What does a CARB EO number mean?

An Executive Order (EO) is CARB's confirmation that a specific aftermarket part, for a specific vehicle application, has been evaluated and does not increase emissions beyond what is allowed. The format looks like D-123-45.

Which parts need a CARB EO?

Generally parts that touch the emissions system: intake systems ahead of emissions sensors, exhaust before and including the catalytic converter, catalytic converters, turbo and supercharger kits, and tunes or ECU changes that affect emissions behavior.

Does every state require CARB EO compliance?

No, this varies by state and can change over time. Some states adopt California's emissions standards for at least some parts and some do not; check current requirements for the specific state a car is registered and driven in rather than assuming.

Is it legal to run a part labeled off-road use only on the street?

Generally no, in a state that requires emissions approval for that type of part. That label is the manufacturer stating the part was not submitted for, or did not receive, street-use emissions approval.

How do I check if a part has a CARB EO?

Look for the specific EO number the manufacturer lists for that part and vehicle application, and check it against CARB's own published Executive Order database rather than relying on a general claim alone.

Does the federal Clean Air Act apply even if my state does not follow CARB rules?

Yes. The federal Clean Air Act's tampering prohibition on emissions control equipment applies nationwide regardless of whether a state separately follows California's CARB standards.

What does "50-state legal" generally mean?

It generally means the part carries a CARB EO for the vehicle in question, since California's standards are historically the strictest a part needs to clear. This is a general description of common usage, not a guarantee for any specific part; the specific EO number is what actually confirms coverage.

Can a part lose its CARB EO after I've already bought it?

Yes, an EO can be modified or withdrawn after it's issued. A part bought with a valid EO at the time of purchase doesn't automatically keep that status forever, which is part of why checking CARB's current database, rather than an old listing or receipt, is more reliable for a used or older part.

Am I responsible for emissions-legal compliance if I buy a car that already has modifications?

Generally yes, in the sense that a state's registration and inspection requirements apply to the car as it currently sits, regardless of who installed the parts. Checking for documented EO coverage on any emissions-related modifications is worth doing at purchase, the same way it would be before installing them yourself.

Does an EO cover the part everywhere it's legal to drive, or just in California?

An EO is a California approval, but because other states can adopt California's standards, an EO'd part is often accepted more broadly than a non-EO'd one. Whether a specific state requires it, or accepts it as sufficient, is a state-specific question that can change over time, so check current requirements for the state of registration rather than assuming.

Guides · Legal and emissions

Emissions-Legal Mods and CARB EO Numbers

Some aftermarket parts that touch emissions systems need a CARB Executive Order to be legal for street use. This guide covers what an EO is and how it generally works, which kinds of parts it applies to, and why the specific rules depend on the state and change over time.

18 min read

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Beginner
Intermediate
Advanced

1 of 3 · Beginner

The basics

What it is, what it changes, and whether you need it.

What CARB is

CARB (the California Air Resources Board) is California's state agency for regulating vehicle emissions. Because California's vehicle emissions rules have historically been stricter than the federal minimum, and other states have the option to adopt California's standards instead of the federal ones, CARB's rules end up affecting more than just cars registered in California.

Whether a given state follows California's rules for a given part varies, and it can change over time, so it is worth checking current rules for the specific state a car is registered and driven in rather than assuming. This guide describes how CARB Executive Orders work in general; it is not a statement about what any particular state currently requires.

CARB's role covers more than aftermarket parts: it also sets standards new vehicles have to meet to be sold in California in the first place, runs programs related to fuels, and oversees California's vehicle inspection requirements. The aftermarket parts side, and the EO system specifically, is only one piece of a broader regulatory body, but it's the piece that matters most directly to someone modifying a car.

What an EO number is

An Executive Order, or EO, is CARB's approval for a specific aftermarket part, confirming it has been evaluated and found not to increase emissions beyond what is allowed for that vehicle. EO numbers follow a format like D-123-45.

