My AdBlue light is on. What do I do?

The AdBlue lamp is not a fuel gauge. On a modern diesel the engine ECU watches the whole SCR system, the tank sensors, the heater, the pump, the injector and two NOx sensors, and lights the lamp when any of them stops adding up. Low fluid is one reason. A failed sensor, a heater that died on the first frost, a crystallised injector or a pump that cannot hold pressure are the others, and from the driver's seat they look identical.

First, read the message

Every make phrases it differently and the wording tells you a lot. "Add AdBlue" or "Refill AdBlue" with a range means the car believes the tank is low. If you have just filled it and the message is still there after a drive, the car does not believe the sensor, and that is a fault, not a level. "AdBlue system fault", "Emissions fault" or a spanner symbol means it has already decided it is a fault. "No engine start in X miles" means the countdown has begun, whichever caused it.

Second, note the miles

If there is a countdown, that number is how far you can still drive before the car refuses to start after being switched off. On most VW group cars it steps from 1,500 to 1,000 to zero with the lamp changing colour on the way. Mercedes and others give a single figure. At zero the car does not become dangerous; it simply will not restart, and it stays that way until the fault is fixed. It does not reset on its own and it does not reset by disconnecting the battery.

Two real dashboards from Kent customers. A Skoda reading "No engine start in 350 mi" and a Mercedes at "No start in 47 mi". Both had full tanks, and both turned out to be sensors.

Third, get the codes read before you spend anything

A generic scanner will give you a code beginning P20 or P22: P203F is the level sensor, P20E8 the pressure, P204F the dosing performance, P20EE the catalyst efficiency, P229F a NOx sensor. The code tells you which part of the system complained; it does not tell you which part failed, because one code can have several causes. But it does tell you whether a top-up is even worth trying, and it stops a garage replacing a pump for what was a sensor fault.

A cheap reader, or whoever reads it for you, will give you the code. Send it to us with the message and we will tell you which part of the system it points at and what your two options are.

What fixes it

One of two things. A repair, where the failed part is found and replaced, the code cleared and the system proven working on a road test, so the car stays as it was built. Or a software delete, which switches the SCR system off in the ECU so the warning, the countdown and the limp mode stop for good. The delete is done on your driveway by us, priced from the registration. A repair means the car going to JF Automotive in Maidstone, the workshop we are part of, where it is diagnosed on the ramp and fixed. Both are priced before any work starts. The legal position is here, and the choice is yours.

Can I keep driving it?

Usually, for now. The car may drop into reduced power. An illuminated emissions warning is an MOT failure, and if a countdown is running you are on a clock, so do not leave it to the last fifty miles. A car that will not start in a supermarket car park means a recovery bill on top of the repair. Driving to have the emissions equipment repaired is one of the lawful uses set out in the regulations.