AdBlue is a urea solution that the SCR system injects into the exhaust of Euro 6 diesels to reduce nitrogen oxides. When the system has a fault the car signals an error, reduces power and counts the remaining starts: past the limit the engine will not restart.
How the SCR system works
SCR stands for Selective Catalytic Reduction. A pump draws AdBlue from the tank and an injector sprays it into the exhaust, upstream of the SCR catalyst; there the ammonia released turns NOx into nitrogen and water vapour. A downstream NOx sensor checks the result is as expected. Average consumption is of the order of 1 litre per 1,000 km, varying by model and use.
Tank, range and cold
- The AdBlue tank typically holds between 10 and 25 litres, depending on the model: the car warns when about 2,000-2,500 km remain.
- AdBlue freezes at around -11 °C: systems have a heater, and in the cold activation can take a few minutes. It is not a fault.
- Topping up is done at the dedicated filler (blue cap), with the engine off: it never goes into the diesel tank.
Most common errors and messages
- "Repair AdBlue system" / "Emission control system fault"
- Countdown of remaining starts or kilometres
- Reduced power and engine light
- "AdBlue running low": low level, not a fault
More in AdBlue error and power reduction and faulty AdBlue pump.
The most frequent faults
| Component | Typical symptom |
|---|---|
| NOx sensor | Efficiency error, countdown of starts |
| AdBlue injector | Urea crystallisation, poor dosing |
| Pump / tank module | Insufficient pressure, noises, pump error |
| Line / tank heater | Errors only in the cold |
| Quality/level sensor | Signals contaminated AdBlue or wrong level |
| SCR catalyst | Persistent low efficiency, rare |
Indicative costs
| Component | Indicative cost |
|---|---|
| NOx sensor / injector | €150-400 |
| Filter or line | €50-200 |
| Pump / tank module | €600-1,500 |
| SCR catalyst | over €2,000 |
Full picture: how much it costs to repair the AdBlue system.
If the message appears: what to do, in order
- Read the message: if it talks about low level, top up and carry on; if it talks about a "fault", note the starts or km remaining.
- Do not wait for the count to reach zero: after the lock the car will not restart.
- Avoid overfilling or using products of doubtful origin.
- Have the errors read: diagnosis tells NOx sensor, pump, injector and heater apart.
Topping up and consumption
- Use AdBlue complying with ISO 22241
- Do not store it for months or expose it to heat and direct sun
- Do not pour it into the diesel tank (or vice versa)
- Wipe up spills: urea crystallises and can ruin plastics and connectors
What the law says
The SCR system is an approved emission control device and disabling it is not permitted on vehicles driven on the road. It is reserved for competition, off-road or export vehicles. On the road the solution is to repair and restore operation.
How MasterTuning helps
We read the real values of the control unit, not just the code: that is the difference between finding the faulty component and replacing parts by trial and error. After the repair we clear the error and check that it does not come back. Find the nearest centre or contact us.
Is the AdBlue message on? Book a diagnosis at the nearest MasterTuning centre.
Frequently asked questions
What is AdBlue?
A 32.5% urea solution in demineralised water that the SCR system injects into the exhaust of diesels to reduce nitrogen oxides.
What happens when AdBlue runs out?
The car signals low level and, if you do not top up, limits power and then will not start. After topping up, the system resumes working.
What does the AdBlue error with starts remaining mean?
The system has detected a fault and is counting the starts allowed before the lock. It must be repaired before it reaches zero.
How much does it cost to repair the AdBlue system?
From €150-400 for a NOx sensor or injector to €600-1,500 for the pump; the SCR catalyst exceeds €2,000.
How much AdBlue does it use?
On average about 1 litre per 1,000 km, varying with model, load and driving style.
Can AdBlue be eliminated?
Not on road vehicles. Disabling is allowed only for competition, off-road or export vehicles.
