The Administrator shall test, or require to be tested in such manner as he deems appropriate, any new motor vehicle or new motor vehicle engine submitted by a manufacturer to determine whether such vehicle or engine conforms with the regulations prescribed under
section 7521 of this title. If such vehicle or engine conforms to such regulations, the Administrator shall issue a certificate of conformity upon such terms, and for such period (not in excess of one year), as he may prescribe. In the case of any original equipment manufacturer (as defined by the Administrator in regulations promulgated before
November 15, 1990) of vehicles or vehicle engines whose projected sales in the United States for any model year (as determined by the Administrator) will not exceed 300, the Administrator shall not require, for purposes of determining compliance with regulations under
section 7521 of this title for the useful life of the vehicle or engine, operation of any vehicle or engine manufactured during such model year for more than 5,000 miles or 160 hours, respectively, unless the Administrator, by regulation, prescribes otherwise. The Administrator shall apply any adjustment factors that the Administrator deems appropriate to assure that each vehicle or engine will comply during its useful life (as determined under
section 7521(d) of this title) with the regulations prescribed under
section 7521 of this title.