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    The temperature of sensor electrolyte affects its conductivity and hence the output voltage. The optimum operation temperature for the oxygen sensor is between 300 and 600º C. During engine warm-up the unheated EGO sensor is not operative. Electrically heated exhaust gas oxygen (HEGO) sensors are therefore used.
     For meeting ULEV emission requirements, the performance of 3-way catalyst is to be further improved and oscillations in air-fuel ratio are to be minimized. For this, a new sensor which measures actual oxygen content in the exhaust and the engine control unit responds gradually to the changes in air-fuel ratio is used. This sensor is known as universal exhaust gas oxygen (UGEO). With the use UEGO, a better control of air-fuel ratio is obtained and 3-Way catalyst operates in a very narrow window of high conversion efficiency (Fig 5.18).