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Reductant that has degraded with age or heat, so its concentration is outside specification. It is a precisely made solution with a limited life, typically around a year, and it degrades faster stored warm or in sunlight.

The quality sensor detects it and the system responds as it would to the wrong fluid — warnings, then derating, then the inducement sequence toward a no-start. So old fluid produces exactly the codes of a contaminated tank, and the diagnosis has to distinguish degraded from wrong.

A refractometer reads concentration directly and settles it in a minute, which is faster and more certain than any code. Check the container's date and where it was stored — fluid bought in bulk and kept through a hot summer is a common cause, and a partly used container left open degrades faster still. Drain, flush and refill with fresh fluid rather than topping up, since diluting good fluid into bad leaves the concentration wrong.