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When it comes to cars today, the computerized engine control systems within your car rely on a variety of sensors in order to regulate the overall performance of your engine, emissions, and other incredibly important functions and tasks.

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One of the most important key sensing tools in the system is the oxygen sensor. Often, it is referred to simply as the “O2” sensor, simply because O2 is the chemical formula for oxygen. The first O2 sensor was brought into the car world in 1976 when it was used in a Volvo 240. Now, they are prevalent throughout the industry and are considered a necessary part of every modern engine.

This vehicle part is mounted right in the exhaust manifold of your car so that it can track how much unburned oxygen is in the exhaust of your car as the exhaust exits the engine. Essentially, this system is used to gauge the fuel mixture and it tells the car’s computer whether the fuel is burning less oxygen (rich) or more oxygen (lean).

Let’s go over some of the basics of oxygen sensors. Of course, if you want to learn more about it, you can click here.

Feedback control loops

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One of the most useful things to know about your sensing O2 equipment is how it actually works. The computer of your car uses this device’s input so that it can regulate the fuel mixture. This is referred to as the feedback control loop. The computer takes information and cues from the O2 and responds in kind by changing the fuel mixture.

This then produces subsequent changes in the O2 sensor reading. This is referred to as a “closed loop” operation due to the fact that the computer is using information it has gotten from the device to regulate the fuel mixture. The result of this is a back and forth between running rich to lean, which in turn allows the catalytic converter to operate at its most efficient while also keeping the proper fuel mixture in order to minimize emissions.

If you are receiving no signal from the equipment, which typically either occurs when the engine has just started up and is still cold, or when the sensor is damaged or failing, the computer will then order an unchanging rich mixture of fuel. This is called an “open loop” operation because zero input from the sensor is being used to regulate the fuel mixture. Ultimately, an open loop operation leads to an increased fuel consumption and emissions as your fuel continues to run rich.

How your O2 sensor works

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To put it simply, our sensing element operates like a little generator and produces its over voltage when it heats up. Within the vented cover on the end of the sensor that is connected to your car’s exhaust manifold is a zirconium bulb. Within that bold are two platinum strips that operate as contacts or electrodes.

The exterior of that bulb is exposed to the gases that are funneling in the exhaust while the inside is vented internally through the body of the sensing device to the outside atmosphere. While these devices used to actually have holes in them to complete the same process, the vents were an innovation that reduced the risk of dirt or water contamination getting in that could negatively impact the part from the inside and cause it to fail.

The difference between the oxygen levels inside the exhaust compared to the outside air within the device is actually what causes voltage to flow through the zirconium ceramic bulb. The bigger the difference is in those two levels, the greater the higher voltage reading is.

An oxygen sensor is known to typically generate somewhere are .9 volts when the fuel mixture is rich to reduce the equipment’s reading. When the sensor is reading lean, the computer then reverses to bring the fuel back to rich. This flip-flopping, which we have already gone over, can occur with different speeds depending on the fuel system overall. Overall, the transition will be slowest on cars with feedback carburetors. Engines with throttle body injection are somewhat faster. The fastest come in engines with multiport injection, with that flip-flopping taking place as many as 7 times per 2500 rpm.

In order to work fully, the O2 must be at a temperature of about 600-degree Fahrenheit or higher for it to generate its voltage signal. For that reason some sensing tools will actually have a small heating device within them so that they can reach their required operating temperature more quickly. This heat unit is also used when the car has been idle for a long time in order to avoid a reversion to open loop.

How to diagnose a broken oxygen sensor

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Overall, they are impressively rugged and are typically able to outlast years of driving and variable environments. That being said, O2 wires will eventually wear out and do have to be replaced from time to time. Typically, deterioration of a sensing device takes place when substances such as water, road salt, oil and dirt find their way into the device.

The only way to truly know if a sensing device is doing its job correctly is to inspect it on a regular basis. That is why some vehicles even have a maintenance reminder light found on the dash. If your car does not have that, a good time to inspect the O2 is when you are getting your car’s spark plugs replaced.

One way to diagnose your oxygen sensor and see if it is working as it should is to read its output with a digital voltmeter. That being said, the transitions can be hard to see due to the numbers jumping around at a high rate. That’s why the PC-based scan tools are often used. Certain soft wares can be used to graph out the transitions of the device’s voltage, making it very clear if something is not working as desired.

O2 sensor replacement

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Any O2 sensing equipment that is defective will have to be replaced. That being said, there are also benefits to simply replacing them periodically as a preventative measure. Replacing an again sensor can restore your vehicle to peak fuel efficiency and performance.

For cars manufactured During and after 1996, replacement is recommended at 100,000 miles.


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