California’s Car Modification Laws: What You Should Know

California’s Car Modification Guidelines Overview

The regulations surrounding car modification in California are primarily governed by the California Vehicle Code (CVC). This body of law outlines the guidelines for vehicle equipment, emissions control, and safety requirements. When a car is modified, these elements must be taken into consideration to ensure compliance with both state and federal laws.
Essentially, the CVC sets out the minimum standards for car modifications, and any fitment or adjustment should be completed in accordance with this code. The code covers numerous aspects of car design and function and can be confusing, especially if you’re a car enthusiast who may not be aware that specific changes might be unlawful.
For example, The California Air Resources Board has implemented the Smog Check program, which affects the registration of modified cars with new engines and other new components. State officials work with law enforcement to identify vehicles that have been tampered with and remove them from the road. In fact, a vehicle can be seized and destroyed if it doesn’t adhere to all emissions standards — even if you only recently purchased it.
The CVC covers not only emissions, but most parts of the vehicle that are now required to be state certified. All modifications must be inspected by a certified smog technician, but it’s also your responsibility to research the laws related to specific modifications before ordering parts or booking an appointment for a fit .
Keep in mind that in some cases, a modification that follows the letter of the law may yet be prohibited. For instance, even with a proper certification, a tinted windshield is illegal in the state of California despite being legal in other states. It is highly recommended that you check with a certified smog technician prior to ordering any parts or making changes.
The California Highway Patrol also has a hand in the discipline of car modifications. Specifically, the CHP sets the standards for lighting, including headlamps, stop lights, taillights, turn signals, and other required lights. The reason for this is to ensure public safety. Officers are on the lookout for malfunctions that may pose a danger to others on the road.
Lighting modifications may require CHP approval, and you must also be aware of restrictions on colors, luminosities, and usage. A blue light, for example, is illegal unless you’re a law enforcement officer.
The CHP also regulates the installation of certain equipment, such as bumpers and seatbelts. Whether or not this is necessary for your vehicle depends on the type and model year. If you operate vehicles registered as "historic motor vehicles", alternative rules apply.
There’s always plenty of confusion around the rules governing car modifications, so be sure to do your due diligence before you call up the manufacturer to get those sleek, black out headlights. Even seemingly innocuous changes can result in serious fines and penalties.

Emission Rules and Exhaust Modifications

To understand the legality of modifying your car to have enhanced exhaust systems is to understand the origin of the law. For example, California has two major regulatory bodies regulating emissions: the United States Environmental Protection Agency and the California Air Resources Board (CARB). The federal government sets minimum allowable emission standards for all vehicles and all 50 states must follow those standards. At the state levels, California has different regulations for vehicles. California has opted for a more strict standard in an attempt to promote cleaner vehicles and ultimately reduce air pollution. This means that California has more authority to regulate emissions and sell vehicles.
When looking at personal vehicles and CARBs right to regulate, private individuals have a right to modify their vehicles, except when those modifications would violate state law. So where can someone find the law to determine if their modifications would be legal or not? California Health and Safety Code sections 42301 and 42301.5 identify what modifications are legal in the state of California. Any vehicle that does not conform to federal emission requirements is not street legal unless it was modified for off-road use at the time of sale. This does not mean that you cannot have a modified vehicle on the road; it just means that it cannot make use of a public highway or freeway. This means that in order to drive your modified vehicle, you have to keep it within a private radius at all times.
Even with the private radius, people do modify their vehicles so that they produce a louder noise. According to the law, any person who modifies his car to where it produces a greater noise must have a certificate of compliance for the engine. One of the types of emissions that these emission standards are supposed to reduce is NOx emissions that are produced from diesel engines. There are more specific regulations that affect owners of diesel vehicles but for the purposes of this blog, it is enough to say that diesel vehicles are regulated in an even stricter manner than other vehicles.

