What is a NOx sensor?

2019-9-28 16:01:18

What is a NOx sensor?

1.0 NOx sensor structure

Currently, NOx sensor can be used to measure the value of NOx on the exhaust pipe. It communicates with the ECM via the body bus, and also has a self-diagnostic system inside the NOx sensor assembly. The sensor monitors its own operation and reports to the ECM whether there is a fault.

1.1. Nitrogen Oxide Sensor Type
Classified by brand: Cummins,Scania,Volvo,Man,Weichai, Yuchai, etc.
According to the supply voltage classification: 24V and 12V.
According to the plug classification: 5 pins and 4 pins. 

1.2. Parameters and range of nitrogen oxide sensor measurement

Output parameter



NOx concentration


3000 PPM

O2 concentration



 1.3. Nitrogen oxygen sensor structure

nox sensor structure.jpg

NOx sensor consists of NOx sensor probe, control module, connection plug and connecting cable.

1.4. NOx Sensor plugs

nox sensor pins show.jpg

1.5. NOx Sensor probe


1.6. HD, HF type probe internal structure

hd hf probe.jpg

1.7. Internal structure of the Bc4 probe

 BC4 probe.jpg

2.0  NOx sensor work measurement principle

2.1. Monitoring and diagnosis

The NOx sensor transmits the NOx value in the exhaust gas to the ECU in real time after normal operation. The ECU uses a nitrogen oxide monitoring program to detect whether the NOx value in the exhaust gas exceeds the standard.
The following conditions must be met to operate NOx monitoring
No cooling water temperature sensor related fault code
No environmental pressure sensor related fault code
Cooling water temperature is higher than 70 Celsius degrees
Altitude below 1600 meters

nox pump cell.png

Oxygen in the exhaust gas from the first measurement chamber is pumped out while HC, CO, H2 is oxidized on the platinum material.
In the second measuring chamber, NOx is reduced on the catalytic material while releasing O2.
Measuring the amount of O2 produced by the second measurement chamber represents the NOx content. 

2.2. Nitrogen oxide monitoring - routine monitoring
The conditions for the routine monitoring of nitrogen oxides are: 
The metering injection volume is 800 to 3800 ml/h 6cyl engine.
600 to 3800 ml/h 4cy engine. 
Engine speed range is 1400 C 1700 rpm 6 cylinder engine.
1400 C 1750 rpm 4 cylinder engine. 
Nox sensor gives a relatively stable reading.
Stable fuel injection for a minimum of 15 seconds
After the above conditions are met, the ECU will sample twice, then average the values of the two samples, and compare the average value with the set limit. If the average value obtained is below the limit, the NOx routine monitoring is completed once. If the average value obtained is above the limit, an additional monitoring program will be entered. The Euro 5 engine will not perform an additional monitoring procedure. When the first routine NOx monitoring fails, the ECU will run a routine monitoring routine.
The first chamber pumps out oxygen but leaves NO. Since the first chamber is oxygenated, the reduction reaction cannot occur.
• NO penetrates into the second chamber and is reduced to nitrogen and oxygen.
• Measure the amount of oxygen that is converted in the second chamber.

 2.3. Nitrogen Oxide Monitoring - Additional Monitoring
Run the additional monitor under the following conditions:
The result of running the regular monitor for the first time exceeded the standard.
Engine speed range is -1500 rpm 6 cylinder engine 1500 rpm 4 cylinder engine.
The estimated NOx (NOx in) into the exhaust system is 800 to 1800 ppm, which means that the engine is metered in the injection metering state under certain load conditions.
NOx sensor reading is stable.
After the above conditions are met, the system will monitor the NOx value for 15 seconds and then give an average of the readings, then command the metering pump to stop working, and then monitor for another 20 seconds to give another reading average. The average of the read values obtained twice is compared. If the set threshold is exceeded, the NOX is confirmed to be exceeded.
If the above conditions are not met when the monitor is running, the additional monitor will be exited, and then the additional monitor will be re-executed after the conditions are met again.
Once the additional monitoring program is completed, the ECU will compare the two sets of data: if the difference between the two sets of data is too small, the monitor will record an error. If the difference is too small for two consecutive times, the fault code will be recorded and the MIL light will be illuminated. If the difference between the two samplings are bigger than the preset value, the monitoring is passed. 

Additional monitoring procedures are only performed after the normal monitoring program is out of tolerance. Additional monitoring procedures will not be performed if normal monitoring is passed.To clear the effects of the twistdown, the monitor must be running and the results must be met.The relationship between the percentage load and the amount of injection and the NOx value before the catalyst is as follows:

When the load is greater than 50%, the injection amount exceeds: 800ml/h 6-cylinder engine, 600ml/h 4-cylinder engine.
When the 6-cylinder engine load is greater than 60% and the 4-cylinder engine load is greater than 45%, the NOx value before the catalyst will exceed 800ppm. 

