Oxygen Sensor

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By Reinier (Contact Me)
Last Updated 2022-12-27
Automobile Repair Shop Owner

What Does the Oxygen Sensor Do?

On gasoline engines, oxygen sensors measure/monitor the level of oxygen in gasoline exhaust gas to provide input data to the PCM (Powertrain Control Module) on the relative composition of the air/fuel mixture.

Oxygen sensors are becoming increasingly more common on diesel engines, but unlike on gasoline engines where oxygen sensors monitor the air/fuel mixture, oxygen sensors perform different functions in diesel vehicles. For instance, oxygen sensors in many diesel vehicles assist in controlling the oxygen content in diesel exhaust before and during DPF (Diesel Particulate Filter) regeneration events, although in some cases, oxygen sensors in diesel vehicles are used in fuel control to some extent, as well.

Why is the Oxygen Sensor Needed?

SPECIAL NOTES: As mentioned above, oxygen sensors fulfill different functions in diesel vehicles than they do in gasoline vehicles. These functions on diesel engines are highly technical in nature, and as such, the details of these functions fall outside the scope of this article. Therefore, this article will focus on the application of oxygen sensors in gasoline vehicles. END OF SPECIAL NOTES.

While most modern diesel engines can run efficiently on air/fuel mixtures that are as rich as 8 parts of fuel to one part of air, to as lean as 1 part of fuel to 60 parts of air, all gasoline engines only run efficiently on air/fuel mixtures that contain around 14.7 parts of air to one part of fuel. This ratio is known as a stoichiometric ratio, in which ratio all of the fuel in the mixture is combusted using all of the available air.

While maintaining this (ideal) air/fuel ratio in an operating gasoline engine is relatively easy to achieve if the engine only ever runs at a constant speed and load, the fact is that gasoline engines in normal vehicles don’t ever run at constant speeds and loads for long. All driving conditions require frequent throttle inputs, and regardless of whether any given throttle input is positive or negative, all throttle inputs affect the engine’s demand for fuel, which directly affects the composition of the air/fuel mixture.

As a practical matter, emissions regulations are built around the maximum permissible levels of harmful exhaust emissions any vehicle can emit during normal use, so modern vehicles are fitted with fast-acting sensors, known as oxygen sensors, to monitor their exhaust streams for the presence of harmful exhaust emissions they contain.

However, since these sensors cannot analyze an exhaust stream for the presence of all possible harmful substances, they only measure the concentration of oxygen in exhaust gas, this concentration being a function of the completeness, (or otherwise) of the combustion of mixtures of gasoline and atmospheric air.

Thus, since the completeness (or otherwise) of the combustion process determines the levels of harmful exhaust components any given engine emits, the function of oxygen sensors is to monitor the oxygen content of the exhaust stream as a  direct function of the quality of the combustion process. As such, oxygen sensors are such critically important components in the emission control systems of all modern vehicles that no modern engine can operate efficiently without functional oxygen sensors.

How Does the Oxygen Sensor Work?

NOTE: It is important to point out that modern oxygen sensors are also known as heated wide-band oxygen sensors, to distinguish them from the old-style, unheated narrow-band oxygen sensors that were largely slow acting, relatively inaccurate, and prone to premature failure.

The topic of exactly how modern oxygen sensors work is an exceedingly technical one, and as such, the finer technical details of this topic fall outside the scope of this article. Nonetheless, in their simplest form, modern oxygen sensors contain the following principal components-

  • PCM controlled heating elements that reduce the time a sensor takes to enter closed-loop operation
  • Sensing elements made from various metals that react to the presence of oxygen in the exhaust stream
  • Complex electronic circuitry that converts changes in the level of oxygen in the exhaust stream into electrical signals, which signals the PCM interprets as changes in the oxygen content of the exhaust stream

Here is the short version of how modern heated oxygen sensors work to control the fueling of modern engines-

In all cases, the oxygen sensor is located in the exhaust system in such a way that its sensing element is in direct contact with the exhaust gas that passes through the exhaust system. When the ignition is switched on, the PCM activates the heating element’s circuit(s) to help the exhaust gas (after the engine has started) heat the sensing element to a temperature of between 600 deg F and 650 deg F, at which temperature the sensor begins to react to changes in the oxygen content of the exhaust stream. Note that at lower temperatures, the sensor is inactive and does not react to the presence of oxygen in the exhaust stream in any way.

