How did topping up the AdBlue in my SEAT Alhambra ruin the NOx sensor?

My 2014 SEAT Alhambra has had a problem with the AdBlue warning light coming on regularly. Each time a warning said 'fill before 600 miles', I filled it. Even though it was completely full I got an orange warning light, so I took my car to a SEAT main dealer. They have said it is my fault because I filled the system too regularly and the AdBlue has crystallised meaning that the car needs to have a new NOx sensor fitted at a total cost of £1350. How did this happen? If it is my fault it would be good if manufacturers told their customers not to keep the AdBlue topped up. I drive the SEAT every day, all motorway driving and the mileage is 127,000 with car not even 3 years old yet. Please can you find out if AdBlue does crystallise. If it does what can be done to avoid it happening?
Never heard of that before. AdBlue is basically a urea solution, similar to urine. But a new problem with the AdBlue system in the EA189 Sharan and Alhambra that I've been made aware of in the last week is that the AdBlue heater fails, commonly after around 50,000 miles, and costs about £700 to replace.
Answered by Honest John on

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