When Should the Oxygen Sensor Be Replaced?

Key Points

• Oxygen sensors detect the amount of oxygen in the exhaust.
• Oxygen sensors usually don’t have a replacement schedule.
• A bad oxygen sensor can be diagnosed with a code reader or scan tool.

A car’s oxygen sensor is typically only replaced when it fails, so there is usually no time or mileage interval for replacement on the maintenance schedule. Some sources say they might need replacement in as little as 30,000 miles, while others claim they could last the life of the vehicle.

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Sometimes referred to as an O2 sensor (pronounced “oh-two,” the chemical formula for oxygen), oxygen sensors are typically about the size of an index finger and look a little like spark plugs. Older cars might have just one, but most modern cars have at least two and sometimes as many as four. The O2 sensors screw into holes in the engine’s exhaust manifold or exhaust pipe, which allows their tips to “sniff” the exhaust as it blows by. One is usually placed in the exhaust system before the catalytic converter, another after the converter, and there may be a set for each cylinder bank of a V-6 or V-8 engine.

What an Oxygen Sensor Does

• Takeaway: An oxygen sensor fine-tunes fuel delivery by telling the engine computer how much oxygen is in the exhaust.

The purpose of an oxygen sensor is to detect the amount of oxygen in the exhaust. Fuel needs oxygen from the air to burn, and the amount of oxygen left in the exhaust indicates whether the engine is running too rich (too much fuel), too lean (too little fuel) or just right. The amount of oxygen the sensor detects is fed to the engine computer, which then adjusts fuel delivery accordingly. In this way, the oxygen sensor helps fine-tune the fuel mixture to benefit fuel economy and exhaust emissions.

How to Know if an Oxygen Sensor Needs to Be Replaced

• Takeaway: A check-engine light will usually indicate a problem, and a code reader will pinpoint it as a bad oxygen sensor.

While a bad oxygen sensor can cause an engine misfire or rough running, in many cases, your first indication is that the check-engine light illuminates.

While that light can indicate any number of problems, it means the car’s engine computer has detected one or more issues, and each will usually generate a separate code. The codes can be read by a simple code reader that can usually either pinpoint a problem or at least guide you in the right direction. (You can buy one of these code readers for less than $30, and some parts stores will run a test for free.) Repair shops usually have a more sophisticated (and more expensive) version called a scan tool that is even more capable and precise.

If an oxygen sensor is faulty, it will likely trigger a code that will not only indicate a failure, but also which one of the sensors is bad.

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Why an Oxygen Sensor Goes Bad

• Takeaway: Oxygen sensors either wear out over time or go bad due to an engine burning oil or coolant.

Often, an oxygen sensor just wears out. After all, it lives its life being subjected to very hot exhaust gases whenever the engine is running.

But if the engine is burning oil or coolant that can shorten an oxygen sensor’s useful life. Oil can seep past worn piston rings, while coolant can leak into the cylinder through a blown head gasket. Either can coat the tip of the oxygen sensor that is in contact with the exhaust, and that can make the sensor send the wrong reading to the computer. That, in turn, tells the fuel injectors to deliver the wrong amount of fuel.

In some cases — particularly on older cars with just a sensor or two in the exhaust manifold — you may be able to replace the oxygen sensor yourself. But newer cars with sensors in front of and behind the catalytic converter require getting under the car, and that means jacking it up and using jack stands. These sensors are much easier (and safer) to replace with the car raised on a mechanic’s lift.

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