In modern vehicles, the heating process typically takes considerably less than one minute, but once the sensor reaches its optimal operating temperature, it begins to sense changes in the oxygen content of the exhaust stream that occur as the result of changes in throttle inputs during normal driving. The changes are converted into electrical signals, and based on the voltage of the signals the PCM will either increase or decrease the volume of fuel being injected into the cylinders to suit the current operating conditions. This process is known as closed-loop operation, which is a condition in which the PCM uses inputs from the sensor to make changes to the air/fuel mixture to reduce exhaust emissions.

In practice, heated oxygen sensors operate in a range of voltages; typically from a minimum of about 0.1 volts to a maximum of about .095 volts. In most applications, a low voltage indicates a lean condition, while a high voltage indicates a rich condition. The mid-point of this voltage range, which is about 0.45 to 0.50 volts, represents a condition in which the air/fuel mixture is at, or close to a ratio of 1 part of fuel to every 14.7 parts of air, but this ratio is impossible to maintain during normal driving.

Thus, when the oxygen sensor registers a voltage of around 0.45 volts, and a sudden large positive throttle input occurs that a) greatly enriches the air/fuel mixture, and b) sharply reduces the concentration of oxygen in the exhaust stream, the PCM has to make a large adjustment to the air/fuel mixture to return the air/fuel mixture to a stoichiometric ratio.

In practice, such large adaptations to the air/fuel mixture take a relatively long time to accomplish, so to prevent large adaptations, the PCM switches between reading lean conditions as rich conditions, and vice versa, several times per second regardless of the oxygen sensor’s actual output signal at any given point. The practical advantage of this strategy is that the PCM can make small, but continuous adjustments to the air/fuel mixture to “chase” the ideal fuel mixture.

Since this strategy removes the need for large adjustments to the air/fuel mixture, the many small adjustments made several times per second collectively serve to maintain the air/fuel mixture closer to the ideal stoichiometric ratio than is possible to do with any other strategy. Overall, the strategy of switching between rich and lean conditions improves fuel economy, while reducing harmful exhaust emissions, at the same time.

Moreover, the latest iterations of wide-band oxygen sensors are now integrated into the misfire detection systems of almost all new engines. In practice, the latest oxygen sensors can relate any exhaust pulse to the cylinder that produced the pulse. This means that if any particular exhaust pulse contains more or less oxygen than the pulses that preceded or followed it, the PCM can use the input data from that particular exhaust pulse to refine the process of determining whether (or not) a misfire or cylinder power contribution problem is present in the cylinder that produced the deviant exhaust pulse.

Where is the Oxygen Sensor Located on the Engine?

This diagram shows the location of oxygen sensors on a V-type engine relative to the catalytic converter, which is indicated by the red arrow.

Note that the sensors in this example are labeled “upstream” and “downstream”. These designations are enshrined in the OBD II standard, and since this standard is an Act of Congress, these locations are not typical- they are enshrined in law, and therefore, the “upstream” and “downstream” labels cannot be switched or transposed in published service and/or repair information.

Nonetheless, upstream oxygen sensors are labeled “upstream” because they are always located upstream of the catalytic converter(s). Similarly, “downstream” oxygen sensors are labeled downstream because they are always located downstream, or after the catalytic converter(s).

It is perhaps worth pointing out that although both upstream and downstream oxygen sensors work in the same way, they have different functions. Upstream oxygen sensors are directly involved in the fuel control system, while downstream oxygen sensors are primarily tasked with monitoring the efficiency of the catalytic converter(s).

See the section on possible “Symptoms” for more details on the function of downstream oxygen sensors. 

What Does the Oxygen Sensor Look Like?


This image shows an example of a typical oxygen sensor and its electrical connector. Be aware though, that although the design of all oxygen sensors follows this general pattern, there are significant differences in both the outward appearance and calibrations of oxygen sensors made for different vehicle makes and models.

As a result, oxygen sensors are largely vehicle make and model specific, and are, therefore, not interchangeable even if any particular oxygen sensor fits on multiple different vehicles.

What are the Symptoms that the Oxygen Sensor is Bad?

The most common symptoms of failed, defective, or malfunctioning oxygen sensors are largely similar across all vehicle makes and models, but note that the severity of one or more symptoms listed below may vary between vehicle makes and models. Below are some common symptoms that might appear when an oxygen sensor fails-

  • Stored trouble code and illuminated warning light, but be aware that multiple trouble codes relating to the affected sensor’s wiring, heating element, or general performance could be present.

NOTE: Typical trouble codes that appear when a downstream oxygen sensor fails could relate to the same issues as above but in addition to codes relating to the efficiency of one or more catalytic converters.