3.0 NOx sensor repair guide
3.1. Sensor problem detection

ceramic cracking.jpg

Probe aging detection is not up to standard: When the probe suddenly encounters water when heated at high temperature, it is easy to expand and shrink and cause ceramic cracking.
Circuit problem: When the circuit is subjected to excessive current and voltage, it is easy to damage components. 

4.0 NOx sensor transportation and storage considerations
4.1. Nitrogen oxide sensor storage conditions
The principle of the NOx sensor determines that the operating temperature of the sensor should be higher than 600 C. Therefore, the liquid or water splashed by the ceramic inductor at a high temperature may be very sensitive, and the liquid or water may cause mechanical stress damage caused by instantaneous shrinkage.The transport box must be transported and stored in a sealed environment,keep dry.
The storage temperature range is -40 C to +105 C (without power supply). 

4.2.NOx Sensor storage considerations
Nitrogen oxide sensors are extremely sensitive products. To avoid damage, please strictly observe the following operating guidelines:
Mechanical vibration: avoid any mechanical vibration generated by the sensor. Note: The protective cap is used to protect the sensing unit from dust and dirt, not to protect the mechanical vibration.
Cleaning: The sensor should be operated and stored in a clean, dry, non-polluting and non-bad weather condition (such as water, ice, snow, dust). Try to avoid any dust after removing the protective cap before installing the sensing element to the exhaust system. Water, liquid, and dirt fall on the sensing element.

Painting: Do not paint or graffiti anywhere on the sensor.
Sensor cleaning: Do not use mechanical, chemical or compressed gas to clean any part of the sensor, which will poison or age the sensor. The sensing element and the unconnected connector should not touch any liquid.
Electronic plug: After powering the sensor, do not pull out the connector plug, which will cause serious damage to the engine electronic control unit.
Supply voltage: Do not reverse the supply voltage. The connector has measures to protect against reverse installation.
Anti-static: The operation process must meet the anti-static.
The assembly line is stored and transported: Please store the sensor in a suitable way to avoid any potential mechanical vibration. Please take the sensor alone and avoid bundles (as shown below). 

5.0 NOx sensor assembly and installation operation guide
5.1. Sensor installation and precautions
To install the NOx sensor on the exhaust system with a clean, dry, dry tool. Always use a standard torque wrench to tighten the screw. Please also install in the following order:
1. Fix the electronic control unit of the sensor to the corresponding position of the car with screws, and determine the corresponding torque according to the type of screw. The maximum allowable torque is specified by the installation table (<60N/mm2).
2. Remove the protective cover of the sensor probe and do not allow any dust to stain the lubricated threads. Do not wipe the anti-wear agent on the thread surface.
3. Insert the sensor probe into the exhaust pipe of the exhaust system, and tighten the hex nut by hand. When tightening the hex nut, hold the bottom of the sensor contact and touch the metal buckle to prevent the cable from bending and twisting.
4. Finally, tighten the hex nut with a standard torque wrench (recommended torque between 50 10Nm). The maximum allowable rotation angle of the cable after tightening is 180.
5. Connect the mated plug to the socket of the electronic control unit of the sensor to ensure that the socket is clean. To prevent the plug and socket from being used, the plug indicator is used. 

If multiple sensors are installed in the system, please note that the corresponding plug and socket labels of different installation positions (upstream/downstream) sensors are correctly matched. Different installation locations (upstream/downstream) sensor label matching. 

5.2. When installing the NOx electronic control module
It is important to note that the connecting cable cannot be stretched too tightly, taking into account the release of stress. The sensor module must be considered for protection and the installation location should avoid the effects of high temperatures in the exhaust system. In order to ensure that the gas permeable membrane on the back of the sensor is not blocked, shielded or contaminated by impurities.

5.3. NOx Sensor Disassembly and Precautions
If you need to remove the NOx sensor from the exhaust system, follow the sequence below. 
1. Remove the connector of the electronic control unit of the sensor and the socket. Do not remove the NOx sensor when it is powered on. The removal of the power may cause serious malfunction of the electronic control unit of the engine.
2. Remove the sensor electronic control unit.
3. Remove the sensor probe on the exhaust pipe. 

In order to prevent the probe from being overheated and causing burns, please remove the sensor after at least 15 minutes of power failure. One sensor cannot use more than 2 installation and disassembly cycles, because the anti-wear agent on the sensor probe will gradually wear out when needed more times. Apply more anti-wear agent to the thread during installation. It is recommended that the plug of the electronic control unit connector be inserted and removed no more than 20 times.