Retailers are not required to perform testing in order to have a defense to presumptive liability. However, to establish a defense to a violation distributors must, among other things, conduct a periodic sampling and testing program. For a truck distributor, the best place to take samples may often be the tanks of retail outlets it delivers to.

Retailers should probably sample and test after tank transition from a higher sulfur product to 15 ppm product, to confirm that the fuel in the tank is meeting the 15 ppm standard. Otherwise, if a violation is detected for some time subsequent to a tank transition, it may be difficult for the retailer to demonstrate that it did not cause the violation.

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)