  • Fuel consumption may increase dramatically, and the odor of rotten eggs may be present when the engine is running
  • The idling quality may be poor, or the engine may not idle at all
  • The engine may stall unexpectedly or repeatedly at low engine speeds
  • Varying degrees of power loss may be present
  • The engine may run roughly at some or all engine speeds and loads
  • Varying degrees of engine knocking caused by uncontrolled or premature ignition of the air/fuel mixture may be present at some or all engine speeds and loads, but note that this condition could result in sudden and potentially fatal engine damage
  • Possible fatal damage to the catalytic converter(s) could occur
  • The engine may exhibit random misfires on multiple, or all cylinders under some or all operating conditions
  • The vehicle will fail an emissions test

How do you test the Oxygen Sensor?

Apart from performing a thorough visual inspection to confirm or eliminate damaged, burnt, corroded, disconnected or shorted wiring and/or connectors as the cause of oxygen sensor issues, the only other objective test that is available to most non-professional mechanics is to check the affected or suspect oxygen sensor’s output signals with a suitable multimeter.

This test involves connecting a suitable multimeter’s positive lead to the sensors’ signal wire, and the negative lead to a suitable ground. With the engine running in closed loop operation and the multimeter set to the 1.0V DC (Direct Current) scale, the sensor should produce a rapidly fluctuating voltage between about 0.1V and about 0.9V. If the sensor’s output voltage does not fluctuate rapidly or stays “stuck” at any voltage, the sensor is defective. Similarly, if the sensor does not produce any signal voltage, the sensor is defective.

In the case of downstream oxygen sensors, the signal voltage [with the engine running in closed-loop operation] should hover around the midpoint between 0.1V and 0.9V. If this happens, both the oxygen sensor and the catalytic converter are working as designed. If, however, the downstream oxygen sensor’s signal voltage fluctuates rapidly or fluctuates significantly, the sensor is defective.

Note though, that if trouble codes relating to the catalytic converter’s efficiency are present, further tests are required to determine if the downstream oxygen sensor is defective, or if a failed or damaged catalytic converter is affecting the operation and signaling of the downstream oxygen sensor. Note, also, that these kinds of tests are best left to professional mechanics with a) the required test equipment, and b) the required technical knowledge and experience to interpret the test results correctly.

The only other definitive test of oxygen sensors involves using an oscilloscope to obtain a visual waveform of the sensor’s output signal. While this test is the same as the above test with a multimeter, oscilloscope waveforms may reveal signal dropouts and other defects that may not be visible or apparent with a multimeter due to multimeters’ low refresh or signal processing rates.

NOTE: Although air/fuel ratio sensors resemble oxygen sensors and do the same job, air/fuel ratio sensors work on vastly different principles than oxygen sensors. Moreover, the switching and signal rates of air/fuel ratio sensors are so high that it is impossible to perform any kind of meaningful diagnostic tests on these sensors with multimeters. In practice, the only practical way to test the operation of air/fuel ratio sensors is to obtain oscilloscope waveforms at high zoom values to be able to analyze the obtained waveform(s).

Based on both the above and the fact that service and repair information often use the terms “oxygen sensor” and air/fuel ratio sensor” interchangeably (and incorrectly), we highly recommend that you determine if the affected vehicle uses oxygen sensors or air/fuel ratio sensor before attempting to diagnose issues with these sensors. Be aware that employing ill-considered or incorrect test procedures can, and often does cause damage to both the sensor under investigation and the vehicle’s larger electrical system.

How do you replace the Oxygen Sensor?

In theory, it should be possible to just unscrew a suspect oxygen sensor from its bung in the exhaust system with a suitable tool, and screwing in a replacement with said suitable tool.

However, as with many (other) things automotive, the practice of removing an old oxygen sensor from an exhaust system is often vastly different from the theory. In many cases, the combined effects of corrosion and heat kind of “welds” the threads of the sensor to the threads in the exhaust system, which could, and sometimes does, cause the sensor to break into pieces if excessive force is applied in an attempt to unscrew the sensor- even if suitable and correct tools are used.

In many, if not most other cases, corrosion in the threads of both the sensor and the exhaust bung could, and often does, strips out the thread in the exhaust as the sensor is forced to rotate during unscrewing.

As a practical matter, removing a broken-off piece of an oxygen sensor from an exhaust system, or repairing damaged or stripped-out threads in the exhaust system almost always requires the removal of at least a part of the exhaust system from the vehicle.

Therefore, and since removing parts of the exhaust system is often a challenging affair that could further require the removal or disassembly of unrelated engine components, we do not recommend that non-professional mechanics attempt an oxygen sensor replacement on any vehicle.

Since the possibility of causing significant damage to the vehicle or sustaining potentially serious personal injuries is directly correlated to the difficulty experienced in removing the old sensor, we strongly recommend that you seek professional assistance with diagnosing and/or replacing suspect oxygen sensors.