Oe Scania Vitesco Nox Sensor A2C1784740077

The NOx sensor not only monitors the level of the oxides of nitrogen being released into the air, but it also plays an essential role in controlling it through the Selective Catalytic Reduction (SCR) system in diesel vehicles.

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 much 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.

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 the one sensor is more common. 

Do you need a NOx sensor?

Yes, NOx sensors are a necessity, especially for modern diesel vehicles.

With oxides of nitrogen causing harmful effects on human health and indirectly damaging the environment, emissions controls are tightly regulated.

NOx sensors have been fitted in vehicles since the early 2000s. To be Euro 6 compliant, a modern diesel vehicle cannot emit more than 80mg/km of NOx gases, which would be impossible without a NOx sensor to accurately monitor the levels.

Common NOx sensor faults

Precision components such as NOx sensors have limited lifespans, so it is fairly common to have to replace them at some point during vehicle ownership. 

One of the main causes of a NOx sensor fault is buildup of soot from the combustion process. Soot is not only abrasive, meaning that it can damage the sensor over time, it can also coat the sensor, inhibiting it from measuring the gas accurately.

Can you test a NOx sensor?

If the vehicle is exhibiting NOx sensor fail symptoms such as: 

• engine warning light illumination
• increased fuel consumption
• reduced power
• going into limp mode
• reduced acceleration
• reduced performance