Why are oil catch cans illegal?

Oil catch cans are not illegal in every situation, but using them on public roads is often prohibited because they can bypass or defeat a vehicle’s emission-control systems.

The following report explains what catch cans do, why regulators treat them as potential tampering, and how legality differs by country and region. It also outlines practical considerations for drivers who are curious about installing one.

What catch cans do and why regulators care

At their core, a catch can sits in the crankcase ventilation (PCV) line to capture oil vapor and condensate before it re-enters the engine intake. When used properly, some designs are intended to reduce intake oil buildup. When they vent to the atmosphere or alter the PCV pathway, they can change how emissions are produced and tested, which regulators scrutinize closely.

Key regulatory concepts

Before examining regional differences, it helps to understand the main issues regulators consider when evaluating catch cans:

• Defeating or bypassing the PCV/emissions-control system by venting blow-by gases and oil vapor to the atmosphere.


• Allowing higher hydrocarbon or pollutant emissions than the vehicle’s certification allows.


• Interfering with onboard diagnostics (OBD) or other emissions-monitoring components during inspections.


• Potentially voiding factory warranties or violating manufacturer guidelines by altering critical emission hardware.

The practical effect is that many catch cans designed for street use are treated as illegal or improper because they can enable emissions increases or bypass the tested emissions system.

Regional snapshots: how legality varies around the world

The rules governing catch cans differ by country and, in some cases, by state or province. Here is a concise snapshot of common regimes and what they typically require for street use.

• United States (federal perspective): The Environmental Protection Agency (EPA) prohibits devices that defeat or circumvent the vehicle’s emissions-control system. On-road use of a catch can that vents to the atmosphere is generally illegal. Some configurations that maintain the PCV flow back into the intake may be allowed in limited contexts, but many are not compliant with federal rules or inspection regimes. California’s CARB standards are typically stricter and aligned with, or stricter than, federal rules.


• California (CARB region): Emission-control integrity is tightly enforced. Modifications that bypass or disable PCV or other controls are typically not permitted for street use and can fail smog tests and inspections. Track or off-road use may be permitted under specific conditions.


• Canada and other North American jurisdictions: Regulations generally mirror the spirit of the U.S. stance—tampering with emission controls is discouraged or prohibited for street use, with off-road or sanctioned-event use sometimes allowed. Always check provincial or territorial rules and inspection regimes.


• United Kingdom and the European Union: Vehicle type-approval and ongoing compliance require that emission-control systems function as designed. Tampering with PCV or related devices to vent hydrocarbons can lead to MOT failures or penalties, unless the device remains fully compliant or is used only in permitted contexts (e.g., track use or special exemptions).

Because laws and enforcement practices can change, drivers should verify current rules with local environmental agencies, road traffic authorities, and vehicle inspection programs before installing a catch can on a street-driven vehicle.

What to consider if you still want a catch can

If you’re weighing a catch can, keep in mind that legality hinges on whether the device affects emissions and how it’s installed. The guidance below is meant to help you assess risk and stay compliant where possible.

1. Check your jurisdiction's rules on emissions-control modifications and defeat devices, and verify how inspections or tests (smog checks, MOT) view catch cans.


2. Choose a configuration that minimizes the risk of emissions defeat, such as designs that maintain the PCV pathway without venting hydrocarbons to the atmosphere. Some drivers opt for setups that keep the PCV system intact and route any condensate back into the intake, if allowed by local rules.


3. Have the installation performed by a qualified technician who understands emissions regulations and vehicle diagnostics to avoid creating leak paths, sensor disturbances, or other unintended issues.


4. Keep documentation from the device manufacturer about compliance, recommended usage, and any disclaimers about road use. Be prepared to provide proof of compliance if required during inspections.

Even with careful choices, you should expect that many catch-can configurations are not street-legal in strict jurisdictions. If your intent is to maximize engine cleanliness without risking regulatory trouble, you may prefer to invest in a car with a well-maintained PCV system and established service intervals for oil management, or to use a device only in sanctioned off-road or track contexts where permitted.

Summary

Oil catch cans sit at the intersection of vehicle maintenance and environmental law. While they can be beneficial for reducing oil vapor entering the intake, many catch-can configurations are treated as defeat devices under emission regulations when they vent gases to the atmosphere or bypass the PCV system. Legality varies widely: in the United States, federal rules target defeat devices, with California and other regions enforcing even stricter standards; in Europe and the UK, emissions compliance and type approvals restrict such devices for road use. If you’re considering a catch can, research your local rules first, choose a compliant design, and seek professional installation to avoid regulatory or warranty pitfalls. Summary: legality depends on device design and jurisdiction, and for on-road use, many catch cans are not permitted. Off-road or track use may offer permitted exceptions.

