Selective Catalyst Reduction (SCR)


Selective Catalyst Reduction

The last component in the aftertreatment system is a flow through Selective Catalyst Reduction (SCR) catalyst which introduces Diesel Emissions Fluid (DEF) to the process. Diesel exhaust fluid is an aqueous urea solution made with 32.5% urea and 67.5% deionized water This fluid contributes to the further break down of nitrogen oxides that pass through to the SCR filter. The SCR filter generally doesn’t need maintenance EXCEPT in rare events where there is a component failure related to the DEF fluid.

The largest sector for use of SCR technology is medium to heavy-duty commercial vehicles although some European manufacturers have incorporated this technology into their passenger and light commercial vehicles to improve emissions above and beyond regulated standards.

Diagram of how a SCR works in conjunction with a DPF system.


How often should you clean your SCR? As an owner/driver of a Diesel-powered vehicle you need to clean the SCR regularly as this will save you money in operating costs and prevent costly downtime in the long run.

Maintenance schedules or cleaning cycles vary according to the type of vehicle and how your vehicle is being used.

To keep your SCR’s optimal operating performance, we suggest a clean at least every 12 months. Shorter timeframes may be needed if your vehicle is exposed to:

  • peak hour traffic at low speeds
  • numerous short stop/start trips (less than 20-minute trips)
  • extended periods where engine is at idle
  • If your vehicle has had a failure with fuel injectors, turbo, head gasket, EGR system malfunction/failure
  • Diesel Exhaust Fluid (DEF)has been contaminated
  • DEF tank is empty!
  • SCR injector nozzle or lines are blocked
  • If too much DEF is injected, the excess is deposited onto the catalytic substrate, where it can crystallise in a layer that prevents contact between the exhaust gas stream and the substrate, which obviously prevents the conversion process from taking place and causes blockages.
  • On the other hand, if too little reductant is injected, the available reductant cannot fully coat the catalytic substrate, which results in an inefficient conversion process.
  • An incorrect “performance tune” has been performed increasing combustion temperatures which in turn cause high exhaust gas temps damaging the catalytic convertor or causing the system to cease working altogether.

For vehicles that operate on the highway at least once a week or more for periods of at least 30 minutes at a time, we recommend a full SCR service and clean every 100,000km providing the below issues have not affected the operation of the SCR.

  • Poor fuel consumption
  • Noticeable decline in performance and torque
  • SCR light keeps illuminating on the dash at shorter and shorter intervals notifying of issue or low DEF (diesel exhaust fluid)
  • Vehicle goes into limp mode after SCR dash light comes on and stays on
  • If your vehicle has had a failure with fuel injectors, turbo, head gasket, EGR system malfunction/failure

Exhaust Clean Australia guarantees the fastest turnaround times in the market due to our revolutionary Flash Clean technology. This patented Italian made machine is the best of the best, guaranteeing the fastest clean and the very best results or your money back.

We can pick-up from your workshop and deliver your SCR back in as little as 3 hours. We offer a range of services, including:

  • Priority Service (next in line)
  • Same day service
  • Standby budget service

If your SCR is oil soaked due to an engine component failure allow extra time for an additional step in the cleaning process to return your SCR back to its original state.

Exhaust Clean Australia offer warranty on all our workmanship where the vehicle is operated according to manufacturer’s specification.

Pricing structure here:

From as little as $350. Compare that the cost of a replacement SCR that could set you back anywhere from $1,500 - $15,000 for cars, and in excess of $25,000 for large trucks and machinery. Cleaning your SCR is a very cheap cost alternative for your car, truck or machine.

  • Diesel particulate filters – from $450 + gst
  • Cat Convertors – from $340 + gst
  • Diesel oxidising catalysts – from $340 + gst
  • Selective catalyst reductors – from $390 + gst
  • Intercoolers – from $110 + gst
  • Exhaust gas recirculation – from $100 + gst

There are some chemical fuel additives on the market that claim to be able to clean SCR’s. They are extremely ineffective and a waste of your hard-earned money. Exhaust Clean Australia’s revolutionary Flash Clean technology will clean your SCR and make it like new again!

  • To help prevent your SCR from being contaminated, ensure that the vehicle’s service schedule is adhered to, and that the correct engine oil and DEF is used .
  • Find an independent mechanic that is familiar with the make and model of your vehicle, and is experienced in servicing modern diesels. This will ensure that they have a thorough knowledge of your vehicle and understand all the tips and tricks in fault finding any problems.

SCR systems are very technical, and some mechanics may not fully understand their complex operation. In our experience, that also includes some dealerships, who will prefer to “replace” your SCR at significant cost to you without necessarily solving the original problem.

Some other workshops may keep using the old “replace the parts” method, hoping it’ll eventually solve the fault, but inevitably, your bank balance will cop a hiding! Try to find a diesel specialist workshop that frequently deal with SCR systems, and who understand how they work. They’ll be able to pinpoint the cause of the fault and only replace or repair the parts that are necessary. Call us for recommended mechanics in your local area.

Exhaust Clean Australia can also provide a removal and refit service of your SCR at our workshop for all vehicle’s sizes, and can even provide a list of recommended mechanics in your area that can perform the removal and fitting service for you, while ensuring that your SCR is still being cleaned by the best in the business.

No, it’s illegal to remove or delete your SCR. Your vehicle has been designed by the manufacturer to operate with anSCR system. It is illegal, and will also void the manufacturer’s warranty on your vehicle along with all Australian consumer law protections. Removal or deletion of your SCR will also impact your vehicle’s insurance policy.

SCR’s are fitted to enable the vehicle to comply with the Australian Design Rules (ADR) emission requirement levels.

The relevant ADR for light vehicles is ADR79/03

The relevant ADR for heavy vehicles is ADR80/01

It is illegal to operate a vehicle that does not meet the ADR requirements so removing or deleting a SCR will result in non-compliance with all relevant emissions ADR’s.

In addition to commonwealth legislation, each state and territory also have additional legislation that require SCR’s to be fitted and operated in accordance with manufacturer requirements.


Don’t be fooled by dodgy operators that claim to electronically delete the operation of your SCR without detection. Authorities are now cracking down on engine tuning shops and vehicle owners who employ this practice, using sophisticated tools to identify vehicles suspected of having no SCR or deleted SCR’s originally fitted as OEM equipment. Stiff prison sentences are now being applied for this practice, with numerous prison sentences already handed down.

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