What's an SCR Light and What Does It Mean?

What's an SCR System?

SCR stands for Selective Catalytic Reduction.  The systems is used to reduce the amount of NOx (mono-nitrogen oxides NO and NO2) that is released in diesel exhaust.  Trucks newer than 2010 are required to have a NOx limit of no more than 0.2g/bhp-hr for all heavy-duty diesel engines.

How Does the SCR System Work?

http://www.dieselforum.org/about-clean-diesel/what-is-scr-

As the exhaust leaves the oxidation catalyst there is an injector that sprays urea (DEF Fluid) in the chamber.  This combination causes a chemical reaction that converts the nitrogen oxides in to nitrogen, water and trace amounts of CO2.  Few people may know that breathing diesel exhaust is far less harmful than exhaust from a regular gas engine (which will kill you!)  

What is DEF Fluid?

DEF stands for Diesel Exhaust Fluid which is urea.  Most equipment with an SCR system will have a DEF tank behind the gas tank.  DEF fluid should be checked regularly even though it is consumed far slower than gas.

What To Do If the SCR Light Comes On? 

When the SCR light comes on, breathe calmly knowing that your truck isn't going to explode.  It just means your after treatment system has some issues that need to be resolved.  The bad news is that if the truck runs for too long with the light on, the computer system will de-rate your power and possible shut the engine off .  This is particularly aggravating because often it doesn't have anything to do with how the engine runs.  The SCR light is something that should be taken to a dealership to be fixed since there usually isn't a clear-cut answer to what is causing the light.  Often we see trucks come in with an SCR light multiple times and it is hardly ever the same problem twice.  A good mechanic is worth their weight in gold.

Find parts for SCR systems