FAQ : Is My Exhaust Legal?

FAQ : Is My Exhaust Legal?


Some of the most common questions we receive here regarding legality revolves around California’s strict smog laws and the legality of bolt on exhaust systems. While the laws surrounding exhausts have changed in recent years, including the advent of the Gold Smog Stations, we’re taking some time to educate everyone on smog laws here in California.


Q : Is my exhaust legal? What options do I have when I’m pulled over and told my exhaust is illegal?

California’s main vehicle code is 27151 which shows that there are other characteristics that the exhaust system must conform to truly show a violation. This also shows that other vehicle codes must be considered when in violation of this vehicle code. 

CA. V.C. 27151 – 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 such 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  of this chapter. No person shall operate a motor vehicle with an exhaust system so modified.

The important section here has been highlighted in red, so long as your vehicle conforms to the noise limit specified in Article 2.5, you are in the right side of the law.

(d) Exhaust Sound Level Measurement – The recorded exhaust system sound level of a stationary vehicle shall be the highest reading obtained during the test, disregarding unrelated peaks due to extraneous ambient noise. When there is more than one exhaust outlet, the reported sound level shall be for the loudest outlet. When there are two or more exhaust outlets separated by less than 12 in. (305mm), measurements shall be made on the outlet closest to the side or rear of the vehicle.


The statement stating extraneous ambient noise refers to noise coming from places other than the exhaust. For this reason, testing cannot be done in a sound-contaminated environment. It would be logical that a roadside or a highway would be a sound-contaminated environment.

(1) Sound Level Limit. The exhaust noise shall not exceed 95 dB(A) – Ask the policeman if he has a measuring device, or what evidence he has that your exhaust exceeds the noise limits. Speak to your legal representative to ensure that your response is within your rights. Remember it’s always within your rights to appeal any decision made in the court system.

Q : Is my External Wastegate legal?

CA. V.C. 27150 (a) Every motor vehicle subject to registration shall 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.

(b) Except as provided in Division 16.5 (commencing with Section 38000) with respect to off-highway motor vehicles subject to identification, every passenger vehicle operated off the highways shall at all times be equipped with an adequate muffler in constant operation and proper maintained so as to meet the requirements of Article 2.5 (commencing with Section 27200), and no muffler or exhaust system shall be equipped with a cutout, bypass, or similar device. (c) The provision of subdivision (b) shall not be applicable to passenger vehicles being operated off the highways in an organized racing or competitive event conducted under the auspices of a recognized sanctioning body or by permit issued by the local governmental authority having jurisdiction.

Part (a) states that the muffler must suppress any excessive or unusual noise. This refers to part (b) which states that the muffler must meet the requirements of Article 2.5, which refers to noise levels. Part (a) and (b) also refer to a cut-away, bypass or similar device commonly known as a wastegate dump or screamer pipe.

Q: Where can I ask for my vehicle to be tested for noise limits?

Exhaust systems for motor vehicles, other than motorcycles, gross vehicle weight rating of less than 6,000 LB (2,722 kg) shall comply with the specified noise limits when tested in accordance with the following specifications:

(a) Measuring Site. The vehicle under test shall be positioned either on outdoor pavement or on a shop floor ( but not over a hoist or pit) in a location where the exhaust outlets are near an open shop door. No sound-reflecting surface other than the pavement and the vehicle being measured shall be within 10 ft (3.0m) of any part of the vehicle.

The testing is done in an environment in which there is minimal sound contamination. Please note that there is nothing stating highways or other public roads as being an exception to this criterion.


(b) Microphone Location. The microphone for the sound level meter shall be at the same height as the center of the exhaust outlet. The microphone shall be no closer to the pavement than 8 in. (203mm) when the exhaust outlet is lower than this height. The microphone shall be positioned with its longitudinal axis parallel to the ground, 20 inches plus or minus 1 inch (508mm plus or minus 24mm) from the nearest edge of the exhaust outlet, and 45 degrees plus or minus 10 degrees from the axis of the outlet.

For exhaust outlets located under the vehicle body, the microphone shall be located at the specified angle and at least 8 in., (203mm) from the nearest part of the vehicle. For exhaust outlets that make an angle of 45 degree or less with the longitudinal axis of the vehicle, the microphone shall be outboard of the exhaust axis. For exhaust outlet angles of more than 45 degrees, the microphone shall be on the side of the exhaust axis to the rear of the vehicle.

This denotes where the sound measuring device is relatively close to the exhaust outlet. Notice that it does not make any references to provisions for sound level testing done from distances greater than 21 inches. < 


(c) Engine Operation. The engine shall be at a normal running temperature with the transmission in neutral. System sound level measurements conducted for certification by exhaust manufacturers shall be made at an average steady-state engine speed of three-quarters of maximum rpm. Sound level measurements conducted by licensed muffler certification stations shall be made at an average steady-state engine speed of 3,000 rpm for vehicles manufactured before 1972 for which the station does not have a maximum rpm data.

Please notice that the engine is supposed to be running and the law has no provisions for parked vehicles. Note that the certification can be done by the manufacturer of the exhaust system. There is no requirement that the manufacturer sound test each individual vehicle. Re: “licensed muffler certification stations,” the letter from the California Highway Patrol file Number 60.A9190.A9181.062.9610186 states in the third paragraph that these facilities were canceled in 1978 due to budgetary cut-backs. 

Q: Is my vehicle exempt from these smog laws?

(2) Exemption. The exhaust noise of vehicles manufactured after 1967 may exceed 95 dB(A) if the replacement parts are no louder than the original equipment or “factory duplicate” system and the total sound level of the vehicle when tested in accordance with Sections 1040 through 1049 of this title complies with the limits in Vehicle Code Section 27205 for the year of manufacture of that vehicle.

For more stay tuned for our primer into California Gold Stations and what they mean to you.


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