Why are oil catch cans banned?

why you should not install oil catch can and your car ( 1 ) Although a catch can could help your car's engine run well for a long time, modifying the PCV system is illegal since it's part of the engine's emissions system.

Why don't manufacturers install oil catch cans?

The reason catch cans are not fitted to passenger vehicles is purely from a user experience. Most people don't even check their oil. So they would never empty their catch can. This would lead to a lot of issues when people start having problems and try to claim warranty.

Are oil catch cans bad for your engine?

An oil catch can is not inherently bad for an engine; however, a poorly designed, incorrectly installed, or poorly maintained catch can can cause problems. Issues arise when a catch can becomes clogged or is not emptied, which can lead to pressure buildup in the crankcase, potentially causing oil leaks or other engine damage. When installed correctly and maintained regularly by being emptied, an oil catch can can protect the engine by preventing oil vapors from entering the intake system, which keeps the intake and throttle plates cleaner.
 
This video explains how a clogged oil catch can cause engine problems: 59sAussie ArvosYouTube · Oct 1, 2022
Potential issues with oil catch cans 
This video demonstrates how an oil catch can collects oil and water vapor: 55sRepair GeekYouTube · Sep 22, 2024

• Clogging and pressure buildup: A catch can's filter can become clogged with oil, dirt, and moisture. If it is not emptied regularly, it can become full. This blockage can cause pressure to build up in the crankcase, which can force oil out of seals and gaskets, leading to leaks and potential engine damage. 
• Incorrect installation: Some catch can systems are designed to create a vacuum, which can be detrimental if the system is not correctly plumbed. A poorly designed system can also restrict the engine's Positive Crankcase Ventilation (PCV) system, leading to more issues than it solves. 
• Moisture and freezing: In cold climates, moisture collected in the catch can can freeze and block the system, causing a pressure buildup similar to a clogged filter. 

This video explains why oil catch cans are beneficial: 1mEngineering ExplainedYouTube · Apr 17, 2019
Benefits of a correctly functioning oil catch can

• Cleaner intake: It traps oil vapors that would otherwise get into the intake manifold, preventing carbon buildup on the intake valves and throttle body. 
• Reduced knock: By preventing oil from entering the intake charge, it helps maintain a cleaner fuel-air mix, which can reduce the risk of engine knock (detonation). 
• Improved engine longevity: By keeping contaminants out of the intake and combustion chamber, a catch can can help prolong engine life, especially for turbocharged or high-performance engines. 
• Pressure regulation: A properly functioning catch can can help manage crankcase pressure, which can improve piston ring sealing, horsepower, and fuel economy. 

Are catch cans carb legal?

No, standard aftermarket oil catch cans are not CARB legal because they are modifications to a vehicle's emissions system that have not been evaluated and approved by the California Air Resources Board (CARB). Even sealed cans are illegal if they modify the positive crankcase ventilation (PCV) system without a CARB Executive Order (EO). Some companies are working to get their specific catch can models CARB-exempt, which would require them to have an EO number for legal sale and use in California. 
Why catch cans are not CARB legal

• Modification of emissions system: The PCV system is part of a vehicle's emissions control system, and any modification to it is illegal in California unless the part has a CARB EO number. 
• Lack of an Executive Order (EO): For a part to be legal, it must be tested by CARB and shown to not increase emissions. If it passes, it receives an EO number that can be verified by a smog station. 
• Catch cans remove and/or redirect emissions: Catch cans capture oil that would normally be routed back into the engine for reburning. This is considered a modification of the emissions system, which is why it's illegal without an EO. 
• Products are often marked as "Not for sale in California": Many manufacturers explicitly state that their oil catch cans are not legal for use in California or other states that adopt California's emissions standards. 

Exceptions and considerations

• Sealed vs. vented: Some argue that "sealed" systems are more legal, but even a sealed system that modifies the factory PCV setup is likely illegal without a CARB EO. 
• Future availability: Some companies are seeking CARB approval for specific products, so it's possible that some models could become legal in the future if they receive an EO. 
• Checking for legality: If you are in California and have a catch can, you should verify that it has a specific CARB EO number to ensure it is legal for your vehicle.