Lighting and Window Tint Laws

California’s Vehicle Code sets limits on the use of certain lighting sources and window tinting on vehicles. While vehicles may come with clear or colored lights, they must still conform to California’s Vehicle Code. According to Vehicle Code sections 25250, 25251, and 25270, motor vehicles and trailers are only allowed to use certain types of lights and reflectors, including those that perform the functions of turning signals, stop lights, hazard lights, and tail lights. Other lights that are allowed under the Vehicle Code are headlights, auxiliary lights and outdoor lights, such as fog lamps and off-road lamps. These lights must also be approved by the Department of Transportation.
Cal. Vehicle Code section 26100 lists lights that are unlawful for a motor vehicle to have. The following lights are prohibited for use on any vehicle upon a highway: The use of red lighting equipment at the front of a motor vehicle is also restricted. Cal. Vehicle Code section 25258 states that every motor vehicle, operated upon a highway, except emergency vehicles, shall be equipped with at least two head lamps with at least one of those head lamps being equipped so that when its beam remains on continuously while the vehicle is traveling on a highway, the lamp will illuminate the road surface directly in front of the vehicle together with the area between the vehicle and the center of the highway and, if possible, no more than 50 feet ahead of that part of the road surface so as to reveal persons and vehicles on foot or traveling with and adjacent to the vehicle.
California law allows commercial vehicles to use red and white flashers as additional warning lighting. According to Cal. Vehicle Code section 25270.5, any vehicle that is required to carry the flashers may not use them unless it is working on the freeway or engaged in emergency operations. Violations of sections 25270.5(a) and 25270.5(b) are subject to a $250 fine or 8 hours of community service.
A local public agency may also adopt ordinances permitting the use of flashing lights and other warning lights on permitted oversized construction vehicles and other vehicles with a gross weight of 10,000 lbs. or more, provided those vehicles travel at less than 25 miles per hour when operated on a highway. (Cal. Vehicle Code section 25270(c)). These vehicles are also subject to the $250 fine or 8 hours of community service provision for violations of section 25270(c).
The limits on window tinting in California can be found in Cal. Vehicle Code section 26708. In general, this section prohibits drivers from operating vehicles with obstructions upon the front windshield, side windows, rear window, and any other window. However, the law does contain some exceptions. Section 26708 does not apply to driver and passenger side windows of authorized emergency vehicles, panel trucks exceeding 10,000 lbs, buses, taxicabs, limousines, and vans manufactured after 1986 that are not equipped with tinted glass.
The law also provides an exception for tinting that is applied to the windshield that allows 70 percent light transmittance and tint above the AS-1 line or the manufacturer’s tint line. Although these permits may allow for some tinted windows, violations can lead to fines of $25 to $250. Some violations may also result in moving violations. Section 26706 also allows law enforcement officers and members of the California Highway Patrol to stop drivers they suspect have violated the window tinting requirements. Additionally, commercial vehicle operators will need to be careful about tinting on oversized construction vehicles, which are subject to the same fines as commercial vehicles with violation hazards lights.

Suspension and Ride Height Guidelines

In order to ensure that vehicles can interact safely with other cars and trucks on the road, California law prohibits certain modifications to suspension and ride height. These restrictions are designed to keep vehicles at an appropriate ride height, maintain proper suspension travel and limit wear and tear on tires.
This section will focus on the effect of modifications to a vehicle’s front and rear suspension on the vehicle’s ride-height measurement. In order to keep your suspension properly tuned and avoid costly penalties, it is important to stay within the legal limits.
Vehicle Ride Height: Measuring the Approximate Ride Height
California law places restrictions on vehicle ride height during daily operation. These restrictions limit how high or low certain civilian vehicles are allowed to ride. These rules apply to all cars operating in California and have nothing to do with modifying the suspension or adding tires to increase the ride height.
The first step in complying with the ride height restrictions on civilian passenger cars is to measure the vehicle’s legal ride height. California law defines the legal ride height for the front and rear of the vehicle as the distance from the bottom of the tire to the bottom of the body at a vertical reference plane located at the center line of each front and rear wheel position. For some vehicles, the exact location of the reference plane may be more difficult to determine. For example, the following vehicles have different locations for this reference plane: the Tesla Roadster (front wheel), the Honda CR-V (rear wheel), the Nissan 350Z (front wheel), the Chevrolet Corvette Z06 (front and rear wheel), the Cadillac XLR-V (front and rear wheel) and the Ford Mustang GT-C (rear wheel).
In addition, California law provides one rule for measuring ride height when the vehicle is equipped with an adjustable suspension. If the vehicle has an adjustable suspension, you must use the maximum possible height of the vehicle’s suspension to measure the vehicle’s legal ride height.
Legal Ride Height
For civilian passenger cars, California law defines the maximum legal ride height as follows:

