NOx sensor: Setup, troubleshooting and repair instructions

Here you will find valuable and useful workshop tips about the design, function and diagnostics of the NOx sensor.

The nitrogen oxide sensor comprises a probe and a control unit. These are firmly connected together using a cable harness to form a unit. This unit is installed in the exhaust gas system and is used to recognise nitrogen oxides in the flow of exhaust gas. Nitrogen oxides are harmful to health and are produced by high temperatures, high pressure or excess oxygen during combustion in the engine. The NOx sensor is thus an important component in the exhaust gas management system of modern motor vehicles with combustion engines.

Important safety note
The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their work. The information provided on this website is intended for use by suitably qualified personnel only.

NOx sensor: Design and function

The NOx sensor is installed in both petrol and diesel vehicles from Euro 5/6 and enables compliance with the strict emission values. The sensor data is required by the respective engine management systems to calculate the exhaust gas recirculation rate, the air-fuel mixture or the urea injection quantity. The sensor is necessary for vehicles with direct petrol injection, as these produce a larger quantity of nitrogen oxides due to the stratified charging operation. These vehicles also have a NOx storage catalytic converter.

In diesel vehicles, the sensor is used in conjunction with a selective catalytic reduction (SCR) system. Here, urea is introduced into the exhaust gas flow and reduces the nitrogen oxides to harmless nitrogen (N2) and water (H2O). By recording exhaust measurement data, the NOx sensor enables the engine management system to provide an optimal dosage of AdBlue®, effectively reducing nitrogen oxides, which are harmful to the environment. As soon as the required operating temperature is reached, the NOx sensor permanently measures the nitrogen oxide content in the exhaust gas. The values determined are processed by the NOx sensor's control unit and forwarded to higher-level control units, such as the SCR or engine control unit, via the CAN data bus. Based on the information received, these control units can calculate how much AdBlue® needs to be injected upstream of the SCR catalytic converter to achieve optimum nitrogen oxide reduction. A heating element integrated directly into the probe also ensures the required operating temperature of approx. 300 ° for the sensor. The NOx sensor unit can be installed individually or as a system pair in the exhaust system. This depends on which system version is installed in the respective vehicle. If two sensors are used, one is located upstream and the other downstream of the SCR catalytic converter. The downstream sensor has the task of monitoring the effect of the SCR catalytic converter. This ensures system function and more precise control of the exhaust gas purification systems. This arrangement contributes to compliance with the increasingly stringent emission limits.

Functional principle of the sensor: Interesting facts

Exhaust gas enters the first chamber via the diffusion barrier. This houses the first pump cell and a measuring cell. The residual oxygen in the exhaust gas is determined using the measuring cell in the first chamber. Another measuring cell with a connection to the outside air serves as a reference. The difference between the oxygen content in the exhaust gas and the reference air creates a voltage between the two measuring cells, which the control unit of the sensor unit uses as a measured variable, thereby controlling the current of the first pump cell. The pump cell transports the residual oxygen out of the first measuring chamber. The remaining nitrogen oxides (NOx) pass through another diffusion barrier into chamber two, which contains a coated electrode. This electrode has the property of catalytically splitting nitrogen oxides (NOx) into nitrogen (N²) and oxygen (O²).

The resulting nitrogen components (N²) diffuse outwards through a porous layer. The oxygen components (O²) are conveyed to the outside air by the second pump cell. The control unit of the sensor unit records the pump current of the second pump cell and sends the processed information to the engine control unit via the data bus. This sensor signal is processed there and can thus monitor and control the NOx reduction.

Integrated heating element in the NOx sensor: Interesting facts

The integrated heating element enables a constant and optimum operating temperature to be maintained in the sensor. This allows the sensor to be heated to the predefined operating temperature regardless of the ambient temperature and engine temperature. This ensures that the NOx sensor can react optimally even at low temperatures. The temperature of the heating element is usually regulated by the engine control. The engine control unit adapts the heating output to the ambient conditions. This not only improves the accuracy of the nitrogen oxide measurement, but also has a positive effect on the service life of the sensor.

Causes of failure and symptoms: Important information

Due to the installation position in the exhaust system and the ambient conditions there, the functional life of the sensor is not unlimited.

