Modern diesel trucks rely heavily on the NOx (Nitrogen Oxide) sensor to help meet emissions standards and maintain peak engine performance. When a NOx sensor begins to fail, it can trigger warning lights, increase fuel consumption, and eventually lead to costly downtime if ignored.
Whether you operate a workshop, run a fleet, or maintain your own vehicle, recognising the early signs of NOx sensor failure can save you money and prevent unnecessary repairs. Here’s everything you need to know about when to replace your NOx sensor.
What Does a NOx Sensor Do?
A NOx sensor is part of the truck’s emissions control system, most commonly in Euro 5 and Euro 6 vehicles. It measures the level of nitrogen oxides in the exhaust before and/or after the SCR (Selective Catalytic Reduction) system. This data helps the ECU adjust fuel, AdBlue dosing, and engine performance to reduce harmful emissions.
When a sensor fails, the truck’s system can no longer manage emissions correctly — triggering issues that drivers and mechanics quickly notice.
Key Symptoms of a Faulty NOx Sensor
Below are the most common warning signs that your NOx sensor is nearing the end of its life and may need replacing.
1. NOx Sensor Fault Codes Displayed
The first sign is usually a dash warning light or a diagnostic trouble code (DTC).
Common NOx-related codes include:
- P2201
- P2202
- P2203
- P229F
- P20EE
- Manufacturer-specific codes for Volvo, Scania, DAF, MAN, Iveco & Mercedes. If these codes return even after clearing, the NOx sensor is a likely culprit.
2. Poor Fuel Economy
A failing NOx sensor disrupts the air-fuel ratio and AdBlue dosing. The result? Noticeably higher fuel consumption. If you’ve ruled out obvious causes, such as load, driving conditions, or tyre pressure, the sensor may be sending inaccurate readings.
3. SCR or AdBlue System Warnings
Because the NOx sensor works closely with the SCR system, a faulty sensor often triggers:
- AdBlue system errors
- SCR malfunction warnings
- Emission control system alerts
These should be investigated promptly to prevent derating.
4. Loss of Engine Power or Derate Mode
Modern trucks may enter limp mode or engine derate when the ECU detects incorrect emission data. This is designed to protect the engine and reduce emissions, but it also means immediate loss of power, making the truck unsafe or unable to complete jobs. A faulty NOx sensor is one of the most common causes.
5. Increased Exhaust Smoke or Odour
Inaccurate NOx readings can cause unburnt fuel or incorrect SCR operation, sometimes leading to:
- Increased exhaust smoke
- Strong diesel or chemical smell
- Poor idle quality
Although these symptoms can stem from various issues, a faulty NOx sensor should be considered, especially on Euro 5/6 vehicles.
Why Do NOx Sensors Fail?
NOx sensors operate in extreme conditions. Common causes of failure include:
- High exhaust temperatures
- Contamination from soot or diesel
- Wiring harness damage
- Internal electronic failure
- Moisture intrusion
- Worn-out heaters in the sensor
Over time, even high-quality sensors wear out and must be replaced.
Diagnostic Tips Before Replacing a NOx Sensor
To avoid unnecessary replacement, run through the following checks:
- Check wiring & connectors: Corrosion, heat damage, or loose pins can mimic sensor failure.
- Check for upstream issues: EGR problems, injector faults, or exhaust leaks can influence readings.
- Confirm with live data: If the sensor output does not fluctuate with engine load, it’s likely faulty.
- Check for SCR issues: A malfunctioning SCR/AdBlue system can generate misleading NOx codes.
Once other causes are ruled out, replacing the NOx sensor is typically the correct fix.
How Often Should a NOx Sensor Be Replaced?
There is no exact interval, but in most European heavy trucks:
- Many sensors fail between 200,000–400,000 km
- Trucks with high idle time or short-haul cycles may experience earlier failure
- Heat-exposed downstream sensors tend to fail first
If one sensor fails, the other (upstream or downstream) may soon follow.
Avoid Downtime. Replace with Quality Aftermarket NOx Sensors. A faulty NOx sensor can quickly escalate from a warning light to a derated engine and expensive repairs. Replacing the sensor early is the best way to keep your truck performing efficiently and meeting emissions standards.
As your go to Independent European Truck Parts Supplier, we carry a full range of high-quality aftermarket NOx sensors for:
- Volvo
- Scania
- DAF
- MAN
- Iveco
- Mercedes
🛒 Shop the full NOx Sensor range here
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