A legitimate emissions-related aftermarket part sold for street use in a state that requires one should be able to point to a specific EO number covering that exact part and vehicle application, not just a general claim of being broadly legal everywhere. The EO is issued to the part's manufacturer for that specific design, not to the buyer or the individual car.

Getting an EO generally means the manufacturer submits the part for testing and evaluation against the vehicle's original emissions certification, demonstrating the part doesn't push emissions output, or the function of the emissions control systems, outside what that certification allows. This is a process the manufacturer goes through once per part and vehicle application, not something an individual owner does after buying the part.

What happens without one

A part that affects emissions equipment and lacks a CARB EO where one is required, or is labeled off-road use only or race use only, is not legal for use on a vehicle driven on public roads in a state that requires it. These labels are not a loophole; they are the manufacturer's own acknowledgment that the part has not been approved for street use.

See no CARB EO on record for how this shows up on a build sheet.

It's worth being clear about what an EO does not cover, too: it doesn't certify that a part is well made, reliable, or a good value; it certifies emissions compliance specifically. A part can be well built and still lack an EO simply because the manufacturer never submitted it for that specific vehicle application, which is a business decision as much as anything else.

How this differs from most other modifications

Most bolt-on modifications, like lowering springs, a different wheel and tire package, or a brake upgrade, don't involve emissions equipment at all, and CARB EO requirements simply don't apply to them; a state's vehicle code may still have its own rules for things like ride height, tint or noise, but those are separate from emissions regulation. What sets emissions-related parts apart is that they can change how much pollution the engine produces or how well the car's own emissions controls monitor themselves, which is specifically what CARB and similar programs exist to regulate.

It helps to think of a car's modifications in two rough groups: parts that change how the car looks, rides or sounds, and parts that touch the intake, exhaust ahead of the catalytic converter, the catalytic converter itself, forced induction, or the engine's own tune. The second group is where CARB EO questions come up; the first group generally sits outside emissions regulation entirely, even though it may have its own separate state rules to check.

A build that mixes both groups, for example a lowering kit and an intake system installed together, needs each part judged on its own terms: the lowering kit against whatever ride-height or vehicle-code rules the state has, and the intake against emissions requirements. Treating the whole build as one legal question, rather than checking each relevant part against the rules that actually apply to it, is a common way an otherwise fine modification gets lumped in with a genuinely questionable one.

Risk of not checking

Running an emissions-related part without confirming its legal status in the state where the car is registered can carry consequences beyond a single ticket, depending on the state and its inspection program: a failed inspection, a requirement to remove or replace the part before the car can be registered again, or, in the more serious federal cases described later in this guide, broader enforcement against the part's sale.

None of this is a reason to avoid emissions-related parts altogether, since many are sold with a valid EO for common platforms. It's a reason to check the specific part and specific state before installing one, rather than assuming a part is fine because it's popular or widely sold.

Buying a used car that already has emissions-related modifications carries the same considerations as installing them yourself: the parts don't come with a documented EO just because they're already on the car, and whoever registers or re-registers the car in a state with inspection requirements may need to show the same documentation a new installation would.

2 of 3 · Intermediate

Choosing and fitting

How to pick one, what it needs alongside it, and the usual mistakes.

Which parts typically need one

Parts that need a CARB EO in states that require it are generally ones that touch the emissions system directly, rather than every bolt-on part. This commonly includes intake systems (particularly ones that replace factory components ahead of emissions sensors), exhaust components installed before the catalytic converter, catalytic converters themselves, turbo and supercharger kits, and tunes or ECU modifications that change how the engine and its emissions controls behave.

Parts further from the emissions system, like an axle-back exhaust behind the catalytic converter, are less likely to need an EO, though this still depends on the specific part and the specific state's rules; it is not a blanket exemption.

The underlying reason this category of part needs approval is that each one can change how much pollutant the engine produces, how well the catalytic converter can do its job, or how accurately the engine's sensors read what's actually happening, and the emissions control system as a whole depends on all of those staying within the range the vehicle was originally certified for.

