NOx Nitrogen Oxide Sensor

Categories: Technology News

1.What is Nox Sensor?

NOx sensors monitor the level of nitrogen oxide emitted by a diesel vehicle to ensure compliance with emissions regulations. Most engines feature two NOx sensors: an upstream and downstream sensor.
In many applications with exhaust-gas treatment systems, one NOx sensor is used upstream of the exhaust-gas treatment system (upstream) and a second sensor is used downstream of the exhaust-gas treatment system. The upstream sensor is used for the aforementioned feedback loop. Meanwhile, the downstream sensor is used mainly to confirm that the legislated emissions limits have not been exceeded.

NOx Nitrogen Oxide Sensor
Below is a diagram of a generic Selective Catalytic Reduction (SCR) system used on light-duty diesel passenger trucks. The assembly uses two NOx sensors: the first sensor (referred to as NOx sensor 1) is located near the turbo downpipe and measures engine-out NOx. The second sensor (referred to as NOx sensor 2) measures NOx levels exiting the SCR catalyst.

2. Why Nox Sensor is important

The drive to develop a NOx sensor arises from environmental factors. NOx gases can cause various problems such as smog and acid rain. Many governments around the world have passed laws to limit their emissions (along with other combustion gases such as SOx (oxides of sulfur), CO (carbon monoxide) and CO2 (carbon dioxide) and hydrocarbons). Companies have realized that one way of minimizing NOx emissions is to first detect them and then to employ some sort of feedback loop in the combustion process, thereby enabling the minimization of NOx production by, for example, combustion optimization or regeneration of NOx traps. For diesel engines, the emissions situation is different. Owing to their much higher combustion temperatures (resulting from their high cylinder compression ratios as well as turbocharging or supercharging), diesel engines produce much higher engine-out NOx emissions than spark-ignition gasoline engines. 

3. What causes NOx sensor to fail?

Common causes of failure include soot buildup on the sensor, ECU water intrusion, and/or damage to the cable, which will cause the check engine light to illuminate. In the event of the following problems, the NOx sensor should also be checked for correct function and replaced if necessary: 1. Poor engine running due to incorrect adjustment values for mixture adaptation 2. Insufficient emissions values 3. Power loss 4. Clogging of the particle filter 5. Limited AdBlue adjustment


When a nitrogen oxide sensor fails on a truck with a diesel engine, bad things start to happen. The motor may begin to run rough and fuel consumption can drastically increase. Next, the motor may go into derate, or “limp mode,” and finally, the truck may break down and stop running altogether.

5. Why are NOx sensors expensive?

Another drawback of amperometric NOx sensors is their high cost due to the multiple chamber design and the requirement for complex electronics to detect nanoampere current levels.

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