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The heater that keeps the reductant tank above freezing has failed or weakened. Reductant freezes at about −11°C, so without it the emissions system simply cannot work in winter — the fluid cannot be drawn, pumped or injected.

Its faults are seasonal and completely silent until they are not. In warm weather nothing happens at all; the first sustained frost produces a vehicle that cannot dose, and the legally mandated inducement sequence begins counting down towards a refusal to restart. That is why this is repaired before winter rather than during it.

Establish how this vehicle heats the tank before diagnosing. Some circulate engine coolant through it via a module-commanded diverter valve — in which case a thermostat stuck open, an airlock or a stuck valve produces the fault, and the heater is innocent. Others use an electric element, which fails as elements do.

A performance code is often the temperature sensor rather than the heater, since the module judges the result rather than the element. And there is a legitimate non-fault: heating a full tank from well below zero takes a long time, so a vehicle doing only short journeys in a hard frost may never reach target with everything working.