To monitor the oxygen content in the exhaust of each engine bank and check the effectiveness of the catalytic converter (TWC), the car’s computer uses the data provided by the Oxygen (O2) Sensors. Each of these sensors is threaded into an exhaust pipe or manifold. In this way, the exhaust gases that are expelled from the combustion chamber pass over them.
The PCM monitors the negative current driver circuit of the O2 Sensor constantly, and if it fails to control the current, losing the voltage return through the corresponding wire, it recognizes this condition as an open circuit, so it will set the fault code P2251 OBD2.