Mpg improvement after NOX sensor replacement

[Tuctuc]

Hi,
I've just had the nox sensor replaced after an emissions fault, and noticed the mpg has improved from 49mpg to 54mpg.
I did notice on the leadup to the failure the mpg dropped to 46mpg but then came back to 49mpg, so I wonder if this was a warning sign of impending failure of the sensor 

Any idea why this improvement would happen?

[Sol]

No idea, it should have no effect. NOx has no bearing on fuelling.