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Smog Equipment Requirements for Used Vehicles (A Guide for the Used Car Buyer)


An Overview for the Used Car Buyer

When a used vehicle is sold in California, the seller is legally responsible for making sure that the vehicle's emission (smog) control equipment complies with state requirements. There is no provision in the law allowing a seller to sell a vehicle "as is." The buyer must submit the certificate of compliance (smog certificate) to the Department of Motor Vehicles (DMV) when transferring registration of the vehicle. Without the smog certificate, the transfer of ownership cannot be completed.

Are any cars exempt?

Vehicles with two-cycle engines, engines under 50 cubic inch displacement and vehicles powered by electricity are exempt. All 1973 and older model year vehicles are not required to get a smog inspection under any circumstances

Who pays for the Smog Check?

In private party transactions, the seller must provide the buyer with a proof of smog certification prior to the transfer of ownership. If the vehicle is purchased from a dealer, the passing Vehicle Inspection Report should be provided as part of the transfer documents. It is evidence that the vehicle meets state requirements for smog equipment. A vehicle that has not been issued a smog certification cannot be registered.

What if a waiver was issued?

The repair cost waiver allows a vehicle needing a biennial inspection to be registered without passing its Smog Check inspection, as long as the vehicle owner first makes some emissions-related repairs to the vehicle at a licensed repair station. Repair cost waivers cannot be issued on change-of-ownership transactions.

What about missing parts?

A Limited Parts Exemption may be issued if an emission control part was not available at the time of inspection. Although a limited parts exemption may be issued on a change-of-ownership smog inspection, the vehicle must pass all other aspects of the Smog Check. Applicants for a limited parts exemption must utilize BAR's Parts Locator Service and Referee Centers.

Restoring tampered emission control equipment could cost you more than you paid for the vehicle. Your strongest protection is to buy only those vehicles for which the seller provides proof of a current Smog Check, as required by law. A Smog Check is good for 90 days after the test date. A vehicle having a smog check in conjunction with an annual registration need not have a smog check again if the sale takes place within 60 days of the registration renewal date.

Better safe than sorry

This document will help you recognize potential problems before it's too late. Before you agree to buy a vehicle, verify that:

  1. The vehicle's identification number matches the registration documents.
  2. The seller is actually the owner or authorized representative of the owner of the vehicle.
  3. The vehicle conforms to U.S. and/or California emission control standards.
  4. The vehicle has the smog equipment required by law.

1.) Verify that the vehicle's identification number matches the registration documents.

Identify the vehicle

Check the vehicle's registration or owner's certificate. Make sure the vehicle's identification number shown on the registration documents matches the actual identification number on the vehicle. The number is usually located on the vehicle's doorpost or on the driver's side of the dashboard viewed through the windshield from the outside.

2.) Verify that the seller is actually the owner or authorized representative of the owner of the vehicle.

Identify the owner/seller

Ask to see the owner/seller's driver's license and the vehicle registration certificate. If the person selling the vehicle is not listed on the registration certificate, the vehicle may have changed owner several times without being properly registered.

[STOP]This may be an indication that someone has tampered with the smog equipment. The owner may be attempting to sell the car rather than complete the smog inspection and registration process.

3.) Verify that the vehicle conforms to U.S. and/or California emission control standards.

Check for conformity

The vehicle must meet either U.S. Environmental Protection Agency (EPA) or California standards. It is legal in California to buy or sell a used vehicle that meets only federal standards.

Look for an underhood label

1971 and newer vehicles have label showing that they were manufactured to meet either California or U.S. standards. The label is affixed in the engine compartment or in a readily accessible location. At the top of the label are the words VEHICLE EMISSION CONTROL INFORMATION and the corporate name or trademark of the manufacturer.

Gray market (direct import) vehicles not originally designed to meet U.S. specifications may require major modifications and must be certified by the Air Resources Board before registration in California.

 [STOP]If you encounter any vehicle without a label indicating that it meets either U.S. or California standards, watch out. Beware of unsubstantiated claims that the vehicle meets the standards or that it can be easily equipped with pollution control devices.

