How the NOx Sensor in Diesel Engines Works – Diagnosis and Maintenance
Modern diesel engines use advanced exhaust aftertreatment systems to meet the strict Euro 6 standards. One of the most important components in this process is the NOx sensor, which measures the nitrogen oxide content in the exhaust gases and ensures that the SCR system (Selective Catalytic Reduction) operates efficiently. Understanding how NOx sensors work helps drivers appreciate their importance and maintain optimal engine performance.
How the NOx Sensor Works in Diesel Engines: What Is a NOx Sensor?
A NOx sensor (nitrogen oxide sensor) is an electrochemical device designed to measure the concentration of nitrogen oxides in a diesel engine’s exhaust gases. These gases are produced primarily during high-temperature combustion and are among the most harmful emissions. Therefore, monitoring them is crucial for compliance with modern emission standards.
Once the sensor detects the NOx concentration, it continuously transmits precise data to the engine control unit (ECU). Based on this information, the ECU adjusts fuel injection, exhaust gas recirculation (EGR), and the AdBlue dosing in the SCR system. This ensures effective emission reduction while maintaining engine efficiency.
Upstream and Downstream NOx Sensors
Most modern diesel engines use two NOx sensors:
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Upstream NOx Sensor (before the SCR catalyst): This sensor is strategically located before the SCR catalyst and measures the raw NOx level immediately after combustion. This provides the ECU with accurate, real-time information about the actual nitrogen oxide concentration in the exhaust stream. Using this data, the ECU can precisely calculate how much AdBlue (urea solution) is needed to efficiently and environmentally neutralize NOx emissions.
- Downstream NOx Sensor (after the SCR catalyst): Installed behind the catalyst, this sensor monitors how effectively the SCR system reduces NOx emissions. If the readings remain high, the ECU detects potential issues such as malfunctions in the AdBlue system, reduced catalyst efficiency, or a faulty sensor.
How NOx Sensors Control Emissions
The engine control unit (ECU) constantly compares the readings from both sensors to maintain an optimal balance between performance and emission reduction. Based on this data, it can:
- Adjust AdBlue injection to ensure complete conversion of NOx gases into harmless nitrogen and water vapor.
- Modify the injection timing to control combustion temperature and thus reduce NOx formation.
- Detect early signs of catalyst or sensor degradation and trigger diagnostic trouble codes (e.g., P2202, P229F).
- Ensure compliance with emission standards under all driving conditions.
Symptoms of a Faulty NOx Sensor
If a NOx sensor fails, the engine can no longer effectively regulate exhaust emissions. Common symptoms include:
- Increased fuel or AdBlue consumption.
- Illuminated check engine or exhaust warning light.
- Loss of power or activation of limp mode.
- Increased exhaust emissions and possible failure of the emissions test.
Relationship Between NOx Sensors and Diesel Injectors
Fuel injectors and NOx sensors work closely together within the exhaust aftertreatment system. If injectors are faulty and deliver too much or too little fuel, combustion efficiency changes, which leads to deviations in NOx levels. The NOx sensor detects these deviations, and the ECU attempts to correct them. However, persistent injector problems can result in inaccurate NOx readings and put strain on the SCR system. Regular maintenance of both components ensures clean combustion and optimal emission values.
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Conclusion
NOx sensors are essential for precise exhaust control and the efficiency of modern diesel engines. They continuously provide feedback to the ECU, allowing it to accurately regulate AdBlue dosing and fuel injection – ensuring clean and efficient combustion that meets environmental standards. Faulty sensors or injectors can quickly disrupt this balance, leading to increased emissions and performance loss. For consistently clean operation, it is vital to use high-quality components and regularly maintain both the injection and exhaust systems.