1. For front wheel drive cars:

The legal ride height is 18" or less, measured by using a level plane across the top of the wheel opening and measuring vertically from the lowest portion of the wheel opening to the pavement. In cases where the vehicle has a removable cover, such as a car with a removable top or convertible, the vehicle height must be checked with the cover off. Additionally, for any passenger cars with removable roofs or hard tops, the vehicle ride height must be checked with the roof off.

2. For rear wheel drive cars:

The legal ride height is 20" or less, measured by using a level plane across the top of the wheel opening and measuring vertically from the lowest portion of the wheel opening to the pavement. In cases where the vehicle has a removable cover, such as a car with a removable top or convertible, the vehicle height must be checked with the cover off. Additionally, for any passenger cars with removable roofs or hard tops, the vehicle ride height must be checked with the roof off.
As outlined above, the maximum vehicle ride height is 18 inches or less for front wheel drive cars and 20 inches or less for rear wheel drive cars.

Tire and Wheel Requirements

California’s Department of Motor Vehicles (DMV) has specific regulations concerning the diameter and width of tires and wheels that are slightly more complex than those found in federal law. Any vehicle must be stopped if at any wheel its tread depth measures less than the minimum depth of 1/32" (except for motorcycles, for which the limit is different), and if any tire or floating wheel has insufficient tread depth to prevent it from aquaplaning when it’s wet on a road.
But that’s just the start: the wheels must be a certain size to accommodate tires of a given height , and to be able to accommodate the necessary wheelbase necessary to keep the steering, suspension, axle and other elements from being adversely affected by larger or wider tires.
The wheelbase of the vehicle must also be kept in mind. The manufacturer’s standards for the wheelbase are listed on the vehicle’s original Equipment Certification Label. When replacing wheels and tires, you cannot alter the wheelbase by more than ¼-inch from the manufacturer’s specifications. Modifications that result in either a longer or shorter wheelbase than minimum allowe for that make and model will violate California law.

