The NOx sensor is an important part of the exhaust aftertreatment system for reducing the concentration of harmful NOx substances (nitrogen oxides). It is used in diesel vehicles with an SCR (Selective Catalytic Reduction) system based on urea (AdBlue).
The sensor enables compliance with stringent Euro 5 and 6 emissions standards, ensures optimal dosing of AdBlue in the engine exhaust system, and thus effectively reduces environmentally harmful nitrogen oxides.
The NOx sensor consists of a probe and a controller that are permanently connected to each other by means of a wiring harness. The sensor is mounted in the exhaust system and is used to detect nitrogen oxides in the exhaust gases.
The exhaust gas from the diesel engine flows to the diesel oxidation catalyst (DOC) and then to the diesel particulate filter (DPF). If the saturation of the DPF diesel particulate filter reaches a certain value, cleaning should be performed.
Particulate filter regeneration is started automatically and monitored by the relevant system master controller while driving. Regeneration can also be carried out in the service mode triggered by a special procedure.
Behind the DOC/DPF system is the SCR (Selective Catalytic Reduction) system, which reduces nitrogen oxide emissions. If the SCR system is equipped with an upstream and downstream NOx sensor, the downstream sensor monitors the operation of the SCR catalytic converter, which should reduce the amount of pollutants to a certain level, while the upstream sensor measures the amount of pollutants before the exhaust gas enters the SCR system. Based on the difference in the readings of both sensors, the amount of urea in the controller is calculated, which should be injected into the system to reduce the content of harmful substances as much as possible.
By adding urea (AdBlue) to the exhaust system, a reaction takes place in which nitrogen oxides (NOx) are converted into nitrogen ( N2 ) and water ( H2O ). In combination with a scavenging catalyst (ASC), excess ammonia ( NH3 ) can also be converted to nitrogen ( N2 )
For the system to function properly, it is necessary to keep all its components in an impeccable technical condition, because it is very sensitive to all kinds of irregularities, which are often the cause of sensor failure and more.
A very common cause of sensor failure is a faulty AdBlue pump responsible for the appropriate pressure of the injected urea. It should be remembered that this liquid freezes at -11 o C. For this reason, vehicle manufacturers use special heaters in AdBlue tanks. Failure of the heater may damage the pump and the NOx sensor due to the inability to dosing urea in sub-zero temperatures. In addition, the lack of urea injection during system operation affects the composition of the fuel-air mixture, and its incorrect proportions may lead to, for example, faster clogging of the DPF filter.
The quality of the urea itself is extremely important. You should not use a liquid of unknown origin, it is best to buy it from a proven distributor. It is also good practice to check the urea concentration with a refractometer, its nominal concentration is 32.5%. However, it is unacceptable to store AdBlue in unwashed substitute containers for other substances, e.g. engine oil, etc., which unfortunately often takes place.
The replacement of a damaged sensor or sensors should be carried out by a specialist with the appropriate equipment, which can be used to reset the AdBlue quality information stored in the ECU, perform sensor tests or upload new software.
The product range currently includes 16 references for most commercial vehicles found on European roads, with different OE numbers from such popular manufacturers as: DAF, Mercedes-Benz, MAN, Scania, Volvo, Renault, Iveco.
