NOx Sensors Fault Diagnosis and Replacement

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.

Common reasons for NOx Sensor failure

Damage to wiring

Contamination by water, soot or carbon build up can lead to sensor electrical failure. (Do not try to clean sensors with chemicals or compressed air)

NOx sensors contain a ceramic chip that can be damaged if
subjected to heavy and excessive vibration.

Checks to be made before replacing NOx sensors

Any contamination on the sensor nozzle means that the source of this contamination needs to be investigated and the cause rectified before fitting a replacement unit.

Check to see that the DEF/AdBlue is correct specification/ quality and not contaminated.
The installation area should be clean and grease free.
Electrical connectors should be clean and rust free.
All wiring should be free of damage.
Check the Catalytic Convertor for damage/contamination.

Installation

Where possible ensure that like for like by part number is being fitted. If the VM has introduced a supersession check that the supersession is correct.

Once fitted the sensor or sensors will have to be trained or coded to the vehicle. This process will vary by VM and will need to be carried out by a VM-authorised workshop or a specialist workshop equipped with the appropriate equipment.

Please Note:

The term “plug and play” is used quite often for aftermarket units. This is rarely true for 12v NOx sensors unless it is a particularly old application. Always assume that coding will be required.

24v NOx sensors used on commercial vehicles appear to be more likely to be “plug and play” currently. This may change with time of course.

Never attempt to replace the sensor only by splicing the wires on the complete NOx unit to those on a new sensor. Always replace with a complete unit.

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