How to Pass Smog With Aftermarket Parts in California

What the Inspector Looks At

A California smog check on a newer car has two halves. There's a visual check of the emissions parts under the hood. Then there are functional checks, and the big one is a scan of the engine computer through the OBD port.

The visual part is where most hardware mods get caught. BAR's sample inspection report lists the categories the inspector grades one by one.

• Air injection system
• Catalyst
• Computers, sensors, switches and wiring
• Crankcase emission controls
• Exhaust gas recirculation (EGR)
• Fuel evaporative system (EVAP)
• Fuel metering system
• Turbocharger and supercharger
• Vacuum lines to sensors and switches
• Other emission related parts and systems

Each line gets a pass or a fail. A part that isn't stock and has no EO fails its line, and the car fails with it. How clean the tailpipe runs doesn't change that.

This isn't rare. In 2025, 134,222 initial tests on newer cars checked by scan failed the visual part, about 1.28% of them. Another 25,308 older cars failed it too, about 3.11%. That's 159,530 visual failures in one year, from the state's own smog check summary data. Not every one was a mod. Missing and broken parts count here too. But a part with no EO lands in the same pile.

Which Parts Need an EO Number

CARB's own enforcement advisory names the usual suspects. Advisory 292 says exhaust headers, software tuners, air intakes, superchargers and turbochargers typically need an EO to be sold, installed or used on a street car in California.

The key words are for your car. CARB's advice to owners is to buy parts that have an EO for their own vehicle model. An intake with an EO for a Mustang does nothing for your Camaro.

Catalytic converters have their own label rules and their own database. Our catalytic converter guide covers those line by line.

How to Check a Part Before You Buy It

Do this before the part ships, not after the test. It takes a couple of minutes.

1. Open CARB's part check page and pick the type. Most bolt on parts are under Performance and Add on Parts. Converters, motorcycles, auxiliary fuel tanks and fuel conversions have their own lists.
2. Search the maker and the part. Then read the vehicle list on the EO. Your car has to be on it, not just a car like it.
3. Keep the EO number and the paperwork in the glove box. CARB says each exempted part has a number that smog stations can verify.
4. If the seller says "50 state legal" but you can't find an EO, treat it as having none. The lookup is the proof, not the ad.

Bought the parts used or off a forum? Find the maker's part number on the part itself and run the same lookup. No match, no pass.

The Tune Check Since July 2021

For years a flashed engine computer slipped through. BAR's own newsletter said software changes were hard, if not impossible, to catch during a smog check. That changed on July 19, 2021.

Since then, the inspection system reads your car's OBD data and compares it to data from similar cars. If your software doesn't match an approved setup, the car fails. The report says Modified Software as the overall result. BAR's slides show a sample report where every visual line passed and the car still failed for this one reason.

BAR's first guess was that 5 to 10 cars a day would fail this way statewide. The rule itself is older. It's been in the inspection rules since July 1, 2013, in Title 16, section 3340.42.2(c)(8). The 2021 update just gave inspectors a way to see it.

How do I fix a Modified Software fail?

Have a licensed repair shop put the stock software back, or load a CARB EO version for your car. Then retest at any licensed smog station. If the DMV sent you to a STAR station or the Referee, you go back to that same kind of station to finish.

Two more things from BAR's rollout slides are worth knowing. A Modified Software fail counts as tampering, so the car can't get state repair money until it's fixed. And if you swear the software is stock, the Referee will ask for a signed letter from a dealer. That letter has to say the software version, Cal ID and CVN are right for that exact car.

Engine Swaps Go to the Referee Once

BAR splits engine work into two kinds. A replacement is the same make, cylinder count and engine family as the original, with the original emissions parts put back. That tests like stock. An engine change is anything else.

An engine change has to pass a first inspection at a Smog Check Referee center. The Referee puts a label inside the engine bay. After that, regular stations can test the car using the label, and your test schedule doesn't change. That first visit follows the smog rules for the car the engine came from. BAR says bringing the donor car's VIN makes it go smoother. Book it at (800) 622-7733. The steps are on BAR's engine change page.

That citation bar matters for modded cars. A loud exhaust ticket under Vehicle Code 27150 or 27151 is cleared with an exhaust test at a Referee station, and the law lets the Referee run that test only for cars that got the ticket. It's a separate trip from your normal smog check.

What a Failed Mod Costs You

The test fee is the small part. You pay for the retest after you put things back. Shops that post a smog price on their own websites list a median of $45, across 262 shop sites in a Data for Trades price study. Your station may charge more or less.

The bigger costs come from the law. CARB says a street car with a performance part that has no EO is tampered under Vehicle Code 27156 and 38391. That can mean a failed smog check, a DMV registration stop, a proof of correction, and a penalty of up to $37,500 per violation. Those numbers come straight from CARB's 2020 advisory.

There's also money you lose access to. A tampered car can't use the state's repair help until it's corrected, and the $650 repair cost waiver isn't open to tampered cars either. Our waiver and Referee guide has the full rules.

From the Inbox

Will a cold air intake pass smog in California?

Only if that intake has a CARB EO for your exact year, make, model and engine. CARB lists air intakes among the parts that typically need one. With the EO, it passes the visual check. Without it, it fails.

Can a tuned car pass smog in California?

Yes, if the tune is stock software or a CARB EO version for your car. Any other tune fails as Modified Software, a check BAR has run on every scanned car since July 19, 2021.

Will a cat back exhaust pass smog?

CARB's list of parts that typically need an EO names headers, not cat back systems. A muffler and pipes behind the converter that leave the converter and sensors alone usually don't touch the parts the inspector grades. Noise is a separate law. Vehicle Code 27151 sets 95 dBA as the passing level for most cars under 6,000 pounds, and a loud exhaust can get you a ticket even if it passes smog.

What if I bought the car already modified?

You still have to make it legal before it passes. BAR says a buyer who didn't know about illegal software can file a complaint with BAR. Get a quote to return it to stock before you register it, if you can.

Can I call it a race car and skip the rules?

Not if it has plates and drives on the street. California's racing exemption covers competition cars that are not used on a public highway, and that includes public dirt roads and parks. CARB says a true race car should be trailered to events.

Once your car is back to stock or EO parts, you just need a station. We list every licensed station in the state on our smog check finder, and you can filter for STAR if your notice asks for one.