Popular Legal Modifications

When making modifications to a car to increase its performance, be sure they comply with California’s laws and the California Air Resources Board (CARB). Smog checks are no longer required for diesel vehicles weighing over 14,000 pounds and manufactured prior to 1998 (except for heavy-duty vehicles over 26,000 pounds that are not subject to federal Environmental Protection Agency standards). Thus, heavy-duty trucks and buses are typically exempt from California’s smog requirements. Although diesel fuel is still allowed, rapeseed biodiesel has become increasingly popular. The use of diesel fuel in most light-, medium-, and heavy-duty trucks is becoming more commonplace. Generally this fuel is less costly and provides greater power to a vehicle.
Some popular aftermarket components offered to improve emissions or modify a vehicle’s performance include Diesel particulate filters (DPF): Vehicle and engine manufacturers are required to offer a DPF for all 2007 and later diesel engines. DPFs collect soot for subsequent disposal. The cost varies, but a DPF for a super duty truck can cost upwards of $3,000. Motorsports DPFs have become available in the last few years and have seen increased popularity. Catalytic converters: To comply with CARB’s regulations, new catalytic converters are provided with emission warranty and need to be purchased from a CARB-certified dealer or an auto parts store that sells parts for offroad or racing vehicles. There are catalytic converters available at a lower cost but without any warranties or compliance with CARB’s standards. Intercoolers: Intercoolers are often used in today’s diesel engines to help reduce engine temperatures. They are designed to fit over the top of the existing intercooler. These typically cost $300 to $500 and may require some fabrication to install. Those concerned with smog emission will need to place the old intercooler back on the vehicle in the event of an emissions test. Cold air intakes: Cold air intakes are offered as a "high flow air filter" on online retail sites. PURUS, Inc. is an aftermarket company that is known within the industry for its high quality performance parts called the P FLOW. The advantage of the P FLOW system is that it is a direct fit and complies with all CARB standards. Typically these kits retails for $350 to $400. This system promises a 30 to 50 horsepower increase, 1 to 3 miles per gallon, and a four to five percent torque increase. However, these modifications are not protected by any warranties and may void the warranty for emissions control systems. Hoods: Hoods are usually purchased with other aftermarket parts. Replacing the hood may free up horsepower and improve looks, but the downside is that the hoods do not include a shielding device or a scoop that meets CARB standards. Aftermarket hoods are generally not approved for street use and their cost ranges between $50 for fiberglass hood scoops and $120 for overlays to $450 for unpainted carbon fiber hoods. Throttle plumbing: This component helps to allow air into the engine but is likely to push the boundaries of CARB compliance, as it involves moving the throttle body closer to the engine. There may also be a possible better alternative in the Integrated Electronic Throttle Control (IEBC). Factory throttle bodies are typically made of aluminum or plastic. Aftermarket throttle bodies are usually made of steel so they handle higher horsepower requirements and increased boost pressure. Grilles: Grilles improve airflow and allow the engine more air to help increase performance. Many may opt to purchase a larger grill and might be prohibited from using it on a street vehicle with a VECI label. Before making any additional modifications, always consider contacting a local smog professional.

Penalties for Infringement

Violating the laws that govern car modification in California can have real, serious consequences. Auto manufacturers, dealers, and shops that sell or assist with aftermarket products must understand the laws before they become victims themselves of those laws’ enforcement. Consumers must also recognize that they’re breaking the law if they modify their cars outside of these statutes’ guidelines. For especially serious violations, the penalties can add up quickly.
Fine and Penalties
The state can fine individuals or entities that modify vehicles in violation of the law as much as $2,500. Depending on the state in which the violation occurs, penalties can vary. In some states, penalties are more severe for intentional second or third convictions within a five-year period. The maximum fines range from $250 to $5,000 and the maximum day equivalent ranges from one day to 365 days in county jail.
Liability
Those who violate the car modification laws remain liable for the consequences of the violations whether or not they act intentionally or negligently. Those who choose to assist others in violating the law during the course of their jobs also bear legal liability. Violators who commit intentional acts can face charges of unlawful sale or unlawful possession of an aftermarket part. Performing prohibited acts with the intent to defraud secondary buyers or retailers is also illegal. Violators who exploit relationships with consumers , claim to possess technical expertise, use their professional titles, and who access sensitive information may receive new felony charges.
Fines aren’t the only costs violators face. Potential bills and civil liabilities can include:
Driving Unpermitted Vehicles
Driving a car after making illegal modifications can subject the driver to further legal trouble, including a misdemeanor infraction. While the driver remains responsible for budgetary penalties and other costs of prosecutor’s fees and expenses as well as investigation expenses, the law imposes criminal penalties against the driver who chose to allow the unregulated vehicle on the road. The driver may pay up to $500 and spend up to six months in county jail, as well as forfeiting the vehicle, which would mean that the driver would also lose any "sunk costs" associated with it.
Prosecution and Investigation Costs
Becoming the target of prosecution for violating law prohibiting illegal car modifications is only the start of the expenses those found in violation of the law will face. The prosecutor’s fees amount to as much as $70,000 per felony case as of 2016, and the total cost of prosecution can rise to as much as $75,000 or more. On top of prosecutions, investigators can charge up to $2,000 for each additional hour they spend on the case.