Quick overview
NOx sensors measure nitrogen oxide concentrations in exhaust gases and provide real-time data used by emission control systems. This information allows engine control units and aftertreatment systems to regulate combustion and urea dosing to reduce harmful emissions.
Modern engines rely on NOx sensors to maintain compliance with local and global emission regulations. By continuously monitoring exhaust gas composition, these sensors enable Selective Catalytic Reduction (SCR) systems and onboard diagnostics to maintain efficient and compliant operations.
About our NOx sensors: Tau Power Electronics develops precision NOx sensing technologies designed for integration into modern emission control architectures used by OEMs and industrial equipment manufacturers. In November 2025, we inaugurated our NOx sensor production line in Pune, India.
- NOx sensors measure nitrogen oxide concentration directly in exhaust gases
- Accurate measurement allows emission control systems to regulate SCR dosing
- Continuous sensing enables real-time emission monitoring and diagnostics
- Reliable NOx measurement supports compliance with global emission standards
- Precision sensing improves combustion control and aftertreatment performance
- Understanding NOx emissions in modern engines
- What is a NOx sensor?
- How do NOx sensors work?
- The role of NOx sensors in automotive emission control systems
- How NOx sensors support SCR systems
- Automotive and industrial NOx sensor applications
- Emission compliance and global standards
- Key features of high-quality NOx sensors
- The future of NOx sensor technology
- FAQs about NOx sensors
Nitrogen oxides (NOx) are among the most tightly regulated pollutants produced by combustion engines.
It’s for good reason; these gases form when nitrogen and oxygen react at high combustion temperatures inside the engine cylinder. The higher the combustion temperature and pressure, the greater the potential for NOx formation.
Because diesel engines operate at higher compression ratios and combustion temperatures, they are particularly prone to producing NOx emissions.
These emissions contribute to:
- Smog formation
- Air quality degradation
- Respiratory health risks
- Environmental pollution
As emission regulations have tightened globally, engine manufacturers have had to introduce more sophisticated emission control systems to reduce NOx levels while maintaining engine efficiency.
Today, most modern diesel engines rely on Selective Catalytic Reduction (SCR) systems to convert nitrogen oxides into harmless nitrogen and water vapor.
But SCR systems cannot operate effectively without accurate measurement of exhaust emissions.
That measurement is provided by NOx sensors.
A NOx sensor is an electronic device that measures the concentration of nitrogen oxide gases in engine exhaust.
These sensors detect chemical changes in the exhaust stream and convert them into electrical signals that engine control systems can interpret.
Those signals allow emission control systems to regulate aftertreatment processes and maintain compliance with regulatory limits.
Sensor Function
Gas sensing
Signal transmission
System feedback
Measurement
Nitrogen oxide concentration
Electrical data to ECU
Real-time emission data
System Role
Emission monitoring
System diagnostics
SCR dosing control
In most diesel applications, NOx sensors are installed at key locations in the exhaust system to monitor emissions before and after catalytic treatment.
This arrangement serves as an emission compliance solution and allows control systems to verify how effectively emissions are being reduced.
Modern NOx sensors measure exhaust gas composition using electrochemical sensing elements designed to detect nitrogen oxide concentrations in harsh exhaust environments.
A simplified operating process includes the following steps:
- Exhaust gases flow past the sensing element inside the NOx sensor.
- Electrochemical reactions within the sensor detect nitrogen oxide molecules present in the exhaust.
- The sensor converts this chemical measurement into an electrical signal.
- The signal is transmitted to the vehicle’s electronic control unit (ECU).
- The ECU analyzes the data and adjusts emission control systems accordingly.
- The SCR controller calculates the appropriate amount of DEF based on the NOx concentration. It drives the doser to spray to the SCR inlet and obtains gas with very low NOx concentration at the SCR outlet.
This process occurs continuously while the engine operates.
In practice, the sensor may perform thousands of measurements during a single drive cycle.
Because emission control systems rely on these measurements, NOx sensor accuracy directly influences system performance and compliance.
Modern vehicles rely on a network of sensors to monitor engine operation and emissions.
Within this network, NOx sensors play a critical role in providing direct measurement of nitrogen oxide levels in exhaust gases.
This information allows control systems to regulate technologies such as:
- Selective Catalytic Reduction (SCR)
- Diesel particulate filtration
- Onboard diagnostic systems
Emission control systems must respond quickly to changing operating conditions such as engine load, acceleration, and temperature.