Intake systems, especially those ahead of emissions-related sensors
Exhaust components before and including the catalytic converter
Catalytic converters and any part that replaces one
Turbo and supercharger kits
Tunes and ECU modifications that change emissions-related behavior

Finding a part's EO

A manufacturer selling an emissions-related part for street use in a state that requires CARB approval should list the specific EO number for that part and vehicle application, not just a general claim. CARB also publishes its own database of Executive Orders that can be searched by EO number or by part category to confirm a specific part and vehicle combination is actually covered, rather than relying on the seller's claim alone.

Checking the database directly matters because a part's marketing material is written once and doesn't always get updated if an EO is later modified or withdrawn, while CARB's own published record reflects the current status.

Some sellers also print an EO number on a label or in an installation manual without it being easy to independently verify at the point of sale; treating the printed number as a starting point to check, rather than as proof on its own, is the more careful approach, particularly for a part bought secondhand where the label's history is unknown.

"50-state legal" and similar claims

A part marketed as broadly legal everywhere generally means it carries a CARB EO covering the vehicle in question, since California's standards are, historically, the strictest a part would need to clear; a part that clears California's requirements has generally also cleared the less strict standard most other states apply. A part marketed only for use outside California, sometimes described as legal everywhere except California, is usually one that hasn't gone through CARB's approval process for that application, even if it's perfectly legal under the federal baseline described in the advanced section below.

These are general descriptions of how the phrases are commonly used, not guarantees. The only way to confirm a specific part's status for a specific car is to check its actual EO number against CARB's database and against what the state of registration currently requires, rather than relying on marketing language alone.

That phrase can be misleading if read too literally, too: some non-California states have their own emissions rules, separate from CARB's, or have adopted California's standards for at least some categories of part, so "legal everywhere except California" is a common shorthand rather than a guarantee for every other state's rules specifically.

Common part labels and what they generally indicate
Label	What it generally indicates	What to check
50-state legal / CARB EO listed	An EO likely exists for at least some vehicle applications	That the EO covers your exact vehicle and engine code, not just the product line
Off-road use only / race use only	Street-use emissions approval was not sought or not granted	Whether that matches what your state allows for that category of part
No emissions claim stated either way	Status is unconfirmed, not necessarily illegal or legal	Contact the manufacturer or check CARB's database directly before assuming either way

Smog checks and inspections

States that require periodic emissions or smog inspections often include a visual check for tampered or missing emissions equipment, separate from the tailpipe emissions test itself. A missing catalytic converter, a disconnected sensor, or emissions hardware that does not match what the vehicle was equipped with can fail a visual inspection even if the car would otherwise pass a tailpipe test.

What exactly is checked varies by state and by the inspection program in place, and these programs change over time. Some inspection programs also check the status of the vehicle's onboard diagnostic monitors, general-purpose self-checks the engine computer runs on its own emissions systems, though the specifics of what counts as a pass again depend on the program and are not the same everywhere.

Not every state requires periodic emissions inspection at all; some require it only in certain counties, typically the more densely populated ones, while others don't require it anywhere. Whether a car needs to pass one, and on what schedule, is itself a state-and-sometimes-county-level question rather than a nationwide constant.

3 of 3 · Advanced

In depth

The numbers, the trade-offs and what goes wrong.

Federal law vs state rules

The federal Clean Air Act includes a tampering prohibition that applies nationwide, regardless of state: it is illegal to remove, disable, or render inoperative emissions control equipment on a vehicle registered for road use, or to manufacture, sell, or install a part intended for that purpose, unless the part is covered by an appropriate exemption. This federal baseline exists everywhere in the United States.

States then layer their own registration, inspection, and enforcement rules on top of that baseline, and it is that state layer, not the federal law itself, that varies most in practice and where CARB EO requirements typically come in for states that follow California's standards. Because both the federal baseline and the state-level rules can change over time, and because whether a given state currently follows California's standards for a given part is not fixed, treat any specific claim about a state's current requirements as something to confirm rather than something to assume from general knowledge.

See state rules and registration state and use for how this is represented on a build sheet.

