NOx Sensors Fault Diagnosis and Replacement

There are a number of methods used on Diesel Engines to reduce harmful emissions. One of these methods is called SCR (Selective Catalytic Reduction).

SCR is an active emissions control system where hot exhaust gases flow out of the engine and into the SCR system where aqueous urea (known as Diesel Exhaust Fluid, or DEF) is sprayed onto a special
catalyst.

Diesel Exhaust Fluid (DEF) is a non-toxic fluid composed of 32% automotive grade aqueous urea and purified water and is sometimes referred to as AdBlue.

The design of SCR technology is such that it permits nitrogen oxide (NOx) reduction reactions to take place in an oxidizing atmosphere. It is called "selective" because it reduces levels of NOx using ammonia as a reductant within a catalyst system. The DEF is rapidly broken down to produce the oxidizing ammonia in the exhaust stream.

The chemical reaction is known as "reduction" where the DEF is the reducing agent that reacts with NOx to convert the pollutants into nitrogen, water, and tiny amounts of CO2.

The component configuration of a SCR system can vary by the vehicle manufacturer, but the fundamental principle of operation remains the same. SCR systems are often used in conjunction with
a DPF filter.

In order to inject the correct amount of DEF into the catalyst, the system employs one or two NOX Sensors. The NOx sensors are fitted before and after the SCR catalytic convertor and may be
referred to as upstream and downstream.

The NOx sensors are required to accurately measure the amount of NOx in real time and feed this information to an ECU. This information in turn allows the ECU to accurately determine how much DEF needs to be injected into the catalytic convertor to produce the lowest emission.

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