Nox sensor

[Jerry]

I have a NV300 from 2018 with 145 dci engine.

Have the fault message on the display 

"Check anti pollution system" 

When I connect with OBD2 it says

-fault value from Nox sensor "B" -

I have changed the sensor and removed the faulty code but it still comes back. I can start the engine 3-4 times before it get back. Are there several Nox sensors or what does the "B" stand for? 

Any ideas?

[Trevor]

The NOX sensor B is measuring against NOX sensor A (B is usually behind A in the exhaust system) and if there is an exhaust leak then this could cause the issue

[Jerry]

Thanks 

Do you happened to have a link were I can purchase the "B" sensor?

I can only find one type of Nox sensor for NV300.