Summary: Exhaust violations are correctable violations according to CVC 40303.5. All vehicles must have a functioning muffler that is not bypassed or defeated. No motorcycle can have an exhaust system modified to increase the noise greater than 80 dbA for 1985 and newer vehicles. Modified exhausts are completely legal in California as long as these conditions are met. However, there is currently no way to test a vehicle's noise level. No law enforcement agency is issued noise measuring equipment, nor are they trained in measuring or estimating noise levels. This law is, therefore, unenforceable. See the CHP Bulletins in the link below.

Further Reading:
CHP Bulletin No. 98-100  - Open original bulletin as PDF
www.bikersrights.com

27150(a): Every motor vehicle subject to registration shall at all times be equipped with an adequate muffler in constant operation and properly maintained to prevent any excessive or unusual noise, and no muffler or exhaust system shall be equipped with a cutout, bypass, or similar device. 

27151(a): No person shall modify the exhaust system of a motor vehicle in a manner which will amplify or increase the noise emitted by the motor of the vehicle so that the vehicle is not in compliance with the provisions of Section 27150 or exceeds the noise limits established for the type of vehicle in Article 2.5 (commencing with Section 27200). No person shall operate a motor vehicle with an exhaust system so modified.

27200(a): The Department of Motor Vehicles shall not register on a dealer's report of sale a new motor vehicle, except an off-highway motor vehicle subject to identification as provided in Division 16.5 (commencing with Section 38000), which produces a maximum noise exceeding the applicable noise limit at a distance of 50 feet from the centerline of travel under test procedures established by the Department of the California Highway Patrol.

27201: For the purposes of Section 27200, the noise limit of 92 dbA shall apply to any motorcycle manufactured before 1970.

27202: For the purposes of Section 27200, the following noise limits shall apply to any motorcycle, other than a motor-driven cycle, manufactured:

• (1) After 1969, and before 1973 ............  88 dbA   
• (2) After 1972, and before 1975 ............  86 dbA   
• (3) After 1974, and before 1986 ............  83 dbA   
• (4) After 1985 .............................  80 dbA

40303.5: Whenever any person is arrested for any of the following offenses, the arresting officer shall permit the arrested person to execute a notice containing a promise to correct the violation in accordance with the provisions of Section 40610 [see 40610(b) below for disqualifying conditions] unless the arresting officer finds that any of the disqualifying conditions specified in subdivision (b) of Section 40610 exist:
  (a) Any registration infraction set forth in Division 3 (commencing with Section 4000).
  (b) Any driver's license infraction set forth in Division 6 (commencing with Section 12500), and subdivision (a) of Section 12951, relating to possession of driver's license.
  (c) Section 21201, relating to bicycle equipment.
  (d) Any infraction involving equipment set forth in Division 12 (commencing with Section 24000) [Division 12 ranges from 24000 - 28150. Exhaust violations are 27150 or 27151 and are, obviously, within Division 12], Division 13 (commencing with Section 29000), Division 14.8 (commencing with Section 34500), Division 16 (commencing with Section 36000), Division 16.5 (commencing with Section 38000), and Division 16.7 (commencing with Section 39000).

40610(b): Pursuant to subdivision (a), a notice to correct violation shall be issued as provided in this section or a notice to appear shall be issued as provided in Section 40522, unless the officer finds any of the following:

• (1) Evidence of fraud or persistent neglect.  
• (2) The violation presents an immediate safety hazard.  
• (3) The violator does not agree to, or cannot, promptly correct the violation.