Why Does AdBlue Crystallise?
One of the most common problems we see with AdBlue systems is crystallisation.
When AdBlue dries, it can leave behind urea crystals.
These deposits can build up around the AdBlue injector, exhaust components and pipework.
If enough crystallisation occurs, it can restrict the flow of AdBlue or prevent the system from dosing correctly.
This can eventually result in an AdBlue warning light, engine management light or SCR fault code.
AdBlue also has specific temperature requirements.
It can freeze at approximately -11°C, which is why modern vehicles are fitted with heating systems to warm the AdBlue and associated components when required.
What Does the AdBlue Pump Do?
The AdBlue pump supplies the system with the pressure required to deliver AdBlue to the injector.
The exact pressure varies depending on the manufacturer and system design.
If the pump cannot produce or maintain the required pressure, the vehicle may detect an AdBlue system fault.
However, a pressure fault doesn't necessarily mean the pump itself has failed.
The problem could also involve:
- Restrictions or crystallisation
This is another reason why replacing a pump without testing the system first can be an expensive mistake.
What Does the AdBlue Injector Do?
The AdBlue injector doses the required amount of AdBlue into the exhaust system upstream of the SCR catalyst.
Because the injector is exposed to the exhaust environment, it can be affected by crystallisation and deposits.
This has led to a common misconception:
“I've got an AdBlue fault, so I need an AdBlue injector.”
Not necessarily.
The injector is only one part of a much larger system.
At Fresh Motors, we test the system before recommending replacement parts.
NOx Sensors and AdBlue Faults
NOx sensors are another common source of problems on modern diesel vehicles.
These sensors monitor nitrogen oxide levels in the exhaust and provide information to the vehicle's emissions control system.
Depending on the vehicle, there may be NOx sensors positioned before and after the SCR catalyst.
A faulty NOx sensor can provide incorrect information to the vehicle.
This can result in an SCR efficiency fault even though other components may be functioning correctly.
We regularly see NOx sensor-related faults on certain BMW and Mercedes-Benz applications.
However, NOx sensor faults can occur on many different manufacturers.
What Is P20EE?
P20EE is one of the most common fault codes associated with SCR systems.
The code generally refers to:
P20EE – SCR NOx Catalyst Efficiency Below Threshold
In simple terms, the vehicle has detected that the SCR system isn't reducing NOx emissions as effectively as it expects.
But this is extremely important:
P20EE does not automatically mean that the SCR catalytic converter needs replacing.
There are several possible causes.
For example:
- Exhaust temperature problems
- Wiring or electrical faults
- Other emissions-system faults
This is why a P20EE fault code should be diagnosed rather than guessed at.
P20EE Fault Code on Peugeot, Citroën and Vauxhall
At Fresh Motors, we see a large number of AdBlue and SCR faults on Peugeot, Citroën and Vauxhall vehicles.
These vehicles can develop faults involving the SCR system, AdBlue delivery system, NOx sensors and SCR catalyst.
If you have an AdBlue warning, engine management light or P20EE fault code, replacing components without proper testing can become very expensive.
A correct diagnostic process can help identify which part is actually responsible.
BMW and Mercedes AdBlue Faults
BMW and Mercedes-Benz vehicles can also experience AdBlue and SCR-related problems.
One issue we regularly encounter on certain vehicles is NOx sensor failure.
A faulty NOx sensor can cause the vehicle to receive incorrect information about exhaust emissions, potentially resulting in SCR-related fault codes.
Again, the important point is that the fault code is not necessarily the failed component.
Proper testing is required to determine what is actually happening.
How We Diagnose AdBlue Faults at Fresh Motors
When a vehicle arrives with an AdBlue problem, we don't simply clear the fault code or replace the first component that appears suspicious.
We use specialist diagnostic equipment and live data to investigate the system.
Depending on the vehicle and fault, our diagnostic checks can include:
- AdBlue injector operation
- Wiring and electrical signals
- Manufacturer-specific testing procedures
The objective is to establish why the vehicle has detected a problem.
This approach helps us avoid unnecessary parts replacement.
Why AdBlue Fault Diagnosis Can Be Difficult
Modern diesel emissions systems are complicated.
The vehicle may know that the expected NOx reduction isn't being achieved, but it may not know exactly which component is responsible.
This is why two vehicles displaying the same fault code can require completely different repairs.
For example, two vehicles may both display P20EE.
One could have a NOx sensor problem.
Another could have an AdBlue dosing issue.
Another could have a pressure problem.
Another could genuinely have an SCR catalyst problem.
The fault code is therefore only one piece of the puzzle.
What Is the Most Common AdBlue Repair Mistake?
One of the biggest mistakes we see is replacing the AdBlue injector without testing the system.
Because the vehicle has an AdBlue fault, the injector is often blamed immediately.
But replacing an injector won't fix the problem if the actual cause is a NOx sensor, pump, pressure issue, wiring fault or SCR catalyst problem.
This can result in customers spending hundreds or even thousands of pounds replacing parts unnecessarily.
Our advice is simple: diagnose the fault before replacing components.
Can You Delete the AdBlue System?
We regularly get asked:
“Can you do an AdBlue delete?”
Fresh Motors does not offer AdBlue delete services.
The SCR system is part of the vehicle's emissions-control system. Disabling or manipulating the system can increase NOx emissions and may create legal, MOT and insurance issues depending on the vehicle and modification.
Our approach is to diagnose and repair the original system properly.
Should I Use Aftermarket AdBlue Parts?
There are many aftermarket AdBlue and emissions-system components available.
However, our experience has shown that quality and compatibility can vary significantly between suppliers.
Electronic components such as NOx sensors can be particularly sensitive to specification and compatibility.
A cheaper component that fails shortly after installation isn't necessarily cheaper in the long run.
For this reason, where appropriate, Fresh Motors recommends genuine manufacturer-supplied parts.
The correct component, correctly fitted and configured, can help prevent repeat failures and unnecessary additional diagnostic costs.
AdBlue Warning Light: What Should You Do?
If your vehicle displays an AdBlue warning light, don't ignore it.
Depending on the vehicle and the nature of the fault, you may eventually receive a countdown warning telling you that the vehicle may not restart if the problem isn't resolved.
You may also see:
- “AdBlue system fault” message
- “Emissions fault” message
- P20EE or other SCR fault codes
The sooner the problem is diagnosed, the easier it can be to determine what is actually happening.
AdBlue Fault Diagnosis at Fresh Motors
At Fresh Motors, we have experience diagnosing AdBlue faults, SCR faults, NOx sensor faults and diesel emissions problems across a wide range of manufacturers.
We don't believe in simply replacing parts until the warning light disappears.
We use specialist diagnostic equipment, live data and our experience with common manufacturer-specific failures to establish the underlying problem.
Whether your vehicle has a P20EE fault code, AdBlue warning light, NOx sensor fault, SCR efficiency fault or another emissions-system problem, proper diagnosis is the first step towards the correct repair.
Looking for AdBlue Repair?
If you're searching for AdBlue fault diagnosis, AdBlue repair, SCR fault diagnosis, P20EE diagnosis or NOx sensor diagnostics, Fresh Motors can help.
Don't guess which part has failed. Get the system properly diagnosed first.
Fresh Motors — specialist diagnostics and repairs for modern diesel vehicles.