Originally Posted by ZetaTre

The O2 sensors in a diesel only marginally affects fueling, unlike the gasoline equivalent.

The primary reason that is there is for close loop feedback for the EGR operation.

By looking at it, it also seems like a wideband O2 sensor which would make sense given the very broad range of AFR diesel run. In other words, not needed. A traditional narrow band used in manifold injection engines won't give much readings

If you want to do the equivalent of replacing the O2 sensors in a diesel, pull the MAP sensor (mounted on the intake manifold) and give it a good clean since it can get dirty with soot and oil. I use throttle body cleaner. Note that with time the rubber grommet holding it hardens so after removal you may not have a good seal with the old grommet so consider buying one before hand. It's cheap.