كيفية فحص مستشعرات NOx في Mercedes-Benz واستبدالها وضبطها

NOx sensors feature their own control unit with vehicle-specific software. Once installed, the sensor frequently requires calibration to the engine management system, otherwise there will be communication errors within the CAN bus network. Without correct encoding, the SCR control system may malfunction or switch to emergency mode.

Before replacement, check the power supply, ground connection, CAN signals and connectors. An internal sensor or control unit fault is likely if these are nondescript and typical fault codes, such as P229F, or communication faults have been saved. Only then does replacing it make technical sense.

The NOx sensor generates the actual values used to control urea injection. If it fails, the SCR system operates on the basis of the fallback map or it is deactivated altogether. This may cause the emission limit to be exceeded, which triggers the strategy to prevent start-ups.

If there is no NOx feedback or it is implausible, the engine control unit cannot safeguard statutory emissions monitoring. In accordance with OBD and type-approval specifications, a remaining mileage limit activates; the vehicle will not restart on reaching this limit.

NOx sensors vary depending on their installation position, emissions standard and software version. Incorrect article numbers can lead to communication or control errors, even if the mechanical fit is correct. It is therefore essential to check the used part's OE number, particularly in the case of existing replacement chains.

SCN encoding compares the software version, vehicle configuration and control unit network. Only once encoding has been completed will the new NOx sensor be correctly integrated into the complete system. Without this adjustment there may be consequential errors, implausible measured values or further fault entries.

NOx sensors in the exhaust system are constantly exposed to high temperatures, condensation, road salt and splash water. Corroded connectors, damaged cables or internal ageing of the measuring elements are typical causes of failure, particularly in cases where the vehicle has a high mileage.

The sensors in the exhaust stream are contaminated if there is a mechanical engine fault, such as increased oil consumption or incomplete combustion. Unless these causes are eliminated, even a new sensor will soon be damaged or generate implausible readings.

Keep the load conditions consistent during the test drive, with SCR catalytic converter temperatures above 180°C. Only then will it be possible to measure conclusive NOx values upstream and downstream of the catalytic converter. A conversion rate of around 60–90% is considered typical for this system.

Voltage drops during encoding or teach-in value reset may damage control units or interrupt the adaptation process. Consequently, connect a high-performance charger to stabilise the vehicle electrical system during any diagnosis and programming operations.