What Is a NOx Sensor?
The NOx sensor measures the level of nitrogen oxides in the exhaust gas within the SCR system. It sits between the DPF and the SCR catalyst and works together with the AdBlue system.
Introduced on Euro 5 heavy vehicles, it became standard on Euro 6 engines. As part of the European emission standards its use is mandatory in most EU countries.
How Many NOx Sensors Does a Vehicle Have?
There are two in the system:
| Position | Function |
|---|---|
| SCR inlet | Monitors the NOx level of gas entering the catalyst and controls AdBlue dosing |
| SCR outlet | Measures the values leaving the catalyst, confirming the treatment is actually working |
If either fails, the AdBlue system reports a fault — so the "AdBlue fault" on the dashboard may in fact be a NOx sensor.
Why Do NOx Sensors Fail?
- Heat cycling and age. The sensor works continuously at exhaust temperature and its ceramic element fatigues over time.
- Soot and ash build-up. The measuring surface becomes coated and the sensor reads incorrectly.
- Out-of-specification or contaminated AdBlue. Crystallisation affects the sensor.
- Wiring and connector damage. Mounted under the chassis, it is exposed to moisture, salt and impact.
- A blocked DPF. Increased back pressure and soot contaminate the sensor rapidly.
- An exhaust leak. The gas mixture the sensor sees is corrupted.
Failure Symptoms
- AdBlue or emissions warning on the dashboard
- Engine power derate
- Abnormally high or low AdBlue consumption
- Rising emission values
- The vehicle progressively refusing to run
Cost and the Right Approach
A common complaint recently is the high prices quoted by main dealers for NOx sensor faults. Yet the answer is not always a complete sensor replacement.
The correct order is:
- Confirm the fault. Look at live data from both NOx sensors: are inlet and outlet values plausible, is the sensor heating up?
- Check wiring and connector. A significant share of faults are in the harness, not the sensor.
- Find the underlying cause. With a blocked DPF or bad AdBlue, a new sensor will fail quickly too.
- Replace and adapt if needed. A new sensor that is not registered to the vehicle will keep reporting the fault.
Warning: disabling the NOx sensor in software is contrary to emissions regulations on road-going vehicles and will cause a failure at inspection.
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