These are the only Nevada Laws that I could find regarding exhaust/emissions/mufflers. There could be federal laws (Commercial Vehicle) that may apply, some of our trucks might fall under commercial vehicle laws. I'm not an expert with those, sorry. Here's a link to the Nevada Law Library index....
NRS: Titles and Chapters
NRS 484D.415 Mufflers: Prevention of emissions.
1. Every motor vehicle shall at all times be equipped with a muffler in good working order and in constant operation to prevent emissions greater than those allowed by rules and regulations established by the Department. No person shall use a muffler cutout, bypass or similar device upon a motor vehicle on a highway.
2. The engine and power mechanism of every motor vehicle shall be so equipped and adjusted as to prevent the escape of excessive fumes or smoke.
(Added to NRS by 1969, 1208; A 1971, 921)—(Substituted in revision for NRS 484.611)
NRS 484D.535 Device for control of pollution: Use required; disconnection or alteration prohibited; exceptions.
1. Except as provided in subsection 2, a person shall not operate or leave standing on any highway any motor vehicle which is required by state or federal law to be equipped with a device for the control of pollution from motor vehicles unless the device is correctly installed and in operating condition. A person shall not disconnect, alter or modify any such required device.
2. The provisions of this section do not apply to:
(a) An alteration or modification found by the State Environmental Commission not to reduce the effectiveness of the required device.
(b) Motor vehicles which have been licensed by the Department as experimental vehicles.
(c) Any vehicle which has been granted a waiver or exemption from the regulations for the control of emissions from motor vehicles.
(Added to NRS by 1971, 1203; A 1973, 5, 1705; 1979, 568, 1034; 1985, 340, 1954)—(Substituted in revision for NRS 484.644)
NRS 484D.540 Device for control of pollution: Penalty; proof of conformity may be required. Violation of the provisions of NRS 484D.535 is a misdemeanor. Whenever any motor vehicle is found by any peace officer to be in violation of the provisions of NRS 484D.535, and a notice to appear or citation is issued, it may require that the person named therein shall produce in court proof that such vehicle or its equipment has been made to conform to the provisions of NRS 484D.535.
(Added to NRS by 1971, 1203)—(Substituted in revision for NRS 484.6441)