Automotive vehicles with an internal combustion engine have an exhaust system including a pathway for exhaust gas to move away from the engine. Automobile exhaust systems include a catalytic converter that reduces exhaust emissions by chemically converting the exhaust gas into carbon dioxide, nitrogen, and water. The efficiency of the catalytic converter is periodically monitored to prevent excess carbon dioxide, nitrogen oxides and unburned hydrocarbons in the exhaust gas. Typically, the catalytic converter is monitored during steady state engine operation. The amount of air provided to the combustion process is important in establishing efficient combustion and hence reduced levels of pollutants in the exhaust gas. The amount of air introduced into the combustion chamber is frequently controlled by systems which first require determining the oxygen content in the exhaust gas. This information is then utilized to control the respective amounts of fuel and air being supplied to the engine so that the exhaust gases will have the desired combustion. Depending on the desired operating state, internal combustion engines can be operated with fuel/air ratios in which the fuel constituent is present in a stoichiometric surplus (rich range), the oxygen of the air constituent is stoichiometrically predominant (lean range), and the fuel and air constituents satisfy stoichiometric requirements. The composition of the fuel-air mixture determines the composition of the exhaust gas. In an internal combustion engine which includes an exhaust purification system utilizing a catalyst, the oxygen concentration in the exhaust gas of an engine has a direct relationship to the air-to-fuel ratio of the fuel mixture supplied to the engine. It is essential to accurately control an air-fuel ratio of an air-fuel mixture which is combusted in the internal combustion engine such that the catalyst effectively purifies exhaust gas emission. The air-fuel ratio is defined as a weight ratio of the air to the fuel contained in the air-fuel mixture for combustion. In order to improve the efficiency of an internal combustion engine in a motor vehicle, an oxygen sensor is often used to sense the oxygen content of the exhaust gas, and the air-fuel mixture admitted to the engine is adjusted by the engine management system according to the sensed oxygen level of the exhaust gas.

In automotive applications, oxygen sensors are provided in the exhaust system to control air/fuel ratio based on measured concentrations of oxygen in the exhaust gas for feedback. An oxygen senor monitors the ratio of oxygen to gasoline in a vehicle. The oxygen sensor provides a feedback signal in a closed-loop engine control system where an air-fuel mixture ratio is maintained as close to stoichiometric as possible using the signal from the oxygen sensor. Accurate feedback from the oxygen sensor to the fuel metering system is essential for proper regulation of the level of pollutants in motor vehicle exhaust gases. The oxygen sensor output enables optimization of the fuel-air ratio fed into the engine. Optimizing the fuel/air mixture entering the engine changes the combustion conditions and achieves more complete combustion, thereby operating the engine closer to the stoichiometric point. The oxygen sensor generates an output voltage depending on the content of oxygen and the fuel-air mixture at the exhaust. Oxygen sensors provide a voltage output that varies in accordance with the amount of oxygen in the combustion product. If the exhaust gas is rich in oxygen, the sensor will produce a low voltage, close to zero volts. If the exhaust gas is rich in fuel, the sensor will produce a voltage close to one volt. When it is detected that the air-fuel ratio is richer than a target air-fuel ratio based on the output of the oxygen sensor, a decrease correction of a fuel injection amount is performed. Adjustments are made in stepped intervals until the sensor output goes to the opposite sense from its previous signal. When it is detected that the air-fuel ratio is leaner than the air-fuel ratio based on the output of the oxygen sensor, an increase correction of the fuel injection amount is performed. Thus, the air fuel ratio is controlled to the target air/fuel ratio. The correction of the fuel injection amount is generally performed using a skip correction and an integral correction. The oxygen sensor is typically is connected to feed data to a powertrain control module (PCM). The data received from the oxygen sensor provides the PCM with input that may be used to calculate the efficiency of the cylinder burn process. The output voltage and internal resistance of an oxygen sensor varies with the temperature and the age of the sensor. A heater is used with the oxygen sensor to allow the sensor to reach an operating temperature faster than the sensor would if heated only by the engine exhaust gases.

An oxygen sensor generally consists of a metal housing, a spring mounted inside the housing to absorb shocks, a cylindrical ceramic holder supported on the spring inside the housing, a cylindrical ceramic holder supported on the spring inside the housing, a probe installed in the cylindrical ceramic holder inside the housing to detect the amount of oxygen in exhaust gas, and a terminal connected to the probe and extended out of the housing for transmitting the voltage signal from the probe to the electronic control unit in the car. An oxygen sensor for an internal combustion engine typically has at least in part a polygonal housing outer surface as well as two lead wires leading into the housing from a connector. The lead wire inserted into the lead wire insertion hole is sealed and fixed into the above-described lead wire insertion hole by caulking it to the protective cover in the radial direction. The wires may be connected via the connector to the vehicle sensing and control circuitry after the sensor is threaded into the appropriate part of a vehicle engine. In a typical oxygen sensor structure an inner and an outer surfaces of a zirconia element are coated with porous platinum electrodes, detects whether the air-fuel ratio is rich or lean as-compared with the theoretical air-fuel ratio on the basis of the oxygen concentration in the exhaust gas, and transmits a signal to an ECU (electronic control unit). The oxygen sensor includes an oxygen sensor element for detecting the oxygen concentration. The oxygen sensor element includes a solid electrolyte and electrodes provided on the solid electrolyte. The electrodes include an internal electrode exposed to a reference gas and an external electrode exposed to a gas to be measured. The outer surface of the sensor is exposed to the exhaust gases and the interior of the sensor is provided with a reference source of oxygen. In operation, the differential in oxygen concentration between the exhaust gases and the reference source causes conduction of oxygen ions through the ion-conductive body, resulting in an electrical current which is dependent upon the relative content of oxygen in the exhaust gas and the reference source. In the oxygen sensor, generally a ceramic separator is placed in the casing and leads from the oxygen sensing element or a heating element for heating the oxygen sensing element are passed through lead insertion holes as a structure for drawing out the leads from the casing.

In general, two different types of oxygen sensors are available for usage in automotive fuel control. A switching sensor is the most common and least expensive sensor which has a bi-stable output voltage that switches or toggles between first and second states corresponding to lean and rich conditions of the sensed exhaust gas. The switching device outputs less than 0.25 volts when the input air/fuel ratio to the engine is leaner than stoichiometric and outputs greater than 0.75 volts when input air/fuel ratio to the engine is richer than stoichiometric. The other type of oxygen sensor, referred to as a universal exhaust gas oxygen sensor, or wide-range oxygen sensor, has an analog output that varies in amplitude in relation to the deviation of the sensed exhaust gas from the stoichiometric air/fuel ratio. An analog-to-digital (A/D) converter receives the oxygen sensor output and generates a digital value input to a digital microprocessor. The microprocessor controls the air/fuel ratio and constantly adjusts the mixture entering the combustion chamber in order to maintain the engine operating near the stoichiometric point. The most widely used oxygen sensors are based on partially stabilized zirconia (PSZ) as the ion conductor. Such sensors function by monitoring the electromotive force (EMF) developed across an ion conductor which is exposed to different partial pressures of oxygen. The performance of an oxygen sensor comprises both static and transient characteristics. The static characteristics of a zirconia oxygen sensor are defined by the magnitudes of the voltages for rich and lean readings and the switching point of the sensor. The transient characteristics of a zirconia oxygen sensor are defined by the response times.