Low reductant level

Not enough reductant fluid in the tank for the system to dose. It is the simplest cause in the aftertreatment and the one legislation makes most consequential, because a vehicle that cannot dose is required to warn, then derate, then refuse to restart — so an empty tank strands people in a way no other low fluid level does.

The warnings are staged deliberately and are worth explaining to a driver: an early notice at a level still giving hundreds of miles, escalating to a speed limit and finally to a no-start after the vehicle has been switched off. The inducement is a legal requirement rather than a manufacturer's choice, which is the answer to why it cannot simply be cleared.

Distinguish a genuinely low level from a level sensor that says so. A stuck float or a failed quality sensor reports low on a full tank, and refilling changes nothing. Look in the tank rather than at the gauge, and consider a leak if the level falls faster than dosing accounts for. Fluid quality matters as much as quantity — water, diesel or an out-of-date fluid put in the tank will produce quality faults that read very differently from a simple low level.