DTC P0136: O2 Sensor Circuit (Bank 1/Sensor 2): General Description
WARNING: This page is about a different variant/trim than selected.
HO2S (S2) is in the rear side of Catalytic Converter to check the proper operation of catalyst. Oxygen density after the catalytic converter has to be within specific range (around 0.5V when there is no acceleration and deceleration.)
If the oxygen density changes in accordance with HO2S (S1), it means the poor performance of catalytic converter.