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Selective Catalytic Reduction

Damien Coleman
Snap-on® Diagnostic Software Specialist 

Maximising Engine Performance with Selective Catalytic Reduction 

Take a look into some issues you need to be aware of when it comes to dealing with selective catalytic reduction systems, AdBlue and NOx in vehicle emissions.

Modern diesel vehicles produced by Mercedes-Benz balance maintaining vehicle performance with meeting specific environmental standards with BlueTEC technology.

BlueTEC is a combination of technologies which includes a selective catalytic reduction (SCR) system that utilises a urea solution (AdBlue) to further reduce the amount of nitrogen oxides (NOx) in vehicle emissions.

It can be a complex area to work on when it comes to diagnosing problems but Snap-on’s ZEUS+ diagnostic scan tool offers you support and guidance to find and solve issues.

The SCR process is a post-treatment in that it operates after the exhaust gas has left the combustion chamber, which allows the engine to run at its full power potential rather than systems that solely rely on re-introducing exhaust gas (EGR) to reduce combustion temperature, with an inevitable reduction in performance.

A standard catalytic converter can remove some NOx emissions but not enough to meet the latest Euro 6 regulations, so Mercedes has utilised an SCR system using AdBlue with its BlueTEC technology to ensure that no power is lost.

AdBlue is a non-toxic, colourless, odourless and non-flammable aqueous solution consisting of refined urea and de-ionised water pre-mixed to a set percentage level regulated by the DIN70070 standard.

The Mercedes-Benz BlueTEC with SCR system injects AdBlue into the engine’s exhaust gases, which enter the SCR Catalytic Convertor to cause a chemical reaction that then converts up to 90 per cent of the nitrogen oxides in the exhaust into harmless nitrogen and water vapour.

AdBlue needs to be topped up every year/service interval and if a vehicle’s supply does run low, a warning message will alert the driver that the vehicle can only be driven for a specified number of miles before AdBlue completely runs out – at which point the engine will not start.

Common issues in AdBlue systems include air supply valve problems, a blocked exhaust injector, and complications with the temperature and NOx sensors (pre- and post-cat).

If AdBlue pressure is low, that could be caused by the pump or an air supply issue – diagnosing one of those problems when the cause of the low pressure is actually the other is the most common misdiagnosis made when dealing with AdBlue systems.

The latest diagnostic software from Snap-on offers a wide range of codes and data that will help you get to the fix quickly.

Special functions relating to AdBlue in the software package include a visual leak test, a reset of the calculated fill level, resetting of the adaptation values of the metering valve and the delivery pump, and calibration of the fill level sensor.