SCR efficiency explained

Part 1

What is SCR efficiency and why does it fall?

Selective catalytic reduction uses ammonia derived from injected AdBlue to convert nitrogen oxides into nitrogen and water. The control system assesses conversion under defined operating conditions, including suitable exhaust temperatures. Low measured efficiency can result from inadequate dosing, unsuitable or contaminated fluid, exhaust leaks, misleading sensor readings or catalyst deterioration. A fault code alone does not distinguish these causes.

• An engine-management or AdBlue warning may appear.
• Some vehicles apply a restart countdown or performance restriction, depending on the fault and calibration.
• Crystallisation around the dosing injector can indicate a leak or dosing problem but needs inspection.
• Unusual fluid consumption can support diagnosis but is not proof of catalyst failure.

Part 2

AdBlue Delete and Removal: what the terms mean

Delete and Removal usually describe software deactivation of SCR functions. Repair aims to restore the system's operation, while Fix is a broad description whose actual scope should be explained. Deactivation does not restore catalyst efficiency or prevent physical component failures. For an efficiency fault, begin with diagnosis rather than selecting an intervention solely from the dashboard warning. Confirm what work can be completed during mobile attendance; some faults may require workshop facilities or recovery.

Part 3

UK vehicle examples and fitment checks

SCR-equipped Peugeot Partner and Citroen Berlingo 1.6 BlueHDi versions can record efficiency faults, but P20EE alone does not establish an integrated tank or pump failure. SCR-equipped Mercedes-Benz Sprinter and E-Class diesels can also develop dosing, sensor or catalyst faults; fitment varies by engine and production date. Some SCR-equipped Land Rover diesels, including Range Rover Evoque variants, may display a diesel exhaust fluid quality warning. That message does not by itself prove poor fluid quality or a failed catalyst. These model names span different emissions systems, so send your registration to help confirm your vehicle's specification.

Part 4

Diagnostic considerations and fault codes

Many SCR systems use upstream and downstream NOx sensors, but layouts and diagnostic strategies vary. Efficiency monitoring considers operating conditions and expected conversion, not simply whether one reading is lower than the other. A useful assessment includes all stored codes and freeze-frame data, sensor plausibility, exhaust temperature, dosing pressure and delivery, fluid condition, wiring and exhaust leaks. Manufacturer-specific descriptions and test procedures should guide the diagnosis.

• P20EE: SCR NOx catalyst efficiency below threshold, bank 1. This does not by itself condemn the catalyst.
• P204F: reductant system performance, bank 1. This is a broader system-performance fault, not a specific component diagnosis.
• P207F: reductant quality performance. Check fluid condition and system operation rather than assuming contaminated AdBlue.

Part 5

What to send before an assessment

Accurate vehicle and symptom details help narrow the checks needed. If the vehicle shows a restart countdown, report its exact wording and remaining distance; do not assume a refill or code clear will cancel it. Follow the vehicle handbook's warning instructions.

• Registration and, if requested, VIN to help confirm engine and SCR specification.
• Current postcode to check mobile availability.
• Photos of warnings and a complete fault-code report where available.
• Mileage, recent repairs, refill history and any leaks or visible crystallisation.