DTC P0138 Or DTC P0158: Description

WARNING: This page is about a different car, the 2002 Nissan Sentra. However, it is still accessible from the selected car via links, so may be relevant.

HO2S2, located after 3-way catalyst, monitors the oxygen level in the exhaust gas on each bank. Even if switching characteristics of HO2S1 are shifted, the air fuel ratio is controlled to stoichiometric, by the signal from HO2S2. This sensor is made of ceramic zirconia. The zirconia generates voltage from about one volt in richer conditions to zero volts in leaner conditions. Under normal conditions, HO2S2 is not used for engine control. HO2S2 has a much longer switching time between rich and lean than HO2S1. The oxygen storage capacity before the 3-way catalyst causes the longer switching time.

To judge the malfunctions of HO2S2, ECM monitors whether the voltage of sensor is unusually high during the various driving condition such as fuel-cut.