[MrBean]

Here is my take and solution on this warning....

I have owned my Jag XE (180ps, auto, Prestige) for 2 and a bit years and have seen this issue twice on a yearly basis (and once more) the series of events are:

1) Dashboard warning: incorrect fluid detected
2) Drive about 60 miles
3) Dashboard warning changes to: NOx malfunction, 512 miles no engine restarts
4) Fill up with any 10lt forecourt Adblue.
5) Clear fault codes using OBD scanner, note at this point, the "NOx malfunction, 512 miles no engine restarts" warning on the dash is still present. At this stage it is reporting a lower mileage based on the miles I have done since fault first appeared, I.e. it appears the fault has not been reset
6) I continue driving, after about another 100 miles, the fault code goes away. This happens every time. Never changed parts.

My conclusion is that the software (not sure about the newer cars) is very sensitive and gets out of sync after the initial code.

I think for the garages, after resetting the ecu they don't see the fault on the dash go away they start more drastic measures. In my opinion, asking the garage to hold fire at this stage for a 100 miles or so would save swapping out parts and by the sounds of it a lot of cost.

In terms of the give it '100' miles or so after ecu reset to clear codes is either because the software needs a number of ignition cycles to validate the code has gone. Or needs to go through a regen. Not sure the exact logic.

Why every year: my theory is the Adblue starts to crystallise the less there is. Also the percentage of actual urea left in the mix diminishes. So the warning is probably genuine. However, the software does not clear the fault even when the NOx is compliant (I.e. after a refill). It seems to stay in a permanent 'false positive' state after the initial fault is flagged. And only a ecu reset and annoyingly some 100 miles / regen / ignition cycles will change this.