DTC P0138 (2019): General Description
The rear secondary heated oxygen sensor (secondary HO2S (bank 1, sensor 2)) detects the oxygen content in the exhaust gas downstream of the rear warm up three way catalytic converter (WU-TWC) (bank 1) during stoichiometric air/fuel ratio feedback control. The powertrain control module (PCM) controls the air/fuel ratio from the rear air/fuel ratio (A/F) sensor (bank 1, sensor 1) output voltage to optimize the rear WU-TWC (bank 1) efficiency. When power is applied to the rear secondary HO2S (bank 1, sensor 2) heater, if the rear secondary HO2S (bank 1, sensor 2) output continues to exceed the upper limit used during feedback control, the PCM detects a malfunction and stores a DTC.