Hyundai Elantra: Heated Oxygen Sensor (HO2S). Description 
and Operation - Engine Control System - Fuel System - Hyundai Elantra MD 2010-2015 Service manualHyundai Elantra: Heated Oxygen Sensor (HO2S). Description and Operation

Description

Heated Oxygen Sensor (HO2S) consists of zirconium and alumina and is installed both upstream and downstream of the Catalytic Converter. It varies in accordance with the air/fuel ratio.
The sensor must be hot in order to operate normally. To keep it hot, the sensor has a heater which is controlled by the ECM via a duty cycle signal. When the exhaust gas temperature is lower than the specified value, the heater warms the sensor tip.