42 U.S. Code § 7585. Standards for heavy-duty clean-fuel vehicles (GVWR above 8,500 up to 26,000 lbs.)

(a) Model years after 1997; combined NOx and NMHC standard

For classes or categories of heavy-duty vehicles or engines manufactured for the model year 1998 or thereafter and having a GVWR greater than 8,500 lbs. and up to 26,000 lbs. GVWR, the standards under this part for clean-fuel vehicles shall require that combined emissions of oxides of nitrogen (NOx) and nonmethane hydrocarbons (NMHC) shall not exceed 3.15 grams per brake horsepower hour (equivalent to 50 percent of the combined emission standards applicable under section 7521 of this title for such air pollutants in the case of a conventional model year 1994 heavy-duty diesel-fueled vehicle or engine). No standard shall be promulgated as provided in this section for any heavy-duty vehicle of more than 26,000 lbs. GVWR.

(b) Revised standards that are less stringent
The Administrator may promulgate a revised less stringent standard for the vehicles or engines referred to in subsection (a) if the Administrator determines that the 50 percent reduction required under subsection (a) is not technologically feasible for clean diesel-fueled vehicles and engines, taking into account durability, costs, lead time, safety, and other relevant factors. To provide adequate lead time the Administrator shall make a determination with regard to the technological feasibility of such 50 percent reduction before December 31, 1993.
Any person may at any time petition the Administrator to make a determination under paragraph (1). The Administrator shall act on such a petition within 6 months after the petition is filed.
Any revised less stringent standards promulgated as provided in this subsection shall require at least a 30 percent reduction in lieu of the 50 percent reduction referred to in paragraph (1).
(July 14, 1955, ch. 360, title II, § 245, as added Pub. L. 101–549, title II, § 229(a), Nov. 15, 1990, 104 Stat. 2519.)