Diagnostic code P0051 OBD2 refers to Oxygen Sensor (O2) number 1 which is located in bank number 2 opposite the 3-way Catalytic Converter (TWC). It should be noted that there is an Oxygen Sensor which is located on the back of the Three Way Catalyst (TWC) and corresponds to O2 number 2 in bank number 2.
The Oxygen Sensors (O2) detect the portion of oxygen found in the exhaust gases and compare them to the outside of the air. This information is sent to the Powertrain Control Module (PCM) who with this information fit the air-fuel ratio.
If the exhaust gas temperature is low, the function of the Oxygen Sensor is affected, which is why these sensors include a heating element, which is hastied to make the Oxygen Sensor readings more effective.
When this DTC code is set it indicates that the resistance of the heating element circuit is below normal. Generally, this resistance level is lower than 0.8 A for the code to be set.