Section 325.91 - Exhaust systemsA motor vehicle does not conform to the visual exhaust system inspection requirements, 40 CFR 202.22 , of the Interstate Motor Carrier Noise Emission Standards, if inspection of the exhaust system of the motor vehicle discloses that the system-
(a) Has a defect which adversely affects sound reduction, such as exhaust gas leaks or alteration or deterioration of muffler elements, (small traces of soot on flexible exhaust pipe sections shall not constitute a violation of this subpart);(b) Is not equipped with either a muffler or other noise dissipative device; or(c) Is equipped with a cut-out, by-pass, or similar device, unless such device is designed as an exhaust gas driven cargo unloading system.40 FR 42437, Sept. 12, 1975, as amended at 75 FR 57193 , Sept. 20, 2010