W211 om642 2028-4 fault code.

[boxer 01]

I am getting the following fault code on my w211 e320cdi with the om642 engine.
2028-4 stored and current. Pump current (g3/2 (O2 sensor upstream of kat )) of oxygen sensor open circuit. Car has just had a reconditioned hp fuel pump fitted and is running well. What is at fault with this code, is the O2 sensor and why does it say pump current, is it something to do with the fuel pump. Any help would be appreciated.

 

[Supernoodle]

The pump current is effectively the UHEGO (O2) signal that is proportional to the oxygen concentration and is what is measured by ECU.
If that is open circuit either the sensor or wiring is bad.

 

[boxer 01]

Thankyou for enlightening me on the workings of the O2 sensor. I will assume the fuel pump is fine and will look into replacing the sensor.