Why is an NOx sensor in a Mercedes-Benz always replaceable as part of a plug-and-play procedure?

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.

How can you tell that the NOx sensor itself and not peripheral equipment is the cause of the fault?

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.

What role does the NOx sensor play in AdBlue metering within the SCR system?

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.

Why does the “No start-up in … km” message frequently appear within the context of NOx sensor faults?

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.

What role does correct spare part allocation play in terms of NOx sensors?

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.

Why is SCN encoding frequently required after installation?

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.

What kind of typical environmental factors lead to NOx sensor failures?

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.

Why might a new NOx sensor fail again shortly after replacement, despite having been installed correctly?

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.

Which conditions must be met for a successful SCR system functional test?

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.

Why is a stable vehicle electric system voltage essential during the adaptation process?

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.

How workshops can test, replace and adjust NOx sensors at Mercedes-Benz

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

If a fault is diagnosed with the NOx sensor when troubleshooting an exhaust gas purification system, this can present a challenge for a workshop. Because of the complexity of the component and the engine management systems, not every NOx sensor can simply be replaced in the plug and play manner so that everything works smoothly afterwards. In this article, we would therefore like to take a closer look at some of the special features that need to be taken into account when the NOx sensor has to be replaced. On the one hand, these involve the correct allocation of the spare part and, on the other, the component adjustment after the NOx sensor has been installed. We will illustrate this using a Mercedes Benz E350 (212) BlueTEC produced in 2014 as an example.

Error display

Figure 1: The error code P229FFB / NOx sensor 2 bank 1 could be read out from the engine control unit.

Driver information \'AdBlue no start in 800 km\' in the instrument cluster display — Figure 2: Driver information: "AdBlue No start in 800 km" in the instrument cluster display

Figure 3: Before the new NOx sensor is installed, the electrical plug connection on the vehicle should be checked for defects.

Example illustration of the component designation and pin assignment 1. NOx sensor upstream of CAT. 2. NOx sensor downstream of CAT. — Figure 4: Example illustration of the component designation and pin assignment NOx sensor upstream of CAT. NOx sensor downstream of CAT.

Figure 5: Checking of the power supply and signal line directly at the connector. Damage caused by test probes is to be avoided.

Figure 6: CAN signal measured at the NOx sensor control unit

In our case study, the error code P229FFB was read out from the engine control unit. This error code is associated with a malfunction of the NOx sensor 2 downstream of the catalytic converter.
The following causes may be responsible for this error entry:

Cable breaks, short circuits or contact resistances in the circuit
Faulty power supply to the NOx control unit
NOx sensor faulty
Control unit faulty

As our vehicle is equipped with an SCR catalytic converter and urea injection, the warning "AdBlue no start in 800 km" is shown in the instrument cluster display at the same time. As a result of the failure of the NOx sensor, the control unit does not receive important information for calculating the dosing of the urea quantity.

Consequently, other error codes such as DTC 16D300 / urea additive system "Distance travelled limited because of fault" may also be stored in the error memory.

In order to rule out further sources of error, the plug connection on the NOx sensor control unit should be checked for damage or oxidation before replacing the NOx sensor. NOx sensors are usually located on the underbody in the exhaust system and are exposed to environmental influences such as splash water and road salt, substances which can lead to damage. We then recommend checking the power supply and signal line (CAN bus) on the electrical plug connection to the NOx control unit. The best way to check the CAN signal is with an oscilloscope. If all measurement results are satisfactory, then it can be assumed that the NOx sensor is the cause of the error entered. After removing the sensor, the measuring probe should be checked for contamination or damage. Sensors in the exhaust gas system of vehicles with high mileage are often impaired by mechanical engine faults, such as increased oil consumption. If such a defect is diagnosed, it can be assumed that a new NOx sensor will also fail again within a very short time.

If the AdBlue warning in the instrument cluster is ignored and the driver continues to drive anyway, this can result in the vehicle no longer starting when the remaining distance is exceeded at the next start.

Part identification

Figure 1: The correct NOx sensor can only be allocated by comparing it with the OE part number.

