How They Work

Every new car and most cars produced after 1980, have one or multiple oxygen sensors, or O2 sensor. The oxygen sensor is part of the emissions control system and feeds data to the­ engine management computer. The goal of the oxygen sensor is to help the engine run as efficiently as possible and also to produce as few toxic emissions as possible.


­A gasoline engine burns gasoline in the presence of oxygen. It turns out that there is a particular ratio of air and gasoline that is considered "perfect," and that ratio is 14.7:1 (different fuels have different "perfect" ratios - the ratio depends on the amount of hydrogen and carbon found in a given amount of fuel). If there is less air than in this perfect ratio, then there will be fuel left over after combustion. This mixture is called "rich". Rich mixtures are bad because the unignited fuel creates harmful emissions. If there is more air than this perfect ratio, then there is excess oxygen. This mixture is called "lean". A lean mixture tends to produce more nitrogen-oxide pollutants, and, in some cases, it can cause poor performance and even engine damage.


­Th­e oxygen sensor is positioned before and/or after the catalytic converter and detects rich and lean mixtures. The mechanism in most oxygen sensors involves a chemical reaction that generates a voltage. The engine's computer looks at this voltage to determine if the mixture is too lean or too rich, and then it adjusts the amount of fuel entering the engine accordingly to balance the ratio.


The reason why the engine needs an oxygen sensor is because the amount of oxygen that the engine can pull in depends on all sorts of things, such as the air temperature, altitude, engine temperature, barometric pressure, the load on the engine, etc.


When an oxygen sensor fails, the computer can no longer sense the air-to-fuel ratio, and it is forced to guess. Your car will then perform poorly and use more fuel than is necessary.