause

(2003 Audi A4 Quattro)
in Champlain, NY on February 15, 2012
Car passed emmision test last week...Would like to determine the root cause before repairing...don't want to just replace the cats and the o2 sensors...Where can i get an accurate diagnosis?
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