Figure 2: Example illustration of the spare parts search in the OE catalogue

One of the most important points for achieving a successful repair is getting the right spare part. Each NOx sensor has its own control unit with an appropriate software version, so correct identification of the spare part number is crucial to ensure everything works perfectly in the vehicle. In some cases, several NOx sensors may be shown in the catalogue for one vehicle model. This is due, on the one hand, to the individual mounting position of the sensor and, on the other hand, to the supersession chains of the spare part numbers, which are put into effect by the vehicle manufacturer. In any case, a comparison should always be made with the OE part number of the removed NOx sensor. This ensures that the correct sensor is installed in the vehicle. If a clear allocation is not possible because of missing spare part or vehicle information, access to an OE catalogue will help in such a situation. The required OE part number can be researched here by means of the chassis number. If you do not have the vehicle documents to hand, the chassis number can usually be found on a type plate on the bodywork or in the lower windscreen area.

To determine the correct NOx sensor, the data from the vehicle registration document and also the part number of the old sensor should be used as a reference. Finally, a comparison should always be made with the OE part number!

Component customisation

After installation of the NOx sensor with control unit, an adjustment to the vehicle must be made using a suitable diagnostic device. Depending on the vehicle, model and control unit, these adaptations may be designed differently. Sometimes it is sufficient to delete the error code and reset the learned values, but in some cases adjustments can be more elaborate. In our case study, SCN coding must also be carried out in accordance with the vehicle manufacturer's repair guide. (SCN = Software Calibrations Number) This coding is carried out via remote access (remote diagnostics) by connecting the vehicle, diagnostic device and the server of the relevant vehicle manufacturer. This necessary measure allows the control unit software and coding to be recognised and adapted to suit the vehicle in question. This is the only way to ensure secure communication with other control units in the vehicle. If this adjustment is not carried out, communication errors and further error memory entries cannot be ruled out. For this reason, the maintenance and repair instructions of each individual vehicle manufacturer must be strictly observed.

macsRemote Services

The NOx sensor can be customised using macsRemote Services. To do this, a macsRemote device is connected to the OBD interface in the vehicle and a connection to macsRemote Services is established. Once the vehicle data and the required service in the application have been selected and confirmed, the ticket can be created and sent. As soon as the Remote Services technician opens the ticket, the dialogue with the workshop begins. All necessary diagnostic and repair procedures are determined by the service technician on the basis of an initial diagnosis and then carried out online after consultation with the workshop.

In this case, the following services had to be carried out on our example vehicle

Reading out and clearing error memory
Carrying out coding
Resetting NOx sensor learned values
Reading out once again the error memory
Resetting distance warning

The workshop technician has to be available throughout the complete Remote Services work on the vehicle so that it is possible to engage in active dialogue with the remote expert. In addition, manual operations, such as switching the ignition on and off have to be carried out on the vehicle. At the end of the service, the customer receives proof of successful coding and resetting. All necessary activities and legal information will be communicated to the workshop prior to the carrying out of the work and must be confirmed in writing. This procedure ensures that the job order has been carried out as arranged.

NOx sensor: Spare part identification and vehicle adaptation

In this video, we look at two important aspects that should be considered when a NOx sensor is being replaced:
How do I find the right spare part and what do I need to consider after installing the NOx sensor?
We explain this information using a Mercedes-Benz E-Class (model 212) as an example.

The NOx sensor can be customised using macsRemote Services. Once the vehicle has been identified, the required service can be selected in the application and a ticket created.

Functional check and test drive

Parameter display of the NOx sensors upstream and downstream of the SCR catalytic converter.

After the adjustment or repair, an extensive test drive should be carried out. To check the function of the SCR system and the NOx sensors, a test drive of at least 15 minutes at a constant speed (approx. 100 km/h) and at an SCR catalytic converter temperature of over 180 degrees Celsius must be carried out. The values from the NOx sensor upstream and downstream of the catalytic converter are then compared. The NOx sensor values have to change continuously, with the sensor upstream of the catalytic converter always displaying the higher value. The conversion rate here should be around 60-90%. On our sample vehicle, the checking of the parameters and the final readout of the error memory showed no further faults in the exhaust gas aftertreatment.

The various diagnostic options have been illustrated using as an example the mega macs X diagnostic device and macsRemote Services. 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 the document’s text and cannot be used as a basis for vehicle-specific repairs.