What does that mean?
The Heated Oxygen Sensor (2) located rear of catalytic converter produces an output signal relative to oxygen storage capacity of catalytic converter. Ho2S 2 signal is less active than signal produced by front oxygen sensor. This code sets when HO2 Sensor voltage is greater than 999 mV for more than 2 minutes (time depends on model. Could be as high as 4 minutes)
There may be no noticeable symptoms except for MIL illumination. Possible high fuel pressure can over rich the system.
A code P0138 may mean that one or more of the following has happened:
Faulty O2 sensor
Short to battery voltage in O2 sensor signal circuit
High fuel pressure (not as likely)
Here are some potential solutions:
Replace O2 sensor
Repair short to battery voltage in o2 sensor signal circuit