Everything you need to know about NOx Sensor

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If you have a diesel vehicle in your workshop, chances are it will be fitted with a NOx sensor.


What is a NOx sensor?

A NOx sensor is an automotive device used to monitor the amount of nitrogen oxide (NOx) gas in a vehicle's exhaust system.


What does a NOx sensor do?

NOx is a harmful gas produced during combustion in internal combustion engines that contributes to air pollution and smog formation. The NOx sensor works by detecting NOx levels in the exhaust stream and providing feedback to the engine control system, which then adjusts fuel injection and exhaust gas recirculation rates to reduce NOx emissions. The NOx sensor is a key part of this system, which also includes the SCR catalyst, Diesel Particulate Filter (DPF), and turbocharger amongst others. If the NOx sensor detects too many NOx gases being emitted, it sends that information to the SCR system, which then adjusts the output to a level that’s compliant with emission regulations. In modern diesel engines, NOx sensors are often placed downstream of diesel particulate filters and selective catalytic reduction (SCR) systems to further reduce NOx emissions. Overall, NOx sensors play a vital role in reducing harmful emissions from internal combustion engines and meeting environmental regulations.


How does a NOx sensor work?

The sensor is made up of a Nernst cell with a current flowing through. This corrects the air/fuel ratio to λ=1. A second cell, usually made of ceramic with a layer of rhodium, breaks down the oxides of nitrogen into nitrogen and oxygen. The NOx controller – usually closely positioned to the NOx sensor – calculates the level of oxides of nitrogen present in the exhaust gas and sends that data to the SCR controller. The SCR controller then adjusts the amount of diesel exhaust fluid injected into the SCR catalyst, which in turn transforms excess oxides of nitrogen into water and nitrogen. 



Where is the NOx sensor located?

The NOx sensor is most commonly found after the SCR catalyst. This allows the SCR controller to check that NOx levels have dropped to the correct amounts. Some vehicles have pre- and post-SCR sensors, although just one sensor is more common.



Are NOx sensors only found in diesel engines?

Most modern vehicles are fitted with NOx sensors – both petrol and diesel.

Even though petrol engines produce significantly less NOx than diesel engines, emission levels are still highly regulated. To pass Euro 6 compliance tests, modern vehicles have at least one NOx sensor to control the vehicle’s output.


How do I know if my NOx sensor is bad?

The NOx sensor is responsible for measuring the NOx emission level in the exhaust system of the vehicle. If the sensor fails, it can result in reduced engine performance, increased emissions, and poor fuel efficiency. Some of the following phenomena may indicate that the NOx sensor is damaged:


CEL illuminated:  The Check Engine Light (CEL) on the instrument panel will illuminate if there is a problem with the NOx sensor or any other component in the emission control system.

Reduced engine performance:  NOx sensor failure will lead to reduced engine performance, such as reduced power and acceleration.

Poor fuel economy:  If the nitrogen oxide sensor fails, it will lead to increased fuel consumption, which will cause the engine to work harder than usual.

Failed emissions test:  If the vehicle fails the emissions test, it may indicate a problem with the NOx sensor or other components in the emission control system.


How to fix the NOx sensor?

Well, if it is suspected that our NOx sensor is not working properly, some first steps can be taken to fix it:

  •  Clean the sensor: Over time, NOx sensors can become dirty and become covered with soot or other contaminants, which can affect their accuracy. You can try cleaning the sensor with a non-abrasive cleaner and a soft brush. However, it's important to exercise caution to avoid causing any harm to the sensor during maintenance.


  •  Check the wiring: The NOx sensor is connected to the vehicle's wiring system, a loose or damaged connection can cause problems. It's important to inspect the wiring and connections for any indications of damage or corrosion. If you detect any issues, it's recommended to perform the necessary repairs or replacements to ensure the proper functioning of the system.


  •  Check the voltage: The NOx sensor works at a specific voltage, if it does not receive the correct voltage, it may not work properly. You can use a multimeter to check the voltage and make sure it is within the manufacturer's specifications.



If you're aware of NOx sensor problems with your machine or have questions about engine replacement components, Our website (DISELMART) has the answers you're looking for – and the products, too! 



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