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Reductant escaping from the tank, its lines or their fittings, so the system loses fluid it believes it still has. The leak is distinctive and easy to identify once you know what to look for: the fluid dries to a white crystalline crust, so the deposit marks the leak path even when nothing is wet.

It produces a level falling faster than dosing explains, which is the diagnostic signal, and eventually the low-level inducement sequence with a tank that was recently filled. Consumption that does not match distance travelled is worth checking against the leak before the dosing system, since an over-dosing fault and a leak both consume fluid.

Where it leaks is predictable. The lines run under the vehicle and are exposed to salt, stones and frost; the fluid expands as it freezes, so a line or a full tank can split over a cold night. Fittings disturbed during other work are the other route. Clean the crystalline residue away after repair — it is mildly corrosive to some metals, it will hide the next leak, and leaving it makes it impossible to tell whether the repair worked.