A malfunction or failure can be caused by the following reasons

  • Sensor function loses efficiency
    • Wear due to ageing. Like a lambda sensor, the NOx sensor unit can also age
    • Due to operating conditions such as exhaust gas composition, temperatures and vibrations
  • Sensor head sooted
    • Short-distance operation, incorrect mixture composition or high oil consumption
  • Environmental influences
    • Moisture, water or road salt
  • Mechanical damage
    • Incorrect installation, accident or marten bite
  • Faulty power supply
    • Cable interruptions
    • External or internal short circuits
  • SCR system faulty
    • Defective components – incorrect dosing of AdBlue® can lead to deposits. These can damage the sensor and cause it to fail

The following symptoms may occur if the NOx sensor fails.

  • Engine warning light comes on
  • SCR system warning in the instrument cluster display
  • Saving an error code in the control unit
  • Malfunction or emergency operation of the SCR system
  • Increased fuel consumption or poor engine performance

Troubleshooting the NOx sensor: Practical tips

The function of the NOX sensor is monitored by the respective higher-level system control unit and thus via the on-board diagnostics (OBD). Component-related faults such as incorrect operational readiness, electrical short circuits or cable interruptions are recognised directly and logged in the fault memory. Therefore, the fault memory of the exhaust-relevant systems should first be read out using a suitable diagnostic device. The data from control unit communication forms the basis for actual troubleshooting and for successful repair work. However, it is recommended that the entire exhaust tract is visually inspected before starting directly with extended control unit diagnostics. External damage can usually be detected when the noise behaviour changes and can be caused by cracks or rusting through at pipes, connections or mufflers. The installed silencers and catalytic converters should also be checked for defects, such as loose parts inside, by shaking or knocking on the respective component. Wiring or electrical plug connections may have been damaged here due to environmental influences such as dirt, water or road salt. The electrical plug connection on the control unit should thus also be included in the troubleshooting process. If no damage is detected, the power supply and data bus communication should be checked using a suitable measuring device in accordance with the manufacturer's specifications.

Example of the control unit diagnostics: Instructions

Depending on the vehicle and system, the fault memory can be read out and additional functions such as parameters or circuit diagrams can be selected and displayed. The following information was provided using a BMW 520D G31 as an example.

Reading out the engine control system’s error memory

In this function, the error codes stored in the engine control can be read out and deleted.

In our case study, an error relating to the NOx sensor downstream of the SCR catalytic converter was stored in the error memory.

  • Error code 2FB 800 "OBD CAN message missing"
    • Error present
    • Error lamp not switched on

Reading off parameters

This function can be used to display current measured values such as engine speed, NOx value before and after SCR catalytic converter, as well as the temperature of the SCR catalytic converter.

This parameter query can be used during operation to check whether the exhaust gas sensors and the SCR system are functioning properly.

As can be seen in our example, no value is displayed for the faulty NOx sensor after SCR catalytic converter.

Basic adjustment

After installing the NOx sensor with control unit, this vehicle must be adapted to the vehicle.
In this function, the stop values of the new turbocharger unit can be programmed into the higher-level control unit.

Circuit diagrams

System-specific circuit diagrams can be taken from vehicle information and used for troubleshooting purposes. Here, for example, the PIN assignment on the NOx sensor can be read and used for further troubleshooting.

Maintenance and repair instructions: Workshop tips

  • Installation should only be carried out by trained, qualified personnel.
  • As part of the troubleshooting process, a visual inspection of the components and peripherals in the engine compartment or on the exhaust system should first be carried out after the control unit diagnosis.
  • The NOx sensor should be replaced if the wiring or sensor housing is damaged.
  • The sensor should always be replaced as a complete unit in the event of repair.
  • Ensure the correct sensor and installation position! Do not mix up the position before and after the catalyser.
  • The thread of the NOx sensor must not be additionally coated with hot screw paste.
  • NOx sensors must be tightened to the specified tightening torque! We recommend a tightening torque of 50 Nm for HELLA exhaust gas temperature sensors.
  • Before installation, it is important to ensure that the product is suitable for the intended application and that it has the required dimensions, connections and properties or features.
  • After installation of the NOx sensor with control unit, it may also be necessary to adjust it to the vehicle by using a suitable diagnostic device!
  • Please always observe the relevant installation and removal instructions provided by the vehicle manufacturer!
  • HELLA accepts no liability for damage caused by the improper handling or incorrect installation of the product!

The various diagnostic options have been illustrated as an example using the mega macs X diagnostic device. The respective test depth and variety of functions can be set out differently depending on the vehicle manufacturer and these are dependent on the relevant system configuration of the control unit. Schematic illustrations, pictures and descriptions serve to explain and illustrate the document text and cannot be used as a basis for vehicle-specific repairs.