On-road motor vehicle noise is regulated by the California Vehicle Code, not Sonoma Municipal Code. CVC § 27150 requires an adequate muffler at all times and § 27151 prohibits exhaust modifications that increase noise above the statutory limits (95 dBA for passenger vehicles under 6,000 lbs GVWR). Sonoma police enforce these state standards.
California has occupied the field of on-road motor vehicle equipment standards. CVC § 27150(a) requires every motor vehicle subject to registration to be equipped with an adequate muffler in constant operation and prohibits cutouts, bypasses, or similar devices. CVC § 27151 prohibits modifying an exhaust system to amplify or increase noise above the limits in Article 2.5 (commencing with CVC § 27200). For motor vehicles with a manufacturer's GVWR under 6,000 lbs (other than motorcycles), 95 dBA or less when tested under the current SAE standard complies with § 27151. Sonoma Municipal Code Chapter 9.56 does not set vehicle-specific dBA limits for on-road operation; instead, the general SMC § 9.56.040 dBA limits and the SMC § 9.56.030 'loud or unusual noises' prohibition apply to off-road noise sources (e.g., idling delivery trucks, generators, refrigerated trailers at the property line). Loud car stereos in public are also enforceable under CVC § 27007.
CVC §§ 27150/27151 violations are correctable equipment infractions handled in Sonoma County Superior Court. Fines start around $25 base plus state penalty assessments, typically totaling $193 for a first violation; correction (re-inspection certifying compliance) clears the charge. Modified exhaust generating noise above 95 dBA is no longer correctable in many cases following AB 1824 reforms — see California Highway Patrol enforcement guidance.
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