Q: How does a smog test work?

asked by on November 18, 2016

how does a smog test work

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

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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.

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.

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