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What is an Oxygen Sensor and How Can It Go Bad?

Updated: Aug 5, 2021

As if you didn’t have enough car problems! Now the check engine light comes on again and this time your Maryland mechanic tells you it’s a bad oxygen sensor. The truth is, an oxygen sensor (O2 sensor) is a fairly common trigger of check engine lights.

The O2 sensor is a wear and tear part in your vehicle, subject to extreme heat. There are numerous reasons why it may go bad, but there is usually only one solution—replacing it. If you are interested in learning more about oxygen sensors or simply want to educate yourself on the subject before calling an auto repair shop for a quote, this article is for you.

What Is An Oxygen Sensor?

An oxygen sensor is a small device located in your vehicle’s exhaust system. Its shape and size resemble that of a spark plug. Depending on its placement in regard to the catalytic converter, it can be located upstream (before the converter) or downstream (after the converter).

Most vehicles manufactured after 1990 have both upstream and downstream oxygen sensors. And dual-exhaust vehicles have a total of 4 O2 sensors.

What Does An Oxygen Sensor Do?

The role of the oxygen sensor is to monitor the amount of oxygen in the exhaust. This is unburnt oxygen that was initially injected into the fuel for proper combustion. Through a voltage signal, the O2 sensor communicates to the car’s computer how much oxygen is in the exhaust, so that the computer can adjust the fuel/oxygen mixture delivered to the engine.

The sensor placement before and after the catalytic converter allows it to keep track of the cleanliness of the exhaust, as well as monitor the efficiency of the converter.

How Does An Oxygen Sensor Go Bad?

The oxygen sensor in modern cars can last up to 100K miles, but typically you would experience problems sooner than that.

Over time, an oxygen sensor may become caked with byproducts of combustion, such as sulfur, lead, fuel additives, oil ash, etc. This contamination causes the sensor to lose its ability to produce voltage and send the right signal.

Using fuel that is not recommended for your vehicle or using low-quality gasoline may also speed up the oxygen sensor failure. And if you are skipping maintenance, especially things like timely spark plug and air filter replacement, you are increasing the likelihood of incomplete fuel combustion, which in turn leads to more dirt and grime in your emissions system.

What Are The Signs Of A Bad Oxygen Sensor?

In most cases, a bad oxygen sensor will trigger a check engine light. P0138 and P0135 are some of the codes you may expect to see on the OBD II reader, if you have one. Other than that, it’s difficult to spot a failing oxygen sensor.

It will inevitably lead to decreased gas mileage, but it’s usually not drastic enough for an average driver to notice. The decrease is gradual and happens over time, so unless you are keeping tabs on your MPGs, you will likely miss these signs. A bad or failing O2 sensor can also cause you to fail your emissions test.

How Much Does It Cost To Replace An Oxygen Sensor?

It will typically cost between $200 and $400 dollars to replace a single oxygen sensor. Most of it is the cost of the sensor itself, which can run anywhere from $30 to $200 depending on the year, make and model of your car. The cost of labor will depend on how accessible the sensor is in your specific vehicle.

In some cases, a bad oxygen sensor may indicate other problems, such as a failing catalytic converter, in which case the repair can get expensive.

Replacing a Bad Oxygen Sensor

Replacing an oxygen sensor is a relatively simple process, but it’s not a DIY job unless you have a lift and have done auto maintenance before. The sensor is located on the underside of your vehicle and may be hard to reach depending on how your vehicle was built. Besides, you would need to have an OBD II reader to know exactly which sensor to replace. If this sounds like too much trouble, just bring your car to one of the four Hillmuth Certified Automotive locations in Clarksville, Columbia, Glenwood or Gaithersburg.


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