Diesel: DPF, AdBlue and glow plugs

AdBlue warning light on: what it means and what to do

The AdBlue warning light means the tank level is low, or the system that reduces nitrogen oxides (SCR) has a fault or detects poor-quality AdBlue. If ignored, the car shows a countdown and may refuse to restart when it reaches zero.

See a garage soonDiesel: DPF, AdBlue and glow plugs

Updated on October 13, 2026

Reviewed by Daniel Morales, founder of Reprorace SL, on October 13, 2026

Is it serious? Can I keep driving?

Carefully, and only as far as the garage. You can drive, but deal with it soon: top up if it is the level and, if it is a fault, get to a garage before the countdown runs out. The engine does not stop on the move because of this, but it may not start again.

A careful, general answer: only someone who inspects the car can say whether it is safe to drive. When in doubt, stop somewhere safe and call for assistance.

Most likely causes

Low level: AdBlue is used up with driving and has to be topped up between services.
Poor-quality or contaminated AdBlue: the quality sensor or the NOx sensor detects that emissions are not being reduced.
AdBlue pump, injector or heater: crystallisation or electrical faults, especially in the cold.
NOx sensor: a faulty sensor gives false efficiency readings.
SCR catalyst: loses efficiency over time or with unsuitable AdBlue.

Listed from more to less common in general; on your car they may be different. A fault-code reading and a check at the garage confirm which one it is.

To understand it better: What is AdBlue (DEF) and what happens if it runs out? What is a particulate filter and how long does a regeneration take?

Car system: Exhaust and emissions

What you can check yourself

Read the dashboard message: whether it says low level and how many miles are left.
Top up with AdBlue that meets ISO 22241, in its own tank (blue cap), never in the diesel tank.
After topping up, leave the ignition on for a few minutes or drive a little, as the handbook says, so the system detects it.
If you see white crystals around the cap or the line, mention it at the garage.

Safe checks only: with the engine off and cold whenever you look under the hood, without taking anything apart and without touching hot or moving parts.

Related fault codes

P20EESCR NOx catalyst (bank 1, catalyst 1): efficiency below threshold
P204FReductant system (bank 1): performance
P20E8Reductant pressure: too low
P2047Reductant injection valve (bank 1, unit 1), circuit open
P20B9Reductant heater A, control circuit open
P2BADNOx limit exceeded: root cause unknown
P2200NOx sensor (bank 1, sensor 1), circuit: malfunction

Generic codes that often come with this symptom. Seeing one does not confirm the cause: it is where the diagnosis starts.

All fault codes

How the ECO-LITE helps diagnose it

Plugged into the OBD socket, the ECO-LITE reads the fault codes of the control units that answer, with the freeze-frame data captured when the fault happened, and shows live data on your phone or computer. With your permission, your garage can see them remotely before you bring the car in.

What to look at here: SCR system codes, AdBlue level and temperature and NOx sensor readings, depending on what the car reports.

See the ECO-LITE See it in the shop

Frequently asked questions

What happens if the AdBlue runs out?

The car warns in advance with a countdown; when it reaches zero, diesels with SCR may not restart until it is refilled.

What if I put AdBlue in the diesel tank or the other way round?

Do not start the engine and call for assistance: the system must be drained and cleaned to avoid damaging the pump and injectors.

I topped up and the light is still on. Why?

Some cars take a while to detect the new level. If it persists, the system has a fault: read the codes, such as P20EE or P204F.