Actually if you think about it, the EGR's job is to richen the mixture. That's why when it isn't working the engine runs too lean, usually pings, and fails for NOX due to increased cumbustion temps. What's interesting about his failure is that a failed CO could often mean different things in different applications. Any poor combustion could raise CO's (incomplete burn of fuel mixture) but his high NOX also tells a story of a lean mixture causing high combustion temps. Were his readings at idle, at elevated rpms? High combustion temps caused by excessively lean mixtures due to a faulty EGR could also cause poor combustion events causing CO's to increase. There are a lot of variables to this kind of stuff, it's not always as simple as 2+2=4. I agreed with the possibility of a faulty EGR as a possible cause but not because it causes a rich condtion. Rich conditions lower NOX readings.
PS. I don't think anyone said to un-hook vacuum advance, and yes if he is in a state that requires openning the hood like during an IM240 test he must leave pcv in the valve cover.
