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The sensor checking that the fluid in the tank really is reductant of the right concentration. It exists because the system cannot work with anything else and because the consequences of the wrong fluid are expensive — diesel or screenwash in the tank destroys the pump, the injector and eventually the catalyst.

A quality fault and genuinely poor fluid look identical from the driver's seat, and both trigger the same staged inducement toward a no-start. So establish which you have before ordering anything: the fluid can be checked independently with a refractometer, which reads concentration directly and settles the question in a minute.

Concentration drifts on its own, which is worth knowing. Reductant is a precise solution and it degrades with heat and age, so fluid stored through a hot summer or long past its date genuinely fails a quality check. It is also frequently a combined level, temperature and quality unit, so a single failure produces several unrelated-looking codes — a cluster naming all three usually means one part rather than three faults.