The Oxygen Sensor (O2) inputs are used by the PCM (Powertrain Control Module) to calculate the oxygen content in the exhaust. These sensors have a heating element that allows the engine to enter the closed circuit in less time. In this way, cold starts are diminished.
The PCM constantly monitors the circuits of that heater to check for out-of-range voltages, or in some cases abnormal amperages. The PCM activates the heater under various circumstances. If a low voltage condition is found in the circuit, the DTC code P0057 OBDII will be set. This code refers to Oxygen Sensor number 2 found in bank 2.