Massachusetts General Laws Part I. Administration of the Government (Ch. 1-182) Ch. 90, § 7O

No person shall remove or render inoperative, except temporarily for maintenance purposes, any device or element of design installed on or in a motor vehicle or motor vehicle engine in compliance with regulations under the National Emission Standards Act (42 USC 1857f-1), unless such device or element is to be replaced by or modified by another device or element, approved by the registrar, the installation of which will result in a reduction of pollutant emission.