40 CFR 91.111 - Requirement of certification
(a) An engine may not be equipped with a defeat device.
(b) For purposes of this section, defeat device means any device, system, or element of design which senses operation outside normal emission test conditions and reduces emission control effectiveness.
(1) Defeat device includes any auxiliary emission control device (AECD) that reduces the effectiveness of the emission control system under conditions which may reasonably be expected to be encountered in normal operation and use, unless such conditions are included in the test procedure.
(2) Defeat device does not include such items which either operate only during engine starting or are necessary to protect the engine (or vehicle or equipment in which it is installed) against damage or accident during its operation.