Underglow and auxiliary lamps under the California Vehicle Code

Start with the gate. Vehicle Code section 24003 provides that "No vehicle shall be equipped with any lamp or illuminating device not required or permitted in this code, nor shall any lamp or illuminating device be mounted inside a vehicle unless specifically permitted by this code." Its only exceptions are interior lamps such as door, brake and instrument lamps and map, dash and dome lamps used to light the inside of the vehicle, lamps needed to operate the vehicles listed in section 25801 or utility service vehicles or to light a cargo space during loading, and warning lamps mounted inside an authorized emergency vehicle. So the question for any accessory light in California is not whether a statute bans it but whether a statute permits it.

Underglow is permitted, within measured limits. Section 25400(a) allows any vehicle to carry a lamp or device on the exterior that emits a diffused nonglaring light of not more than 0.05 candela per square inch of area. Subdivision (b) sets the color and placement rules: the light may not display red to the front, may display other colors, and may not resemble, be installed within 12 inches of, or sit in a position that interferes with the visibility or effectiveness of any required lamp, reflector or other device on the vehicle. Subdivision (c) caps the lamp or device at 720 square inches, and where a lease, rental or donation is involved it restricts installation to vehicles operated primarily within business or residential districts or municipalities, or between such districts in close proximity. Section 25401 adds that no diffused nonglaring light may resemble any official traffic control device.

Color is governed separately by section 25950, amended by Stats. 2025, Ch. 415 (SB 480) effective January 1, 2026. Light visible from in front of a vehicle must be white or yellow, subject to narrow exceptions for rear side marker lamps required by section 25100, foglamps under section 24403 anywhere in the white to yellow spectrum, infrared illuminating devices permitted by section 24255, and automated driving system marker lamps described in section 38750(i). Light visible from the rear must be red, with exceptions for turn signals, backup lamps showing white, front side marker lamps showing yellow, stoplamps on vehicles built before January 1, 1979, and reflectors installed under section 24611. Blue is not available to a private vehicle at all: section 25258(b)(1) reserves a steady or flashing blue warning light visible from the front, sides or rear to authorized emergency vehicles operated by the peace officers and probation officers it lists, and section 25250 states flatly that flashing lights are prohibited on vehicles except as otherwise permitted.

The forward facing accessory lamps that the code does permit come with counts and heights. Section 24402(a) allows not more than two auxiliary driving lamps mounted on the front between 16 and 42 inches, designed to supplement the upper beam and not to be lit with the lower beam. Section 24402(b) allows not more than two auxiliary passing lamps mounted between 24 and 42 inches, which supplement the lower beam and may also burn with the upper. Section 24403 allows not more than two foglamps, used with but never in substitution for headlamps, mounted on the front between 12 and 30 inches on a motor vehicle other than a motorcycle and between 12 and 40 inches on a motorcycle, aimed so that no high intensity portion of the light left of the vehicle centerline rises above a level four inches below the center of the lamp at 25 feet ahead. Section 24404 allows not more than two white spotlamps, none exceeding 32 standard candlepower or 30 watts, none projecting glaring light into an approaching driver eyes, with the beam kept off the roadway to the left of the prolongation of the vehicle left side line and its top not striking the roadway beyond 300 feet, and no spotlamp in use may be aimed to illuminate another moving vehicle.

Lighting equipment offenses are Vehicle Code infractions, so section 42001(a) caps the fine at $100 for a first infraction, $200 for a second within a year of a conviction and $250 for a third or subsequent within a year of two or more convictions, before penalty assessments. 5(e) puts them inside the correctable violation scheme and section 40610(a)(1) requires the officer to take a signed promise to correct rather than a straight citation, with a stated correction period not exceeding 30 days. That protection is conditional.

Section 40610(b)(2) lets the officer refuse the notice where the violation presents an immediate safety hazard, which is the natural finding for a forward facing red lamp, an unpermitted blue light or an aimed light bar burning on a public road, and section 40610(b)(1) does the same where there is evidence of fraud or persistent neglect. When a notice does issue, proof of correction may be certified by a police department, the California Highway Patrol, a sheriff, a marshal or another agency regularly engaged in Vehicle Code enforcement under section 40616(c), and the clerk collects a $25 transaction fee for each violation under section 40611(a). Willfully violating the promise to correct, or failing to deliver proof, is a misdemeanor under section 40616.