Richard

An EGR that is not opening, will cause elevated NOx readings ( that's what it's there for ).

An EGR that is opening too fast, or not closing properly, would cause elevated HC readings.

I am really interested to see what the outcome of this is, once John goes for his diagnostics on Thursday.
It'll probably turn out to be some kind od " head slapper ".

And yes, you are right as far as an exhaust leak being identified during an emissions test. Air dilution is measured and must be within certain limits.

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2007 C 230 Sport.