4.) Verify that the vehicle has the smog equipment required by law.

Check for required emission control equipment

To be sure that all required emission control equipment is actually on the vehicle, have it checked by a licensed Smog check technician.

Abbreviation of major emission control systems

These systems are identified on the underhood label by the following abbreviations:


Air injection reactor


Air injection valve (pulse air injection)


Catalytic Converter


Closed crankcase ventilation


Closed loop


Diesel injection


Electronic fuel injection


Exhaust gas recirculation


Exhaust gas sensor (oxygen sensor)


Engine modification (includes systems with oxidation catalysts)


Fuel injection (gasoline)


Heated oxygen sensor


Oxidation catalyst


Oxygen sensor


Pulse air injection reactor


Port fuel injection


Thermal reactor


Three-way catalyst system (includes systems combined with oxidation catalyst)

Smog control equipment requirements

All vehicles in the Smog Check Program (1976 and newer) were originally equipped with crankcase and exhaust emission controls.

Computer Control

The Computer Control System

Computerized engine controls were introduced as early as 1977 and by the early 80's most vehicles had such systems. The computer controls the air/fuel mixture, ignition timing, and various emission devices. Some of the major components are the oxygen (O2) sensor, temperature sensor, mixture control solenoid, and the microprocessor (computer).

Tampered computer control system

[STOP]If a system has been tampered with, you may find the mixture control solenoid disconnected or removed, a non-computer control carburetor installed, or the O2 sensor disconnected. Correcting a tampered computer control system could be very costly.

Air Injection Reaction

Air Injection Reaction (AIR)

The air injection reaction system is used on almost all late model vehicles and may be found on some vehicles as early as 1966.

The air injection reaction system is designed to supply more air to the exhaust manifold and to actually burn the unburned portion of the exhaust gases.

Engine view, air injection reaction system picture

Pump Type Air Injection Reaction System (PAIR)

In a pump type air injection reaction system that is intact, the belt is on the pump, the hoses and lines are installed, and no abnormal noise is emitted during operation.

Exhaust pulse air injection reaction system

Some late model vehicles with air injection systems use exhaust pulse air injection. Such vehicles do not have air pumps. These systems may be identified by the terms Pulse Air Injection Reaction (PAIR), Aspirator, Thermactor 11, Suction and Reed Air Injection.

A small number of vehicles are equipped with electronically driven air pumps, and do not use a drive belt.

Tampered air injection reaction system

[STOP]In a system that has been tampered with, you may find that the air pump, distribution hoses, valves, and manifold(s) have been removed and that the manifold holes have been plugged with pipe plugs. Replacement of any of these items could be costly.

Exhaust Gas Recirculation

The Exhaust Gas Recirculation (EGR) system

The EGR system reduces oxides of nitrogen (NOx) emissions by recirculating controlled amounts of exhaust gases into the air/fuel mixture before they enter the combustion chamber.

Tampered EGR system

[STOP]Check to see whether the EGR valve has been removed, damaged or plugged. Most EGR tampering is done by plugging the EGR signal line. A plugged line can usually be repaired quickly. However, prolonged plugging of the EGR system can create expensive engine damage.

The Catalytic Converter

The catalytic converter system

Most 1975 and later vehicles are originally equipped with a catalytic converter system.

After exhaust gases leave the engine, they react with material in the catalytic converter, where secondary burning (oxidizing) continues.

Tampered catalytic converter system

[STOP]Restoring a tampered catalytic converter system can be an expensive job.

Check to see whether the converter has been removed. Don't mistake the muffler or resonator for the converter.

If the converter has been removed, if a bypass tube has been installed, or if the inside of the unit has been gutted, a replacement converter will have to be installed.

Tampered fuel inlet

[STOP]The converter requires use of unleaded gasoline. Check the fuel filler inlet (gas tank opening) for damage or enlargement. A tampered fuel inlet is a good indication of fuel switching.

