Vehicle Code section 521

A “retarder” is a device, other than a brake, which, when activated by the driver, applies a retarding force to the wheels of a vehicle without the use of friction. A retarder may be installed in or on the engine, exhaust system, drive train, or wheels of a motor vehicle, or an axle or wheels of a towed vehicle. A retarder may operate by altering the valve timing of the engine, by controlling the flow of a circulating fluid, by applying an electromagnetic force, by controlling the release of gases from the exhaust system, or by other means. A retarder may or may not be capable of stopping the vehicle upon which it is installed.

Source: Section 521, https://leginfo.­legislature.­ca.­gov/faces/codes_displaySection.­xhtml?lawCode=VEH§ionNum=521.­ (updated Jan. 1, 1991; accessed Jun. 25, 2025).

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Verified:
Jun. 25, 2025

§ 521's source at ca​.gov