Oxygen Sensor O2

Oxygen Sensor or Lambda Sensor

The oxygen sensor in your vehicle is an electronic component that is designed to measure levels of oxygen in the engine exhaust. Typically, the oxygen sensor is mounted to the exhaust system tube, with the sensor part inside the tube. This measures the oxygen mixture by generating a small amount of electricity due to the difference in atmosphere, oxygen and carbon dioxide. The computer PCM monitors this voltage and adjusts fuel delivery accordingly.

Oxygen Sensor
Oxygen Sensor (02)

The oxygen sensor or Lambda sensor (AKA O2 Sensor) in your vehicle is one of the key components in the fuel injection system. Its job is to measure the amount of oxygen required to burn any fuel remaining in the exhaust stream and relay that information back to the computer PCM (Powertrain Control Module) where it is compared with other live information so that adjustments can be made to maximize fuel efficiency and power via proper air-fuel mixture and ignition timing in the engine. Oxygen sensors do this through a chemical reaction inside the sensor itself; in this article we will explain the evolution and application of this very important piece of the fuel injection puzzle

Early oxygen sensors were simple one or two wire sensors that gave feedback to the computer through a chemical reaction within the sensor that creates voltage. These early sensors had to warm up before they became active, which means they didn’t work until they had reached the operating temperature in the exhaust system required for the chemical reaction to work properly as well as the engine reaching a close-to-operating coolant temperature. While extremely simple, they worked fine with the basic fuel injection systems of the time, which had extremely slow BAUD rates (rate at which information is processed in the computer). As technology improved, the sensors had to be improved upon as well.

Oxygen sensors work through a chemical reaction. The core or element of the sensor is Zirconia ceramic with a thin layer of platinum. Since these materials are reactive and are applied as layers they will eventually wear out reducing their efficiency, you should follow your vehicle manufacturers recommendations as far as replacement. A common misconception is that the sensors measure the actual amount of oxygen in the exhaust, when in reality they measure the amount of oxygen required to burn any fuel that is remaining in the exhaust stream. For instance a rich condition (too much fuel) will cause a higher voltage reading since it is creating a demand for oxygen within the sensor to burn the fuel, whereas a lean condition will do the exact opposite. The voltage created by the sensor is then relayed to the computer where it will compare it with other live information to make the necessary mixture and timing adjustments.

As fuel injection evolved, so did the oxygen sensor. They have gone from one-wire sensors that ground through the outer case to four-wire sensors that ground externally and have built in heaters to bring them up to the required temperature quickly so that the computer can begin adjusting the mixture as soon a possible to improve emissions as well as performance. Oxygen sensors are now also being used to measure the efficiency of the catalytic converter to be sure that it is working properly. By placing an oxygen sensor in the exhaust system in front (Primary or upstream) of the converter and one behind it (secondary or downstream) the computer can see if the converter is reducing emissions as designed while it is adjusting for optimum performance.

Oxygen sensors can usually be found in the exhaust pipe near the engine (Primary sensor) although sometimes they are mounted in the exhaust manifold itself where the exhaust pipe connects. The secondary sensor will be found behind or in the catalytic converter so as to measure its efficiency. Early systems would use just one primary sensor and adjust the entire engine based on that reading, whereas newer fuel injection systems use a sensor for each side of the engine for V-6 and V-8 engines and even will sometimes use 2 primary sensors on an inline 4 or 6 cylinder to adjust the mixture more accurately. The number of secondary sensors will depend on how many catalytic converters the vehicle has. A dual exhaust system would require 2, but a single only needs one since all it does is measure catalyst efficiency.

Oxygen sensors come in two basic designs, narrow band and wide band. To work properly, narrow band sensors (most common) use a cycling of rich to lean mixture to achieve a balance close to a stoichiometric mixture (ideal for internal combustion). Wide band sensors use what is called a electrochemical gas pump to keep a constant current in the electromechanical cell, this stability eliminates the rich-lean cycling of the narrow band sensor and makes the mixture adjustments much more accurate and faster. These sensors are fairly rare in production vehicles; they are used more in controlled environments such as dynamometer rooms, but will be seen in Diesel engines to help them meet new emissions standards. Regardless of design changes or differences, all oxygen sensors work on the same principal and do the same thing.

Since oxygen sensors are made of reactive materials, their life span is definitely limited. Aside from mechanical failures such as shorted heating elements or physical damage, these sensors will usually last around 100,00 miles for a heated element sensor and roughly half that (50,000) miles for a non-heated sensor. The difference is basically due to the slow warm up time for the non-heated sensor allowing build up to collect on the sensor, hampering its ability to accurately read the exhaust gasses. If an engine has a mechanical failure such as a head gasket (coolant) or an issue that causes oil consumption (Rings, valve guides), it will drastically shorten its life, if not ruin the sensor all together from heavy contamination of the reactive materials.

When replacing your oxygen sensor, be sure to use a high-quality OEM (Original Equipment manufacturer) part. Cheaper sensors aren’t as accurate and don’t last as long as an OEM sensor will, and can cause performance issues as well as emissions problems. Always consult your service manual for the proper parts and procedures for repairing your vehicle, and also wear the proper safety equipment when working on a vehicle.

Oxygen Sensor Cut Away
Oxygen Sensor Cut Away

The oxygen sensor is in continuous communication with the engine control unit giving it the information necessary to adjust fuel delivery for optimum combustion. When the engine is cold the oxygen sensor reads slowly, a heating element has been installed to correct this problem and help the sensor operate correctly until the engine has reached operating temperature. When the throttle is wide open and under max load the oxygen sensor will go to full voltage output until normal driving conditions return.

Typically changing an oxygen sensor when necessary is a simple process. Most solutions to oxygen sensor problems result in changing the oxygen sensor, but always be sure there are no vacuum leaks present in the intake system, this can give a false oxygen sensor trouble code. Due to the severe usage environment the sensor endures, it is common for most sensors to last approximately 75,000 miles, however it is not uncommon for an oxygen sensor to last only 40,000 miles depending on your driving habits and vehicle operating conditions.

Over the life of the oxygen sensor soot build up can occur on the sensing probe which can result in inaccurate readings. This can cause your engine to run rough and consume excessive fuel. If further technical assistance is needed, ask our team of certified car repair technicians.

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Written by
Co-Founder and CEO of
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2014-04-07)