What Do Oxygen Sensors Do, and Why Are They Important?

Closeup on car engine parts

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We all hate seeing that little orange light come on behind our steering wheel — you know, the one that says, “Check Engine.” It gets even more annoying when you pull the codes and they indicate one or more of your oxygen sensors has failed. What are oxygen sensors, anyway? Let’s take a closer look at what oxygen sensors are, what they do and why it is important to replace them if they fail.

What Are Oxygen Sensors?

First, what are oxygen sensors?

Oxygen sensors, as their name suggests, are designed to sense oxygen — in this case, in your car’s exhaust. Depending on the year of your car and the design of your exhaust system, you will have between one and four of these devices installed on your exhaust, both before and after the catalytic converter. Upstream oxygen sensors are installed before the converter, while downstream sensors are installed after.

Cars that were manufactured before 1990 may only have one oxygen sensor installed. Those manufactured later will have at least two — one upstream and one downstream — and cars or trucks with dual exhaust will have four. These sensors are specially designed to be able to withstand the high heat of car exhaust.

What Do Oxygen Sensors Do?

If you look under your car or truck, you’ll probably see these sparkplug-shaped sensors screwed directly into your exhaust pipes, but what does an oxygen sensor do? To explain that, we have to go all the way back into the engine.

To create combustion, a mixture of fuel and air is injected into each cylinder. This mixture is optimized to create the perfect miniature explosion, allowing your car to move forward. Ideally, the majority of both the fuel and the oxygen should be burned away, leaving just carbon dioxide and other exhaust gases.

If the oxygen sensor detects a higher level of oxygen than is normal in your exhaust, it can send a signal to the engine control module (ECM) to adjust the fuel-air mixture so it burns more efficiently.

How to Tell If Your O2 Sensor Has Failed

What are the signs your oxygen sensor has failed or is not working as efficiently as it could be?

The most obvious sign is that your “check engine” light will come on. If your car was built after 1996, it should be equipped with an OBDII sensor port somewhere under the driver’s side dash — just drive to your local auto parts store or repair shop and have them pull the codes for you. These codes will tell you which oxygen sensors you need to replace. The code will even tell you which one has failed — for example, Bank 1, Sensor 1 in a truck with dual exhaust will indicate the upstream sensor on the driver’s side.

One less obvious symptom is that your car will start having poor gas mileage, due to the fact the engine is no longer burning fuel efficiently. The ECM is basically guessing to create a fuel-air mixture because it’s getting no feedback from the oxygen sensors.

How to Replace Oxygen Sensors

Once you’ve determined which sensor is faulty, the next step is to learn how to replace oxygen sensors. Thankfully, these are pretty simple to replace, though they can be tricky to get to, depending on the design of your vehicle’s exhaust system.

When it comes to tools, all you’ll need is an open-ended box wrench or an oxygen sensor socket —  these sockets have a large slit up the side to accommodate the sensor’s wiring harness.

Locate the sensor and disconnect the wire harness from the plug. Make sure you mark which plug the sensor is connected to — if you swap the plugs, your car will start throwing even more error codes. Once the wiring harness is unplugged, just use your tool of choice to unscrew the oxygen sensor from the exhaust pipe. Discard the old sensor and reverse the steps to put the new one in place.

It’s that simple.

A failed oxygen sensor might not seem like the end of the world, but it can negatively affect your car’s engine in several ways. If you start seeing warning lights that indicate your O2 sensors are failing, don’t wait — replace them as soon as possible.

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