It's worth being specific about what "tampering" means under that federal prohibition: it generally covers removing or disabling equipment the vehicle was originally certified with, like a catalytic converter, or a sensor the emissions system relies on, not modifications that don't touch certified emissions equipment at all. This is a general description of the prohibition's scope, not a ruling on any specific part; how it applies to a particular modification can be a genuinely complex legal question.

How EOs are scoped

A CARB Executive Order is not a blanket approval for a part in general; it is scoped to a specific part number, sometimes to a specific configuration of that part, and to specific vehicle makes, models, model years, and often engine or emissions calibration combinations. The same physical part sold for two different vehicle applications can have an EO covering one and not the other, and a part's EO can be limited to certain years of a model even within the same generation.

Confirming coverage means matching the EO to the exact vehicle it will go on, not just the part's general description.

CARB's evaluation is tied to a specific vehicle's original emissions certification, the baseline the factory car was tested and approved against
A different engine code, even in an outwardly similar car, was very likely certified separately, with its own sensor calibration, catalytic converter sizing and tuning
A part evaluated against one certification baseline isn't automatically evaluated against another, even a closely related one
An aftermarket part's EO can lag behind a brand-new model year, since testing against that year's certification has to complete before an EO for it is issued

Worked example

Checking whether an EO covers a specific car

Say an intake system's EO documentation lists coverage for a specific four-cylinder engine code across model years 2016 through 2019. This is a hypothetical example for illustration, not a real EO record.

A 2018 model of that same car, with that same engine code, falls inside the listed range, so the EO would cover it, assuming nothing else about that specific car changes the calculation, like a separate engine swap.

A 2020 model that looks like the same generation of car, but received a mid-cycle engine update with a different engine code, would fall outside that EO's coverage even though the car looks nearly identical from the outside, because the EO is scoped to the engine code and model years listed, not to the car's body style or generation as a whole.

This is why checking an EO by part number and general description alone isn't enough; the exact engine code and model year both have to match what the EO actually lists.

How EO status is tracked over the life of a part

An EO can also be modified or withdrawn after it's issued, for reasons that can include the manufacturer changing the part's design, a compliance issue found after the fact, or the manufacturer discontinuing the part. A part bought new with a valid EO at the time of purchase does not necessarily keep that status forever if the underlying EO is later withdrawn, though enforcement in that situation depends on the state and the specific circumstances.

This is part of why checking CARB's own published database, rather than relying on a listing or a receipt from when the part was purchased, is the more reliable way to confirm current status, especially for a part bought used or some time ago.

A withdrawn EO doesn't necessarily mean the part itself changed; it can also follow from a manufacturer discontinuing support, a business closing, or CARB updating its own testing requirements in a way older approvals no longer meet. None of these reasons are visible from the part alone, which is another reason the database check matters more than the part's physical condition or apparent quality.

Other emissions-adjacent checks

Beyond the tailpipe test and the visual inspection for tampered or missing equipment, some inspection programs also look at whether the engine computer's onboard diagnostic monitors have completed and are reporting ready, a general concept used across modern emissions inspection programs, though the specific pass criteria and how many monitors need to be ready vary by program and are worth checking for the specific state and inspection type.

A tune or ECU change that alters emissions-related behavior can affect these monitors, which is one more reason tunes are commonly grouped with the other emissions-related categories described earlier in this guide, alongside intake, exhaust, catalytic converter and forced-induction changes. See ECU tuning explained for what a tune generally changes.

A car's malfunction indicator lamp, the dashboard warning light tied to the onboard diagnostic system, is a related signal worth understanding on its own: it illuminates when the system detects a fault it considers relevant to emissions, which can come from a genuine mechanical issue or from a modification that the system's baseline calibration doesn't expect. Either way, an illuminated light is generally treated as a fail condition by inspection programs that check for it, separate from whatever the tailpipe or visual check finds.

What this means for a build

For anyone planning a build with emissions-related parts, the practical approach is to check EO coverage for the specific part and vehicle before buying, confirm what the state of registration currently requires (since this is one of the areas most likely to differ from general assumptions and to change over time), and keep in mind that off-road or race labeling on a part is the manufacturer saying street-legal approval was not sought or not granted, not a marketing term to ignore.

