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FAQ - Simple smog check answers

Frequently Asked Questions


Do I really need a smog check?


Not all vehicles must get a Smog Check. Additionally, some vehicles only need a Smog Check when they are being sold or being registered in California after previously being registered in another state. Whether or not a vehicle needs a Smog Check depends on the type of vehicle, the model-year, and the area in which the vehicle is registered.

Some vehicles are exempt from the Smog Check program

All vehicles model year 1975 and older are exempt from all aspects of the Smog Check program, but owners of these vehicles are required by law to keep their emissions systems intact. Vehicles built in 1975 for the 1976 model-year are 1976 vehicles and must be tested accordingly.

Additionally, vehicles with diesel engines, vehicles with two-cycle engines, vehicles with engines smaller than 50 cubic inches of displacement, electric vehicles, and motorcycles are exempt from the Smog Check program.

New vehicles exempt until their sixth year

Vehicles six model-years old and newer (model-years 2005-2010, for registrations due after 1/1/2010 ) are not required to have a biennial (every other year) Smog Check performed until their fifth year. However, these vehicles must have a Smog Check performed if the vehicle is sold, or being registered in California for the first time. Some additional vehicles might not be re quired to have a biennial Smog Check performed if the specifications in the following document are met. Extension of Smog Check Exemptions

How do I know if my vehicle needs a Smog Check?

The Department of Motor Vehicles (DMV) will note on your DMV Renewal Notice whether a Smog Check is required to reregister your vehicle. In addition, an information insert explaining the Smog Check Program requirements should be included in that mailing.

No Smog Check required on sales within immediate family

Section 4000.1 (d)(2) of the California Vehicle Code exempts from the change of ownership Smog Check requirement vehicles being sold or transferred between certain family members, such as parents, grandparents, children, grandchildren, siblings, or spouses.

Current law exempts vehicles 30-model-years old and older from the Smog Check program.



What exactly is smog?*

Smog is formed when nitrogen oxides ( NOx ) and hydrocarbon gases ( HC ) are exposed to sunlight.

NOx is produced when there is to much heat and/or pressure in the combustion chamber.This can be caused by numerous engine conditions or by Smog component failure.

HC is fuel that went through the engine without being burned.This emission also can be caused by various engine conditions or by smog component failure.

There are five gases that are monitored when doing a Smog Check.They are as follows.

HC   =  Hydrocarbons
CO   = Carbon Monoxide
NOx  = Nitrogen Oxides
CO2  = Carbon Dioxide
O2    =  Oxygen  

For Smog Check purposes  HC,CO, NOx are the ones that result in a pass or fail condition. Gases, CO2 and O2 are generally interpreted for diagnostic purposes.

Hycrocarbons (HC)*
A list of common causes of HC related smog check failure:

1. Ignition Timing Advanced above specifications.Timing is measured in degrees. before or after Top dead center. If your vehicle is required to be at 10b degrees and instead is set at 16b degrees your HC emissions will increase. ( b = before top dead center ) . 

2. Ignition system problems. Parts like the ignition coil, distributor cap, distributor rotor, ignition wires, spark plugs.

3. Vacuum leaks, Gasket leaks that cause vacuum leaks. Broken, disconnected or misrouted vacuum hoses.Vacuum component failure such as a power brake booster.This causes a very large vacuum leak. 

4. Catalytic converter failure.  (Avoid using a catalytic converter as a bandage. Fix the source problem and then move to the converter.)

5. Air injection system problems. ( Smog pump and related components )

6. Engine damage (burned valve, low or no compression in one or more cylinders).

7. Sensor problems and or computer problems can cause a Smog Check failure.


Note - when your CO level is high it will usually bring HC along with it.

Carbon Monoxide (CO)*
CO is created when gasoline is not completely burned. High CO ( running rich ). 

A list of common causes of CO related smog check failure:

1. Timing is not a big factor in CO production but check it anyway. Everything helps.  

2. On older Carbureted vehicles you can have problems like a partially stuck choke plate.This is the moving steel plate on the top of your carburetor. 

3. A leaking power valve on your carburetor can cause high CO and Smog Check failure.

4. A faulty carburetor can cause high CO and Smog Check failure. 

5. A dirty air filter can cause high CO. 

6. A common cause for high CO is a faulty Oxygen Sensor. This sensor tells the computer how to fine tune the air fuel ratio.

7. Other sensors like the M.A.P.sensor , C.T.S. sensor or the Air flow meter can also effect the CO level.

8. A faulty air injection system can cause high CO. This is the Smog pump and related components.  

9. A faulty Catalytic Converter can cause high CO and Smog Check failure.

10. If your car has been running rich ( high CO ) you should change your oil after you get it repaired. When you have rich condition all the fuel cannot be burned, you start to saturate the motor oil with CO and HC. This in itself can also cause a rich condition.

Nitrogen Oxides (NOx)*
NOx is formed inside the combustion chamber when excessive heat is present.

A list of common causes of NOx related smog check failure:

1. As usual check the timing first. Advanced timing can cause extra NOx.

2. The next thing to check is the EGR system.This system is designed to reduce Nox. It consists of a EGR valve, vacuum hoses, one or more vacuum switching valves or solenoids. Its job is to reroute a small amount of exhaust gas back into the engine to help reduce combustion chamber temperature. Not all vehicles have an EGR system.

3. Next thing to check is the air / fuel ratio. If the vehicle is running to lean, NOx emissions will increase.

4. Some possibilities are a restricted fuel filter, low fuel pressure, vacuum leaks, oxygen sensor, load sensor such as a map sensor, air flow meter.

5. Check the cooling system. An extra increase in water temperature will increase NOx production.

6. A defective catalytic converter can also increase NOx. The Cat. reduces NOx that has already been produced.

7. Check to see that the air coming into the engine is not over heated. Some vehicles have a vacuum controlled air valve which switches the incoming air to hot air from the outside of your exhaust manifold.This should only happen when the engine is cold. If this system malfunctions and sends hot air all the time.Your Nox could go up. This system is called  T.A.C.  ( Thermostatic air cleaner )

8. There is another system similar to the one above called  E.F.E. ( Early fuel evaporation ). This system routes hot air under the intake manifold. This helps keep the fuel in the vapor state. If stuck in the hot mode Nox emissions could increase and cause a Smog Check failure.      

*Data obtained from www.smogchecksrus.com



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