What did it fail? HC? CO? NOx?

If you could post your #'s it may help.

From what you said so far, could be fuel pressure escaping from a leak or via accumulator diaphragm.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k