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What is an O2 Sensor? When Should I Replace It?

A car requires gasoline to run, but it also needs oxygen. Your vehicle requires oxygen to create the spark to ignite the engine and burn the gasoline which will create the fuel needed. Most cars require a ratio of 14 grams of oxygen for every gram of gas. How do you know if you have the correct balance to keep your vehicle running efficiently?


Oxygen, or O2, sensor

An oxygen sensor (O2 sensor) is a small, but important part of your vehicle’s emissions systems.

What Is An O2 Sensor?

An O2 sensor is generally mounted in the exhaust stream to measure the oxygen content of the exhaust gases. The sensor will compare the oxygen content to the oxygen proportion in the air and passes that information back to your vehicle’s engine computer (yes, your car has a computer).


Newer vehicles will have multiple sensors to provide the engine computer with two readings and more data.

Why Do I Need to Replace The O2 Sensor?

The oxygen sensors are critical to your vehicle’s emissions, which has an impact on air pollution and smog. This is why most states require your vehicle to have an emissions test at least once a year. Every state has a specific emission standard that your vehicle has to meet to pass the test.


Car oxygen sensors help with emissions

A properly functioning O2 sensor will help your car to not only pass an emissions test but will also help your car’s fuel efficiency.

Signs That You Need to Replace the O2 Sensor

Since O2 sensors are so critical to your car’s system, it’s recommended to replace your sensors every 60,000 to 90,000 miles. There are a few signs that will help you know when it’s time to take your vehicle in to have the sensor replaced.

Check Engine Light Is On

This is the simplest sign. Those warning indicators on your dash are not just pretty pictures, but they tell you when something is wrong with your vehicle. If you see a check engine light, you might want to use your car monitoring system to quickly diagnose issues.

Bad Gas Mileage

Are you filling up your gasoline tank more often than usual? This is a sign that something isn’t working right and your O2 sensor could potentially be one of the causes. Your gas mileage efficiency will decrease over time, so you’ll need to monitor this to recognize the pattern.

Emission Test Failure

Everyone hates failing, especially in a test, but your emissions test is one that could be costly to pay for (especially when you’d have to pay for a second one). Poor oxygen sensors are one of the most common reasons for failure during an emissions test. Make sure to have your sensors checked by a mechanic prior to your first test to save you time and money.

Rotten Egg Smell

You’ll know it as soon as you smell it. This smell is from burning sulfur and is a symptom of damage in the emission system or a problem with the catalytic converter. The oxygen sensor can fail and lead to a poor fuel/air mixture. If there is not the right balance, it can damage your catalytic converter in your exhaust system, leading to the rotten egg smell.

Rough Idling and Stalling

If you notice that your engine is a little jumpy, bumpy, or stalls and starts, you should definitely have your car checked out, and one of the important things to have your mechanic check while they’re looking at everything are your oxygen sensors.


It is a good idea to not only have the O2 sensors checked with the car isn’t performing well, but also when you do your regular maintenance like oil changes. You can find the problems before they have a chance to potentially damage more parts of your vehicle’s engine.

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