Skip the auto shop - Our mechanics make house calls

Q: How does a smog test work?

asked by on

how does a smog test work

My car has 150000 miles.
My car has an automatic transmission.

A: How a smog test works depends on where you ...

How a smog test works depends on where you live. If you live in the United States different states have different requirements and some parts of states may have as well. Usually the more densely populated areas require more stringent inspections in an effort to control local air pollution. The more lenient form of the test requires a visual inspection to be sure that the emission control devices that the car is supposed to have are in place. Then the car is hooked up to the state’s computer system via the diagnostic link. The vehicle’s diagnostic system can tell the inspection computer if everything is in working order. If the car has any stored trouble codes, or if the codes have been cleared so recently that the car’s active test cycles have not yet run, The car will fail the inspection. The more strict procedure actually involves the use of a dynomometer and a four gas analyzer. The car is run through a series of accelerations and decelerations while the exhaust is sampled and tested. Any excess over the prescribed levels of HC, CO, or NOX will cause the car to fail. If you have any trouble getting your car past your local inspectors, contact Your Mechanic. They can send a technician to your home or office to [check out your car]https://www.yourmechanic.com/services/tune-up) and tell you what you need to pass the test.

Was this answer helpful?

A: Hi there: Smog test regulations and proced...

Hi there:

Smog test regulations and procedures vary based on the specific US state in which your vehicle is registered. It also depends on whether or not the smog or registration inspection in your state requires a basic safety inspection. In general however, the smog inspection is designed to measure a few harmful gases that are created through the combustion process. Smog or particulate matter is created when nitrogen oxides (NOx) and hydrocarbon gases (HC) are exposed to sunlight. The five gasses monitored during a smog check are Hydrocarbons (HC), Carbon Monoxide (CO), Nitrogen Oxides (NOx), Carbon Dioxide (CO2), and Oxygen (O2). NOx is what most people see as "smog" that comes out of a tail pipe. Different states have different levels of passing for each of these emissions. The test typically includes three phases, the start up test, an acceleration test and a post acceleration idle inspection.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Problem with torque converter or camshaft sensor

Having your Volkswagen's code is a great place to start. Reading Error code P0741 or Volkswagen manufacturer code 17125, means that the torque converter is not functioning as it should. The first thing to check is the level of the...

Q: alignment specs?

The tires should not change your alignment unless you altered your suspension system. The tires may put a larger stain on steering components and cause premature wear and stress. The tires being taller will change your speedometer output reading and...

Q: Size for v belt

First of all it is not a V-belt, it would be a multi rib serpentine belt. The belt size can be found by any auto parts store or dealer. The question is why did the belt come off? The belt...

Related articles

How Long Does a Heater Control Valve Last?
Keeping the right amount of coolant in a car is essential in keeping the engine at the right temperature. Failing to have the right amount of coolant or even bad elements...
What are the Car Pool Rules in Hawaii?
Hawaii is widely regarded as a land of vacation and relaxation, and as such, its scenic roads and routes are far better known than the state’s freeways. But, as with all...
How Much Does a Mechanic Make in Vermont?
Automotive technician jobs in Vermont have an average mechanic salary of $37k, with some mechanics earning a salary of $53k.