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Cars are equipped with many sensors that provide feedback to the onboard computer for optimal engine efficiency. There are sensors that read tire pressure, fuel level, oil pressure, and engine temperature. An O2 sensor detects oxygen in vehicle exhaust and other locations, which is an indication of the engine’s efficiency.

What Does an O2 (oxygen) Sensor Do?

What Does an O2 (oxygen) Sensor Do?

Cars now have sophisticated onboard computers with a series of sensors throughout the engine that monitor the vehicle’s systems. That includes a sensor for gasses like O2 in the tailpipe, which detects the level of oxygen in exhaust and can adjust the fuel mixture in the engine by communicating with the onboard computer.

Replacing oxygen sensors is not a common maintenance item like changing the oil or getting new tires. Sensors can last for years, and the only way to know if they’re not working well is if your vehicle shows signs of malfunction. If a O2 sensor is failing, you may experience one of the following symptoms:

  • the engine idling unevenly 
  • misfiring 
  • an unexpected increase in fuel consumption (lower gas mileage)
  • stalling

If the sensor is failing, your check engine light should be illuminated on the dashboard. A mechanic or other qualified person using an OBD tool should be able to diagnose the issue by reading messages sent by the onboard computer. Experts say that replacement of an oxygen sensor costs between $200 and $500.

Where is The O2 Sensor Located?

The Oxygen (O2) sensor is typically located in the vehicle's exhaust system. There are usually at least two O2 sensors in modern vehicles. 

Some vehicles may have additional O2 sensors, especially those with dual exhaust systems, where each exhaust path is monitored separately. The exact location can vary depending on the make, model, and year of the vehicle, but they are generally found screwed into the exhaust manifold, the exhaust pipes, or the catalytic converter.

Why Do We Have Oxygen Sensors?

Sensors that read a vehicle’s O2 emissions are a result of the Clean Air Act, a law passed around 1970. The Clean Air Act sought to reduce pollution that clouded the skies over many cities, caused illnesses like asthma, and even negatively impacted wildlife and farming.

Those who understood the cause of the air pollution had the foresight to take action by finding ways to make internal combustion engines more efficient, which would reduce motor vehicle exhaust. Exhaust was loaded with noxious gasses and heavy metals like lead. A key component of exhaust then was nitrogen oxide, which is 265 times more potent than carbon dioxide in contributing to global warming. 

In 1960 there were 61 million private vehicles registered in the country, and that number was clearly going to increase with population growth and the push to develop interstate highways. By 1970 there were 118 million private vehicles in the country. Now there are over 283 million registered vehicles in the country.

How Was Smog Addressed?

California was the first state to enact local laws about air pollution, in 1946. The state created regional boards with authority to regulate polluting businesses. The federal Clean Air Act was passed in 1970 and included the creation of the Environmental Protection Agency as the administrative agency for that and other conservation laws, like the Clean Water Act.

California’s geography and climate made the southern half of the state particularly prone to the effects of air pollution. The mountains around Los Angeles trapped car exhaust and industrial pollution over the city. When exposed to sunlight many of the gasses became particularly harmful, causing cardiovascular disease, breathing difficulty, and lost productivity. In addition, Los Angeles and the entire Southern California region earned a terrible reputation for filth.

But California wasn’t the only state experiencing poor air quality. The Mid-Atlantic states of Ohio, Illinois, and Indiana had factories and power plants that spread air pollution eastward, and much of that pollution descended from the skies in the form of “acid rain” over New England in the 1970s and 1980s. Better regulation of industries, combined with technology that reduced smokestack emissions, eventually reduced the issue.

For automobile emissions, gasoline was also reformulated to remove lead and introduce additives that would burn cleaner, with fewer harmful pollutants.

Years after the smog was erased from the skies over Los Angeles, California has continued to make progress on reducing vehicle emissions. State laws mandate that increasing proportions of autos sold in the state are plug-in electric vehicles, hydrogen-powered, or hybrids, so that by 2035 no gasoline-powered vehicles will be sold as new cars. Several other states are following their lead on the Advanced Clean Cars II Rule, including:

  • Massachusetts
  • Connecticut
  • Maryland
  • New York
  • New Jersey
  • Rhode Island
  • Washington

In addition, Vermont plans to implement the zero emissions goal by 2030.

Emissions Testing

In 1977 California was the first state to require tailpipe emissions standards. This means that all private vehicles were required to be tested for smog-creating emissions. To do this, a sensor was hooked up to the vehicle’s exhaust pipe during mandatory inspections and all had to meet a threshold limit of gasses. This process removed many older, less efficient vehicles from the roads

Since the Clean Air Act, the federal government has required emissions testing for motor vehicles. It allows states to create their own standards or to copy California’s. Those using California standards include: New Mexico, New Jersey, Delaware, Maine, Maryland, Connecticut, Colorado, New York, Oregon, Pennsylvania, Rhode Island, Washington D.C., Virginia, Vermont, and Washington. 

Oxygen sensors have been required in new cars since 1981, and cars built after 1996 are likely to have several, including after the catalytic converter. Catalytic converters are part of the exhaust manifold that burns up harmful gasses before they’re released via the tailpipe. An O2 sensor here determines how well the converter is working, and whether that part needs replacement.

What Gases Cause Smog?

The primary gas that causes smog is nitrogen dioxide (NO2). When combined with airborne particulates, which are tiny pieces of dust, as well as pollen and smoke from factories or coal burning plants, smog takes on a thick, brownish color as it hangs above a city like a cloud.

Many gasses found in smog can be reduced by requiring more efficient burning of fuels, or by using “scrubbers” in smokestacks to capture the particles.

Sensors are integral components of modern engines and significant in the battle against smog and greenhouse gasses. These small parts monitor the efficiency of internal combustion engines, adjusting the fuel-oxygen mix for optimal functioning.

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