EPA is not in a position to discuss what may constitute an adequate investigation in every conceivable scenario. An adequate investigation is whatever is required to correct the cause of the deviation and to assure that the method performance has been restored to statistical control.
Question and Answer was originally posted at Questions and Answers on the Clean Diesel Fuel Rules (PDF)(135 pp, 888 K, EPA420-B-06-010, July 2006, About PDF)