Another treatise on NOx sensors here.
NOx Sensors
As said before these are essentially two lambda sensors one after the other. You will see from the diagram that the first cell's function is to remove all oxygenated species from entering the second part of the unit other than NOx so it can be accurately measured
Now previously I had said these can be badly affected by excess water in the exhaust so the exhaust stream had to be up to temperature before they were switched on but perhaps that was innaccurate maybe instead of water I should have said " oxygenated species" which would have included ethanol! Ethanol in petrol at 5% levels is pretty much a given in 95 octane in most petrol blends now. [so called E5 fuel possibly to be E10 =10% ethanol shortly!] One of its less desirable properties is its deliquescent -it absorbs water when exposed to air--- [ this is already knocking out scores of carburettors in small petrol driven appliances since its introduction!] so there is scope perhaps to include ethanol containing fuel as a possible culprit for early NOx sensor demise? What about diesel you ask it doesn't contain ethanol??? True --- all I can suggest there is the prolonged engine and exhaust warmup time of diesel exhausts compared to petrol may be a factor in those compression ignition engines.
