P0135 Fault on 2008 Nissan Pathfinder 2.5 dci: Sensor Issues, Wiring Problems, and Power Supply Concerns

Hi everyone, I'm currently experiencing a persistent P0135 fault on my 2008 Nissan Pathfinder 2.5 dCi. My mechanic has already replaced two of the three oxygen sensors, but the code continues to appear. After that, I took the vehicle to a Nissan dealership who cleaned the mass air flow sensor — however, the P0135 code returned. They performed sensor testing and confirmed all sensors are functioning properly. At this point, they’ve advised it might be due to a wiring short, an electrical connection issue, or even a potential ECU fault. The front oxygen sensor has two wires, and I recently tested the power supply — only 6 volts is being delivered to the sensor. My question is: should the oxygen sensor typically receive 12V? Any insights or troubleshooting steps would be greatly appreciated. Thanks in advance for your help!