What Are the Signs That You Need an O2 Sensor?

by Jody L. Campbell

The oxygen sensor(s) in a vehicle, simply put, are clever devices that monitor the amount of pollution the vehicle dispels from the exhaust system. Although the function of the oxygen sensor goes far beyond this simplistic definition, its main purpose is to monitor and communicate its information to the computer system in a car. When an oxygen sensor fails, your vehicle may have obvious side-effects such as loss of power and decreased fuel efficiency. However, this is not always the case. If a sensor goes bad and there are no noticeable side effects, ignoring the "check engine" light may be detrimental to the vehicle.


The oxygen sensor is a small wired unit that is placed into the exhaust stream of the combustible engine of a vehicle. Its purpose is to monitor the burned fuel to air ratio, also known as the "lambda ratio." There is a perfect lambda ratio in the exhaust of the combustible engine and when it changes, the sensor is effected and transmits this information to the engine management computer via voltage signals. Less air in the lambda ratio will create more leftover fuel after combustion occurs and this will result in a rich mixture. More leftover fuel will create more pollution to the atmosphere and decrease the fuel efficiency of the vehicle. Excess oxygen and less fuel creates a lean mixture. This will result in more nitrogen oxide pollutants and can damage the engine or other expensive exhaust components. In all cases where an O2 sensor fails in a vehicle, the engine management computer will detect the information or lack of information coming from the sensor and trigger the "check engine" light to illuminate on the dashboard to alert the driver.


Professor Walter Nernst developed a gas-tight ceramic electrolyte that became conductive at temperatures above 620 degrees in 1899. Because there was little concern for its use back then, it was redeveloped and ingeniously modified in the 1960s by Robert Bosch. When the Environmental Protection Agency mandated stricter exhaust emissions regulations in the 1970s, the lambda sensor or oxygen sensor was introduced to the combustible engines to monitor emissions. The first vehicle to introduce the oxygen sensor was the Volvo 240 model distributed in California in 1977, which also came equipped with the three-way catalyst. By the early 1980s, most all vehicles were mandated to have catalytic converters and oxygen sensors to monitor exhaust emissions.


There are quite a few different types of oxygen sensor, but their primary objective is the same. The first lambda sensor was developed with a single wire (unheated sensor) that simply transmitted voltage to the engine management computer. The problem was that it needed to reach a certain temperature in order to begin to work. In the interim, it would waste fuel and create more pollution. A three and four wire oxygen sensor was later developed (heated sensor) that brought the sensor up to an operating temperature more quickly and thus reduced pollution and increased fuel efficiency. The older single wire or two-wire vehicles may have only had one sensor to monitor, but nowadays, there are at least two heated sensors per vehicle and in some cases as many as four, which makes locating the bad sensor(s) a bit more challenging. The older vehicle's sensor would be located near or in the exhaust manifold (upstream). Some cars were then developed with one after the catalytic converter to monitor the exhaust after it was filtered through the converter (downstream). If the vehicle has split cylinders on the engine with dual manifold and more than one converter, more sensors are integrated into the exhaust system to monitor each bank and each converter.


The benefits of the oxygen sensor are simple. The combustible engine will perform better, it will have better fuel efficiency and dispel less pollution. However, if a bad oxygen sensor is not addressed, the long-term effects of its ineffectiveness can cause costly damage to the engine or other expensive exhaust components. The decrease in fuel efficiency on an annual basis alone would easily pay for the sensor replacement.


The location of the sensor is screwed or bolted into the exhaust system, usually in or near the manifold, and as previously mentioned near or after the catalytic converter. A wire and plug attached to the exterior side of the sensor plugs into a wire harness to communicate via voltage to the computer and heat the sensor up to operating temperature. To locate the sensors, follow the exhaust components from the catalytic converter forward to the front exhaust pipe and you would notice small devices screwed directly into the exhaust pipes with a wire attached to them.


Some shady mechanics and people have discovered ways to trick the oxygen sensor or simply clean the sensor in order to save money and not have to replace them while attempting to pass an emissions inspection. While cleaning a sensor may have a beneficial result, it may be temporary. Tricking the sensor is futile in the long run. The thing to consider when replacing the oxygen sensor is that something else may have caused it to fail. The lean or rich mixture coming from the exhaust has nothing to do with the sensor. The sensor simply monitors it and informs the computer, which then makes adjustments to create the perfect mix. An engine that needs a tune-up or has a leak in the exhaust system can be leading causes of damaging oxygen sensors and just replacing the sensor may not solve the underlying problem.

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