• Based on the control of the front and rear HO2S heaters, a stabilized oxygen concentration is detected even at low exhaust gas temperature and feedback control of fuel injection even during cold engine start is made possible for improved cold temperature exhaust emission performance.
• When exhaust gas temperature is high, the front and rear HO2S are protected from a rise in temperature by the stopping of energization to the front and rear HO2S heaters.
• Through duty control of the front HO2S according to engine operation conditions (emission temperature), both exhaust emission performance improvement and protection of the front HO2S have been achieved.