Basically this diagnostic code refers to the second oxygen sensor in bank 1. It indicates that the voltage of said sensor remains low for a long time. The ECM (Engine Control module) interprets this reading as a low voltage condition and thus establishes the MIL light.
The second sensor is at the rear of the catalytic converter and has the objective of producing an output signal, which is relative to the oxygen storage capacity of the catalytic converter. Although this signal sent by the O2 sensor is less active than the one sent by the front sensor, when the Engine Control Module detects that the sensor is not active, this code is established.
Symptoms of fault code P0137 Generic
There can be no symptoms visible to the driver, except that: