NOx sensor: Setup, troubleshooting and repair instructions

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.

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.

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.