NOx Sensor - Who has had trouble/replaced?

I have a March 2008 E92 325i N53. I purchased the car in November 2011 and had the NOx Sensor replaced and also had seen in the prior owners documentation that he also had it replaced under goodwill a couple of years earlier.

Looking on Realoem, I see the part has gone through a fair few revisions.

NOX sensor 11787565447 ENDED
NOX sensor 11787576908 ENDED
NOX sensor 11787580518 ENDED
NOX sensor 11787582327 ENDED
NOX sensor 11787587128 ENDED
NOX sensor 11787587129 CURRENT

I just had the injectors replaced via dealer and as this NOx showed in diagnostics again the dealer said they would contact BMW and see what they say as this would be the 3rd replacement. This is what the dealer emailed me:

"Bmw replied with regarding the NOX sensor fault, they have suggested running a number of checks on the car including a wiring check with the NOX and inspecting the Nox condition before proceeding any further, they have recommended once doing the checks then reporting back our findings if any before they can give us reasons why the nox has shown faults."

Why do you make of the above? Feels like possible extra cost if no goodwill at the end?

I asked the dealer for the best price on the part to look at just getting it replaced however while they would do it they are saying "because this has been replaced on 2 other occasions I would not recommend on replacing until we do all the other checks that is being recommended, we do not replace a great amount of these, in 2013 we replaced in total 4 Nox sensors on this model. It is an expensive part to replace without doing the checks."

So if I do the checks I potentially have the cost of checks, NOx sensor and diagnostics or I could just replace the sensor?

Interested in peoples experiences and advice?

I've had a related issue with an 07 320i recently...still haven't resolved it.

I started a thread on it here

http://www.bmwhaus.ie/forum/showthread.php?tid=3350

I know the sensor's give trouble but this will be the 3rd sensor and the car only has 87k miles so an average of a new sensor every 30k miles. I am trying to find out if they are that bad or this seems outside the norm?

It's a tough one, our 320i only has 45k miles and the timing chain had to be done, so there
doesn't seem to be a set time for other things to give trouble either.

Won't be doing anything until nct time, car isn't really being driven at the moment.

Keep us updated

Has the MPG dropped or not?
I would not be too worried if MPG has not changed and engine feels the same as usual.

I don't think MPG has dropped anyway significantly that I've noticed.