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One of the heating elements keeping reductant liquid in the lines, failed or weakened. Reductant freezes at about −11°C, so the tank, the supply line, the return line and the metering unit are each heated separately — and the system is only as good as its weakest heater, because one frozen section breaks the path from tank to exhaust and stops dosing as completely as four would.

Line heaters fail more often than the tank heater, and the reason is exposure: a tank holds a large volume in an insulated container, while a line is a thin hose running the length of the underbody in road spray and freezing wind, losing heat continuously while it works.

Their construction decides the repair. The element is resistive wire wound along the hose and moulded into the assembly, so a failed line heater usually means replacing the entire heated line — and those are surprisingly expensive, which is why the connector and the shared supply should be eliminated first.

Several heater codes at once point at a shared supply, earth or control module rather than several failed elements. The faults are seasonal and silent: nothing happens in warm weather, and the first hard frost starts the inducement countdown towards a vehicle that will not restart. Repair before winter, not during it.