What does the oxygen sensor on the exhaust manifold of the Toyota Highlander do?

Answer 1

http://en.wikipedia.org/wiki/Oxygen_sensor Automotive applications A 3-wire oxygen sensor (spare part) suitable for use in a Volvo 240 or similar.

Automotive oxygen sensors, colloquially known as O2 sensors, make modern electronic fuel injection and emission control possible. They help determine, in real time, if the air fuel ratio of a combustion engine is rich or lean. Since oxygen sensors are located in the exhaust stream, they do not directly measure the air or the fuel entering the engine. But when information from oxygen sensors are coupled with information from other sources, they can be used to indirectly determine the air-to-fuel ratio. Closed-loop feedback-controlled fuel injection varies the fuel injector output according to real-time sensor data rather than operating with a predetermined (open-loop) fuel map. In addition to enabling electronic fuel injection to work efficiently, this emissions control technique can reduce the amounts of both unburnt fuel and oxides of nitrogen from entering the atmosphere. Unburnt fuel is pollution in the form of air-borne hydrocarbons, while oxides of nitrogen (NOx gases) are a result of combustion chamber tempuratures exceeding 2000 deg. F due to excess air in the fuel mixture and contribute to smog and acid rain. Volvo was the first automobile manufacturer to employ this technology in the late 1970s, along with the 3-way catalyst used in the catalytic converter. Modern spark-ignited combustion engines use oxygen sensors and catalytic converters as part of an attempt by governments working with automakers to reduce exhaust emissions. Information on oxygen concentration is sent to the engine management computer or ECU, which adjusts the amount of fuel injected into the engine to compensate for excess air or excess fuel. The ECU attempts to maintain, on average, a stoichiometric air-fuel ratio by interpreting the information it gains from the oxygen sensor. The primary goal is to lower the levels of certain by-products in the exhaust stream, namely hydrocarbons (which are released when the fuel is not burnt (a misfire), carbon monoxide (which is the result of running rich) and NOx (which dominate when the mixture is lean). Failure of these sensors, either through normal aging, the use of leaded fuels, or fuel contaminated with silicones or silicates, for example, can lead to damage of an automobile's catalytic converter and expensive repairs. Tampering with or modifying the signal that the oxygen sensor sends to the engine computer can be detrimental to emissions control and can even damage the vehicle. When the engine is under low-load conditions (such as when accelerating very gently, or maintaining a constant speed), it is operating in "closed-loop mode." This refers to a feedback loop between the ECU and the oxygen sensor(s) in which the ECU adjusts the quantity of fuel and expects to see a resulting change in the response of the oxygen sensor. This loop forces the engine to operate both slightly lean and slightly rich on successive loops, as it attempts to maintain a stoichiometric ratio on average. If modifications cause the engine to run moderately lean, there will be a slight increase in fuel economy, sometimes at the expense of increased NOx emissions, slightly higher exhaust gas temperatures, and eventual misfires at ultra-lean air-to-fuel ratios. If modifications cause the engine to run rich, then there will be a slight increase in power, but at the risk of decreased fuel economy, much higher exhaust gas temperatures, an increase in unburned hydrocarbons in the exhaust, and overheating the catalytic converter. Long-term operation at very rich mixtures can cause catastrophic failure of the catalytic converter (see backfire). The ECU also controls the spark engine timing along with the fuel injector pulse width, so modifications which alter the engine to operate either too lean or too rich may result in inefficient fuel consumption whenever fuel is ignited too soon or too late in the combustion cycle. When an internal combustion engine is under high load (e.g. wide open throttle), the output of the oxygen sensor is ignored, and the ECU automatically enriches the mixture to protect the engine. Any changes in the sensor output will be ignored in this open-loop state, as are changes from the air flow meter, which might otherwise lower engine performance due to the mixture being too rich or too lean, and increase the risk of engine damage due to detonation if the mixture is too lean.