DTC P0139: Bank 1 Heated Oxygen Sensor 2 Circuit - Slow Response Or DTC P0159: Bank 2 Heated Oxygen Sensor 2 Circuit - Slow Response : Description

WARNING: This page is about a different car, the 2003 Nissan Pathfinder. However, it is still accessible from the selected car via links, so may be relevant.

HO2S 2, located after 3-way catalyst, monitors the oxygen level in the exhaust gas on each bank. Even if switching characteristics of HO2S 1 are shifted, the air/fuel ratio is controlled to stoichiometric, by the signal from HO2S 2. 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, HO2S 2 is not used for engine control.

HO2S 2 has a much longer switching time between rich and lean than HO2S 1. The oxygen storage capacity before the 3-way catalyst causes the longer switching time. To judge malfunctions of HO2S 2, ECM monitors whether voltage of sensor is unusually high during various driving condition such as during fuel-cut.