Any action taken with respect to any element of design installed on or in a motor vehicle or motor vehicle engine in compliance with regulations under this subchapter (including any alteration or adjustment of such element), shall be treated as not in violation of section 7522(a) of this title if such action is performed in accordance with high altitude adjustment instructions provided by the manufacturer under subsection (b) and approved by the Administrator.

If the Administrator finds that adjustments or modifications made pursuant to instructions of the manufacturer under paragraph (1) will not insure emission control performance with respect to each standard under section 7521 of this title at least equivalent to that which would result if no such adjustments or modifications were made, he shall disapprove such instructions. Such finding shall be based upon minimum engineering evaluations consistent with good engineering practice.