SCR (Selective Catalytic Reduction) is the technology with which Euro 6 diesels reach NOx limits. By injecting urea (AdBlue) upstream of a specific catalyst, the NOx are converted into nitrogen + water.
Components
- AdBlue tank (10-20 liters typically)
- SCR pump
- SCR injector (DEF nozzle)
- SCR catalyst (zeolite)
- NOx sensors (one or two)
- AdBlue quality sensor
Problems and costs
The SCR is the most problematic system in modern diesels. Replacements:
- SCR Pump: €600-€1500
- NOx Sensor: €400-€800 each
- SCR Catalyst: €1500-€4000
SCR Off
The SCR off via software disables the pump, sensors and counter. Illegal road use (NOx out of limit). For export/track it is a common request.
How to reduce NOx
In the SCR catalyst the ammonia released by the AdBlue reacts with the nitrogen oxides transforming them into nitrogen and water vapour. The ECU calculates the quantity of urea to be injected based on temperature, load and the NOx sensors placed before and after the catalyst, maintaining the optimal dosage to leave no NOx or unreacted ammonia.
Reliability and tuning
The SCR is among the systems most subject to failure on recent diesels: NOx sensors, pump and urea injector have high replacement costs. Software deactivation (SCR off), which turns off the pump, sensors and meters, is required for export and track but is illegal on the road because it eliminates the reduction of NOx. In a legal remapping the SCR dosage must be left functioning and consistent with the new calibration.
Hidden costs
The SCR is one of the heaviest expense items in the maintenance of recent diesels: NOx sensors, pump and catalyst have high costs and relatively frequent failures. Knowing these costs helps the customer make informed choices. The legal route remains to repair the system; software deactivation, although required, is only permitted for export or track use, because on the road it eliminates the reduction of NOx and invalidates the approval.