Diagnostic Trouble Code (DTC) P0138 for the bank 1 downstream sensor is being set.
1. Warm the engine and check the Oxygen Sensor (O2S) operation. On this vehicle the voltage should switch between 2.5 - 3.5 volts.
2. If the voltage is normal, this is an intermittent concern. Carefully inspect the wiring harness.
3. If the voltage is high (above 3.99 volts) unplug the O2S and confirm that the scan tool reads around 5 volts for that sensor. If it does measure the voltage on the return circuit using a Digital Volt Ohm Meter (DVOM), a normal voltage is 2.5 volts. If these voltages check good replace the O2S.
Be sure to use an Original Equipment from Manufacturer (OEM) specification oxygen sensor on this vehicle. Most non OEM spec sensors will not work properly with the type of PCM on this vehicle.
Bank one means where the number one cylinder starts and downstream means after the cat.