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The exhaust fluid has frozen, so it cannot be dosed, measured or moved.

The number to know is minus eleven degrees Celsius, and it is not an accident — exhaust fluid is 32.5% urea in water precisely because that mixture has the lowest freezing point available, and the concentration the catalyst wants happens to be the concentration that survives winter best. Any vehicle sold in a cold climate will freeze its tank sooner or later, so this is a designed-for condition rather than a fault.

Which is why the hardware exists to deal with it. The tank and the lines carry heaters, usually fed with engine coolant, a resistive element, or both, and the module's job at start-up is to decide whether there is enough thawed fluid to dose. It also purges the lines back into the tank at shutdown, because fluid left in a pipe expands as it freezes and splits it.

So the fault is normal in the first minutes and abnormal after that. A code that appears on a frosty morning and clears once the vehicle has warmed through is the system working. One that persists on a warm engine points at the heating — a failed element, a coolant valve not opening, a temperature sensor misreporting, or a level so low that the heater is uncovered and cannot work.

Two practical points. A tank filled to the brim in freezing weather can crack it, since the fluid expands as it freezes; a container left in an unheated garage over winter and then poured in brings ice crystals with it. And thawing is slow — several litres of frozen fluid takes far longer to melt than a short journey provides, so a vehicle used only for short trips in a cold spell can carry this fault for weeks.