Use of leaded gasoline in a vehicle may also damage other critical components of the vehicle, such as the oxygen sensor.

Crankcase Ventilation

The Positive Crankcase Ventilation (PCV) system

Blow-by gases collect in the crankcase when a small amount of unburned gases seep past the piston rings during combustion.

Early gasoline engines vented air polluting blow-by gases into the atmosphere. The PCV system vents blow-by gases back into the combustion chamber for burning.

Blow-by gases must be removed from the crankcase to reduce engine wear and lessen pollution.

A spring-loaded valve controls the PCV system. Engine vacuum draws blow-by gases from the crankcase, through the control valve and into the combustion chamber. Filtered air replaces blow-by gases removed from the crankcase. The control valve and filter are interconnected with hoses.

Tampered PCV system

[STOP]Look for disconnected hoses, plugged vacuum hose, or a vented oil filler cap.

Evaporative Emission Control

The Evaporative Emission Control (EVAP) system

The EVAP system is designed to contain and store these harmful vapors. It operates continually while the vehicle is being driven and while parked. The EVAP system contains a sealed fuel tank, nonvented gas cap, liquid-vapor separator, vent lines, and the vapor storage canister. When fuel evaporates within the gas tank, vapors flow through the lines to a storage canister located in the engine compartment. Vapors are purged from the canister to the intake burned while the engine is running. Although all systems work on the same principle, the location of the canister does vary. Non-factory auxiliary fuel tanks must be approved by the California Air Resources Board before installation.

Tampered EEC system

[STOP]It is unlikely that anyone would intentionally remove or modify the components of the EVAP, but look for disconnected lines in the engine compartment due to use of a missing canister. Check the gas cap for proper fit and nonvented type.

Thermostatic Air Cleaner

The thermostatic air cleaner system (TAC)

Most carbureted and some fuel-injected engines preheat the intake air to improve driveability and reduce emissions during cold start-up. Preheated air is routed through the inlet snorkel using a vacuum-operated damper.

A temperature sensor within the air cleaner housing regulates the damper. When the engine reaches operating temperature, the damper blocks the hot air duct and allows cooler air through the snorkel.

Tampered TAC system

[STOP]Look for a missing or torn air duct between the snorkel and the heat stove. Vacuum hoses must be present and connected. Turning the air cleaner lid upside down defeats the purpose of a TAC and is deemed a modification.

Buying a modified Vehicle

[STOP]If a vehicle has been highly modified or equipped with special manifold and carburetion "speed parts," beware of tampering.

If you are in doubt about the condition of the major emission control components or aftermarket modifications, have the vehicle inspected by a licensed Smog Check technician before you buy the car.

Remember--it is the seller's legal responsibility to make sure that the required emissions control systems are on the vehicle and functioning properly and to provide you with a Smog Certificate.

The California Department of Consumer Affairs' Bureau of Automotive Repair

The Bureau of Automotive Repair (BAR) is the branch of the California Department of Consumer Affairs that regulates the auto repair industry. BAR provides consumer information and assistance in the areas of auto repair and automotive smog control.

All California auto repair shops must be registered with BAR. To do smog inspections, shops and technicians must have special licenses issued by BAR.

If you have questions about smog control equipment, you can contact a state-licensed Smog Check station. Look in the yellow pages of your telephone book or search the database of active Smog Check stations on this Web site.

BAR field offices are open between 8 a.m. and 5 p.m Monday through Friday (except state holidays).

Or call toll-free 1-800-952-5210.


  • Does the vehicle's identification number match the registration documents?

  • Is the person selling the vehicle the registered owner?

  • Is the vehicle certified for sale in California or U.S. or is it an imported nonconforming vehicle?

  • Are major emission control components installed, as listed on the underhood label:

    • Air injection reaction system

    • Catalytic converter

    • Exhaust gas recirculation system

    • Thermostatic air cleaner

    • Evaporative emission control

    • Crankcase ventilation system

    • Computer control system

  • Do any aftermarket modifications render the vehicle uncertifiable?

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