What does an oxygen sensor do?

The oxygen sensors on vehicles are very important components in a fuel-injected engine. These sensors work in conjunction with the mass air flow meter, or the manifold absolute pressure sensor in some vehicles, in order to provide the correct mixture of air and fuel into the motor.

The ideal mix of fuel and air uses what is called a stoichiometric ratio of 14.7 parts air to 1 part fuel for ignition in the combustion chamber. This ratio is constantly trying to change due to different demands on the engine. For example, idling and constant speed driving on the highway uses less air and fuel because the throttle on the engine is open a small amount. Converseley, accelerating away from traffic lights and stop signs means the throttle plate is open as much as sixty to eighty percent, drawing in a large amount of air, requiring more fuel to make the 14.7:1 mixture neccessary to keep the engine running. This is also why fuel economy for city and highway driving are different.

The computerized fuel injection system uses the mass air flow meter to measure the incoming air to the throttle body, and send the information to the engine control module. The control module then activates each fuel injector for a predetermined amount of time, metering the fuel load into the intake, or combustion chamber. The mixture ignites, and the exhaust gasses flow into the exhaust manifold, and past the oxygen sensors. The oxygen sensors then read the gasses, and send a signal back to the control module to adjust the quantity of fuel from the injectors. If there is too much fuel, the computer reads this as a rich condition, and will restrict the flow of fuel, bringing the mixture down to the stoichiometric ratio. On the other hand, if there is not enough fuel, this is read as a lean condition, and more fuel is added. All of this information and corresponding adjustments happen in a span of milliseconds.

Also, most late-model vehicles have heated-element oxygen sensors. In order for oxygen sensors to read accurately, they must be at a very high temperature, usually close to the temperature of the exhaust gasses. Since most people do not need to warm up their vehicles like in the days of the carbureted vehicle, manufacturers introduced an electrical heating element to their oxygen sensors to bring up the temperature very quickly. Manufacturers that do not have a heating element calibrate their cars to idle higher for a short time on the first start-up to warm the sensors. Either system is effective.

Finally, oxygen sensors are very durable components. They are built to withstand high heat and long use. When a code for an oxygen sensor sets, most often this is caused by something else in the system, and the oxygen sensor reads the effects as either a lean or rich code. Over time, a sensor can “slow down” and affect the mixture rate, but it rarely outright fails. However, heater elements in so-equipped sensors can fail, requiring a new oxygen sensor.

Copyright © Smitty’s Auto Service, Inc. 10/5/2010

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