Accurate NOx measurement allows these systems to adjust their operation dynamically.
Without reliable sensing, emission control systems would effectively be operating without visibility into actual exhaust emissions.
Selective Catalytic Reduction systems reduce nitrogen oxide emissions by injecting diesel exhaust fluid (DEF) into the exhaust stream.
The DEF reacts within a catalyst to convert nitrogen oxides into nitrogen and water vapor.
However, the effectiveness of SCR depends on precise control of DEF dosing.
If the system injects too little fluid, emission reduction becomes insufficient.
If it injects too much, the system wastes fluid and risks damaging components.
This is where NOx sensors become essential.
Many SCR systems use two NOx sensors positioned at different points in the exhaust system.
Sensor Position
Upstream sensor
Downstream sensor
Purpose
Measures NOx levels before catalytic treatment
Confirms emission reduction after SCR
By comparing these measurements, the control system can determine whether the catalyst is functioning correctly and adjust DEF dosing as needed.
In simple terms, NOx sensors allow the SCR system to “see” the emissions it is trying to reduce.
NOx sensors are used across a wide range of automotive and industrial systems where emission monitoring is required.
Automotive applications
Common vehicle platforms include:
- Heavy-duty trucks
- Light commercial vehicles
- Passenger vehicles
- Agricultural equipment
- Off-highway machinery
These environments expose sensors to vibration, rapid temperature changes, and extremely hot exhaust gases.
Sensors must therefore be designed to operate reliably under demanding conditions.
NOx sensors are also used in equipment such as:
- Diesel generator sets
- Construction equipment
- Marine propulsion systems
- Industrial engines and power units
In these applications, sensors support continuous emission monitoring and help operators maintain regulatory compliance.
Emission regulations around the world continue to evolve as governments seek to reduce pollution from engines and industrial equipment.
Modern vehicles and machines must comply with set standards. At Tau Power Electronics, these include BS VI & VII, CEV V/VI, TREM V, CPCB IV+, as well as China VI, Euro VI & VII, and EPA standards. These regulations require manufacturers to demonstrate that emissions remain within strict limits under real-world operating conditions.
Meeting these requirements depends heavily on accurate sensor data.
NOx sensors provide the measurements needed for emission control systems to regulate combustion, monitor catalyst performance, and verify compliance.
For OEMs, reliable NOx sensing technology is therefore a fundamental component of emission system design. That’s why our NOx sensor solutions integrate with modern emission control architectures while supporting regulatory compliance across automotive and industrial applications.
Other sensors in our portfolio:
Because NOx sensors operate directly in the exhaust stream, they must function reliably in one of the harshest environments within a vehicle or machine.
Sensors that maintain stable readings under these conditions enable emission control systems to operate reliably throughout the vehicle’s lifecycle.
Emission control systems will continue to evolve as environmental regulations become more stringent.
Future developments in NOx sensor technology are expected to focus on:
As engine systems become increasingly complex, the role of precision sensing will only grow.
Accurate measurement remains the foundation that allows emission control systems to operate effectively.
Have a question on how our NOx sensors can work for you? Contact our team.
FAQS
A NOx sensor measures nitrogen oxide concentrations in exhaust gases using electrochemical sensing elements. These elements detect NOx molecules and convert the measurement into an electrical signal. The signal is transmitted to the engine control unit, which uses the data to regulate emission control systems such as the SCR.
Emission regulations require engines to maintain strict limits on nitrogen oxide emissions. NOx sensors provide real-time measurement of exhaust gases, allowing emission control systems to adjust their operation and maintain compliance with regulatory standards such as BS VI and Euro 6/7.
SCR systems reduce nitrogen oxide emissions by injecting diesel exhaust fluid (DEF) into the exhaust stream. NOx sensors measure emission levels, often before and after catalytic treatment, allowing the system to adjust dosing levels and verify that emission reduction targets are being achieved.
Future NOx sensors are expected to offer higher measurement precision, faster response times, and enhanced diagnostic capabilities. These improvements will help engine manufacturers meet increasingly strict global emission standards.
High-quality NOx sensors provide stable and accurate measurements under harsh operating conditions. Key features include durability, resistance to high temperatures and vibration, fast response time, and compatibility with modern engine control systems.