A build sheet that tracks whether a part has an EO on record for a given vehicle is describing this same check, not asserting a specific ruling; see no CARB EO on record and state rules. Planning emissions-related parts early in a build, rather than after other modifications are already installed, also makes it easier to back out a single part if its EO status turns out not to match the car, rather than untangling it from a finished build.

Confirm the EO number covers the exact part and vehicle application, not just the part in general
Check current requirements for the state of registration and use, since this varies and changes over time
Treat off-road use only and race use only labels as the manufacturer's own statement that street approval was not obtained
Remember the federal tampering prohibition applies nationwide independent of any state's specific rules
Recheck EO status periodically for a used or older part, since an EO can be modified or withdrawn after issue

Common part categories and how they generally relate to CARB EO requirements
Part category	Generally emissions-relevant?	Why
Intake ahead of emissions sensors	Often yes	Can change airflow readings the emissions controls rely on
Exhaust before the catalytic converter	Often yes	Sits within the emissions control path
Catalytic converter or replacement	Yes	Directly performs emissions control
Turbo or supercharger kit	Often yes	Changes engine operating conditions the emissions tune assumes
Tune or ECU modification	Often yes, if it changes emissions behavior	Can alter how emissions controls and monitors operate
Axle-back exhaust behind the catalytic converter	Often less likely, but not exempt by default	Sits downstream of the main emissions control equipment
Suspension, wheels, brakes, interior	Generally no	Doesn't touch emissions equipment, though other state rules may still apply

Questions people ask

What does a CARB EO number mean?: An Executive Order (EO) is CARB's confirmation that a specific aftermarket part, for a specific vehicle application, has been evaluated and does not increase emissions beyond what is allowed. The format looks like D-123-45.
Which parts need a CARB EO?: Generally parts that touch the emissions system: intake systems ahead of emissions sensors, exhaust before and including the catalytic converter, catalytic converters, turbo and supercharger kits, and tunes or ECU changes that affect emissions behavior.
Does every state require CARB EO compliance?: No, this varies by state and can change over time. Some states adopt California's emissions standards for at least some parts and some do not; check current requirements for the specific state a car is registered and driven in rather than assuming.
Is it legal to run a part labeled off-road use only on the street?: Generally no, in a state that requires emissions approval for that type of part. That label is the manufacturer stating the part was not submitted for, or did not receive, street-use emissions approval.
How do I check if a part has a CARB EO?: Look for the specific EO number the manufacturer lists for that part and vehicle application, and check it against CARB's own published Executive Order database rather than relying on a general claim alone.
Does the federal Clean Air Act apply even if my state does not follow CARB rules?: Yes. The federal Clean Air Act's tampering prohibition on emissions control equipment applies nationwide regardless of whether a state separately follows California's CARB standards.
What does "50-state legal" generally mean?: It generally means the part carries a CARB EO for the vehicle in question, since California's standards are historically the strictest a part needs to clear. This is a general description of common usage, not a guarantee for any specific part; the specific EO number is what actually confirms coverage.
Can a part lose its CARB EO after I've already bought it?: Yes, an EO can be modified or withdrawn after it's issued. A part bought with a valid EO at the time of purchase doesn't automatically keep that status forever, which is part of why checking CARB's current database, rather than an old listing or receipt, is more reliable for a used or older part.
Am I responsible for emissions-legal compliance if I buy a car that already has modifications?: Generally yes, in the sense that a state's registration and inspection requirements apply to the car as it currently sits, regardless of who installed the parts. Checking for documented EO coverage on any emissions-related modifications is worth doing at purchase, the same way it would be before installing them yourself.
Does an EO cover the part everywhere it's legal to drive, or just in California?: An EO is a California approval, but because other states can adopt California's standards, an EO'd part is often accepted more broadly than a non-EO'd one. Whether a specific state requires it, or accepts it as sufficient, is a state-specific question that can change over time, so check current requirements for the state of registration rather than assuming.