What is the meaning of the Mercedes Benz P229F fault code?

In this blog, we will find out what is the meaning of the Mercedes Benz P229F fault code and answer the following questions: what is a NOX sensor in a Mercedes Benz? What are the symptoms of a bad NOX sensor? 

What is the Meaning of the Mercedes Benz P229F fault code?

The meaning of the Mercedes Benz P229F fault code is that the emissions control unit has detected a performance issue related to the NOX sensor. NOX sensors are mainly present on the latest Mercedes Benz models with diesel engines. However, Mercedes Benz also used them in the stratified version of the M276 petrol engine. It was available only in selected markets as it needed special gasoline with low sulphur content for operation in stratified mode. 

The latest diesel engines OM 654 and OM656 use an integrated diesel particulate filter along with a selective catalytic reduction converter for the exhaust gas after-treatment. An SCR control unit also controls the Adblue injection system which injects urea into the exhaust system upstream of SCR. When the SCR control unit detects a range signal from the NOX sensor then fault code P229F is recorded along with fault freeze frame data. The recorded fault code is stored in the SCR control unit and as this is an emission-related issue, the engine control unit will actuate the engine malfunction light in the instrument cluster.

The further diagnosis of the fault code P229F is only possible at an authorised dealership as the NOX sensor must be checked for proper operation. The NOX sensor is responsible for the feedback in the exhaust gas after-treatment. According to the feedback from the NOX sensor, the SCR control unit determines the injection quantity of the Ad-blue.

The diagnosis steps for fault code P229F are as follows:

  • Checking the actual value of the sensor and comparing it with the specified value.
  • Performing the visual inspection of the electrical line and coupler and checking the electrical contact for corrosion or lose contact.
  • The NOX sensor comes with an electronic controller that sends signals to the engine control unit. The signal is in the form of voltage level, if the voltage is out of range then the electrical or the electronic controller has an internal short circuit.
  • Removing and cleaning the NOX sensor probe is not a remedy as the fault will occur if there is a short circuit in the electrical line or the controller.

Once all the checks are completed, the NOX sensor is replaced. After renewing the NOX sensor, a road test of the vehicle is performed to check if the new sensor is working correctly. 

What is a NOX sensor in a Mercedes Benz?

A NOX sensor in Mercedes Benz is used to monitor the nitrogen oxide levels in the exhaust gases. It is connected to a small control unit that provides a power supply to the sensor probe and converts the sensor signal according to the NOX level in the exhaust system. 

There are two NOX sensors in the latest Mercedes Benz models. One sensor is located upstream of SCR and the other is located downstream. The selective catalytic reduction control unit can monitor the efficiency of the SCR by comparing the signal from both sensors.

If a huge amount of NOX is present then the SCR can increase the dosing of the Adblue to lower the NOX in the exhaust gases. The signal from the two sensors is also used to determine the conversion efficiency of the converter. If the diesel particulate filter is clogged the generation phase can be initiated by the engine control unit.

Mercedes Benz uses a dedicated oxygen sensor to check for oxygen percentage in the exhaust gases. It is also used to recognise a lean-burn or a rich burn operation. The NOX sensor is also capable of measuring the oxygen percentage in its dual-chamber design, the oxygen is removed in the first chamber and only NOX remains in the second chamber. In the second chamber of the sensor probe, the gas reacts with a catalyst made of rare earth elements and the control unit connected to the sensor probe determines the amount of NOX present in the exhaust gases.

What are the symptoms of a bad NOX sensor?

NOX sensor is crucial for the proper working of the SCR exhaust system, the symptoms of a bad NOX sensor are as follows

  • Check engine light

When the NOX sensor reading is not within the specified range the SCR control unit will save a fault code. As the SCR control unit is connected to the engine control unit over a CAN bus, the engine control unit will try to adjust the fuel quantity and activate the ‘check engine light’ in the instrument cluster.

The vehicle can still be driven with the check engine light ON but it is recommended not to drive the vehicle for long distances as it can damage the exhaust system.

  • Poor idling 

One of the symptoms of a faulty NOX sensor is poor idling of the engine along with increased noise. The exhaust back pressure can increase in the exhaust system due to the formation of soot particles. Excess soot particles will also cause blockage in the diesel particulate filter. 

The vehicle can idle at higher rpm as the engine control unit will inject more fuel for the regeneration of the diesel particulate filter. But as the NOX sensor is faulty, the injection quantity is not calculated correctly which affects the idling characteristic of the engine.

  • Erratic throttle response

The Mercedes Benz diesel engine is quality controlled, the angle of the throttle valve determines the amount of diesel injected into the combustion chamber. The feedback from the NOX sensor is also responsible as the SCR calculates the quantity of Adblue to be injected into the converter. 

An incorrect feedback signal from the NOX sensor affects the injection quantity of the fuel injectors and creates a jerky or erratic throttle response. 

  • Black smoke from the exhaust

There are many factors that can cause black smoke from exhaust like incomplete fuel consumption or a leak in a charge air system. However, a vehicle with a faulty NOX sensor will show black smoke intermittently. The NOX monitoring system is working all the time but it does not adjust the injection quantity of the Adblue until the nitrogen oxide levels reach maximum levels. The use of high-pressure EGR in combination with the low-pressure EGR reduces the NOX levels when the engine is not operating under full load conditions.

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NOX are chemical compounds of oxygen and nitrogen formed during the combustion of the fuel at high temperatures and pressure. Nitrogen oxides are very harmful to the environment and the NOX sensor is crucial for the working of the exhaust gas after-treatment system that converts the NOX into nitrogen, water vapours and a low amount of carbon dioxide. The latest diesel engines of Mercedes Benz are cleaner than some of the petrol engines available on the market.