AS-PL: Design and function of NOx sensors

Date: Wednesday 01 March 2023

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NOx sensors are very important elements of the Selective Catalytic Reduction (SCR) system. It is worth knowing what they are made of in order to be able to understand better their role in the process of reducing nitrogen oxide emissions.

A typical sensor consists of the following:

• Measuring probe with an electrochemical sensor (it consists of two measuring chambers)
• The cable connecting the probe to the control module
• Probe data processing module
• Socket connecting the sensor to the ECU computer
• NOx sensors are one of the most important elements of the complex SCR system.

The location of NOx sensors in the exhaust system at the input and output of the SCR system allows for continuous measurement of the concentration of harmful substances. The first sensor receives a measurement sample of exhaust gas, which has already undergone a treatment process in the DOC/DPF system. The NOx sensor at the output is designed to monitor the performance of the SCR catalyst, which should reduce the amount of poisonous compounds to a certain level. Based on both measurements, the required amount of urea (AdBlue) injection is calculated to reduce the harmful substances as much as possible.

NOx sensor defects
One of the most common problems is the malfunctioning AdBlue pump responsible for the proper pressure of the injected urea and the failure of the AdBlue tank heater. Another common problem is soot deposit on the measuring element, caused e.g. by poor fuel quality or increased engine oil consumption.

The condition of NOx sensors often depends on the operation of another component.

Due to the very delicate design, mechanical or electrical damage may also occur. Abrasions of wires, short circuits, or impacts, e.g. with a small stone, can significantly damage the sensor. Moisture also affects the condition of wires and causes corrosion of some components and damage to electronics. The urea alone is also one of the factors to blame for sensor damage. The use of AdBlue fluid of unknown origin or its low quality will make the sensor, as well as probably the entire SCR system, need to be replaced.
The symptoms of the damaged NOx sensor include:

• Fluctuations in speed during a cold start
• Increased petrol consumption
• Increased consumption of engine oil
• Jerks while starting
• Problems with the DPF system.

How to protect yourself from failure during the annual check, attention should be paid to plugs and the condition of the exhaust system. In the event of a larger failure, computer diagnostics will be necessary to completely rule out the problem related to the sensor. The repair of the sensor should be performed by a specialist with appropriate equipment.

NOx sensors should not be packed tight during storage, they cannot be tied, bent, or twisted. They cannot be painted, washed with water, or opened. Contact of sensors with solvents and other chemicals is also prohibited.

There are currently 100 NOx sensors on permanent offer, and more will be constantly added to ensure products meet all customer’s needs. You can find the sensors by clicking here.

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