SCR System Fault Explained: Causes, Symptoms and Solutions
SCR System Fault Explained: Causes, Symptoms and Solutions
What Is an SCR System Fault?
An SCR System Fault is one of the most common emissions-related warnings displayed on modern diesel vehicles. SCR stands for Selective Catalytic Reduction, a system designed to reduce harmful nitrogen oxide (NOx) emissions through the controlled injection of AdBlue into the exhaust system.
When the vehicle detects that the SCR system is not operating correctly, it may display an SCR System Fault warning and store one or more diagnostic fault codes.
An SCR System Fault should not be ignored, as it can eventually lead to vehicle restrictions and no-start countdown warnings.
What Causes an SCR System Fault?
Several issues can trigger an SCR System Fault, including:
- Poor AdBlue quality
- Contaminated AdBlue
- Crystallisation within the SCR system
- Failed NOx sensors
- AdBlue injector faults
- AdBlue pump faults
- Wiring faults
- SCR catalyst efficiency issues
- Exhaust leaks
- Software-related issues
The exact cause varies between manufacturers and vehicle models.
Common SCR System Fault Symptoms
Drivers may experience:
- SCR System Fault warning messages
- Engine management light illuminated
- AdBlue warning lights
- Reduced engine performance
- Increased emissions
- Dashboard countdown warnings
- No-start messages
- Multiple emissions-related fault codes
In many cases the warning appears before any noticeable change in vehicle performance.
Which Vehicles Commonly Experience SCR System Faults?
SCR System Fault warnings are regularly reported on:
- Mercedes Sprinter
- Mercedes Vito
- Ford Transit
- Volkswagen Crafter
- Peugeot Boxer
- Citroën Relay
- Renault Master
- Iveco Daily
- DAF Trucks
- MAN Trucks
Most modern diesel vehicles equipped with AdBlue technology can experience SCR-related faults.
Can Poor AdBlue Cause an SCR System Fault?
Yes. Poor-quality or contaminated AdBlue is one of the most common causes of SCR System Fault warnings.
AdBlue must meet strict quality standards. If contamination occurs, the SCR system may struggle to reduce emissions correctly, triggering warning lights and fault codes.
Common contamination sources include:
- Water ingress
- Dirty containers
- Incorrect storage
- Mixing with other fluids
Even minor contamination can affect SCR performance.
Can NOx Sensors Cause an SCR System Fault?
Yes. NOx sensors are critical components within the SCR system.
These sensors monitor emissions levels and provide feedback to the engine control unit. If a NOx sensor fails or provides inaccurate data, the vehicle may display an SCR System Fault warning.
Common related fault codes include:
- P2200
- P229F
- P20EE
Accurate diagnosis is essential before replacing parts.
What Fault Codes Are Associated With an SCR System Fault?
Common SCR-related fault codes include:
- P20EE
- P204F
- P207F
- P2200
- P229F
Many manufacturers also use their own specific emissions-related codes alongside these generic OBD fault codes.
Can an SCR System Fault Cause a No-Start Countdown?
Yes. Many manufacturers use countdown systems to enforce emissions compliance.
Drivers may see warnings such as:
- No Start In 500 Miles
- No Start In 200 Miles
- Engine Start Prevented
- AdBlue System Fault
If the fault remains unresolved, the vehicle may eventually refuse to start.
How Is an SCR System Fault Diagnosed?
A professional diagnosis may include:
- Reading fault codes.
- Checking live diagnostic data.
- Testing NOx sensors.
- Verifying AdBlue quality.
- Inspecting the injector and pump.
- Checking SCR catalyst performance.
- Inspecting wiring and connectors.
- Checking for exhaust leaks.
A systematic approach helps identify the root cause and avoid unnecessary component replacement.
How Blue Resolve May Help
Where an SCR System Fault is linked to contamination, crystallisation or deposits within the AdBlue system, Blue Resolve may help restore normal SCR operation.
However, faults caused by failed sensors, damaged wiring, defective pumps or catalyst failure will generally require conventional repair.
Preventing Future SCR System Faults
The likelihood of future SCR faults can often be reduced through:
- Using quality AdBlue
- Preventing contamination
- Investigating warning lights early
- Performing regular diagnostics
- Maintaining the emissions system
- Addressing minor faults before they escalate
Early diagnosis remains the most effective way to avoid costly repairs.
Frequently Asked Questions
Is an SCR System Fault serious?
Yes. While many vehicles remain driveable initially, unresolved faults can lead to operating restrictions and no-start warnings.
Can I clear an SCR System Fault myself?
The warning may return if the underlying fault remains present.
Is an SCR System Fault always caused by AdBlue?
No. Sensors, injectors, pumps, wiring and catalyst issues can also trigger SCR-related faults.
Can an SCR System Fault affect an MOT?
Active emissions faults may affect MOT compliance depending on the vehicle and severity of the issue.