Exhaust and emissions
What is AdBlue (DEF) and what happens if it runs out?
AdBlue, called DEF (diesel exhaust fluid) in North America, is a 32.5 % urea solution in deionized water that diesels with an SCR system inject into the exhaust to turn nitrogen oxides into nitrogen and water vapor. If it runs out, the car warns you well ahead with a countdown, and when it reaches zero it may not let you restart until you refill it.
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How does the SCR system work?
A pump carries the fluid from its tank to an injector in the exhaust. With the heat, the urea turns into ammonia, which reacts with the NOx in the SCR catalyst.
NOx sensors before and after the catalyst check that it works. If efficiency drops, the control unit stores a code such as P20EE.
How much is used?
It depends on the engine, the load and how you drive: the instrument cluster or the manual shows the range. In cars it is usually topped up between services; in trucks and vans, more often.
How do you refill it correctly?
Use fluid that meets ISO 22241, in its own tank (usually with a blue cap), never in the diesel tank. Some cars need a minimum amount to recognize the refill and take a moment to update the level.
If you put DEF in the diesel tank, or the other way around, do not start the engine and call for assistance.
What happens in the cold?
The fluid freezes at about 12 °F (−11 °C). The system is heated and the fluid is usable again once thawed, but in severe cold pump, heater and crystallization faults become more frequent.
General guidance only: every manufacturer and model has its own details. Always follow your vehicle's manual and, if in doubt, ask a workshop.
Related
Related trouble 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
- P2200NOx sensor (bank 1, sensor 1), circuit: malfunction
Related symptoms
Learn more
See what your car is saying
With the ECO-LITE, your workshop sees the SCR system's codes remotely and, if the car publishes them, the fluid level and temperature and the NOx readings.
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Frequently asked questions
Can I use water instead of AdBlue?
No: it damages the system and the car detects it as poor quality.
Does AdBlue expire?
It degrades with heat and time. Keep it closed, in the shade, and follow the date on the container.
Do all diesels use AdBlue?
No. Only those with SCR, which are increasingly common on recent diesels, vans and trucks.