Nitrogen oxide sensor

A nitrogen oxide sensor or NO_x sensor is typically a high-temperature device built to detect nitrogen oxides in combustion environments such as an automobile, truck tailpipe or smokestack.

Automotive NOx sensors are primarily of the amperometric type, with two or three electrochemical cells in adjacent chambers. The first cell electrochemically pumps O2 out of the sample so it does not interfere with the NOx measurement in the second cell. Commercial sensors, available from several suppliers, are used for the control of NOx adsorber and SCR aftertreatment. NH3 sensors have been also developed for use in SCR systems.

An oxides of nitrogen sensor is used to monitor the levels of oxides of nitrogen that a vehicle emits to ensure it is compliant with emissions regulations.

Oxides of nitrogen (otherwise known as NOx) are pollutants that have both environmental and health impacts. NOx usually refers to nitric oxide (NO) which becomes nitrogen dioxide (NO2) when it combines with oxygen under high temperatures. This reddish-brown gas makes summer smog look brown and hazy in high-traffic areas.

The gases are poisonous and highly reactive. They can affect and contribute to respiratory infections and asthma, especially in children, as well as cause detrimental effects to local ecosystems and road visibility.

Controlling the levels of NOx emitted into the atmosphere is incredibly important, which makes the NOx sensor a vital vehicle component.

Overview

The term NO_x represents several forms of nitrogen oxides such as NO (nitric oxide), NO₂ (nitrogen dioxide) and N₂O (nitrous oxide, also known as laughing gas). In a gasoline engine, NO is the most common form of NO_x at around 93%, while NO₂ is around 5% and the rest is N₂O. There are other forms of NO_x such as N₂O₄ (the dimer of NO₂), which only exists at lower temperatures, and N₂O₅, for example.

Meanwhile, for diesel engines, the emissions situation is different. Owing to their much higher combustion temperatures (resulting from their high cylinder compression ratios as well as turbocharging or supercharging), diesel engines produce much higher engine-out NO_x emissions than spark-ignition gasoline engines. The recent availability of Selective catalytic reduction (SCR) allows properly equipped diesel engines to emit similar values of NO_x at the tailpipe compared to a typical gasoline engine with a 3-way catalyst. The SCR changes the harmful nitrogen oxides by adding the solution AdBlue which reduces environmental pollution and protects the exhaust system. In addition, the diesel oxidation catalyst significantly increases the fraction of NO₂ in "NO_x" by oxidizing over 50% of NO using the excess oxygen in the diesel exhaust gases.

Advances of sensor technology enable people to monitor air quality through widely distributed low-cost sensors. The drive to develop a NO_x sensors arises from environmental factors. NO_x gases can cause various problems such as smog and acid rain. Many governments around the world have passed laws to limit their emissions (along with other combustion gases such as SOx (oxides of sulfur), CO (carbon monoxide) and CO₂ (carbon dioxide) and hydrocarbons). Companies have realized that one way of minimizing NO_x emissions is to first detect them and then to employ some sort of feedback loop in the combustion process, thereby enabling the minimization of NO_x production by, for example, combustion optimization or regeneration of NO_x traps. Therefore, in many applications with exhaust-gas treatment systems, one NOx sensor is used upstream of the exhaust-gas treatment system (upstream) and a second sensor is used downstream of the exhaust-gas treatment system. The upstream sensor is used for the aforementioned feedback loop. Meanwhile, the downstream sensor is used mainly to confirm that the legislated emissions limits have not been exceeded.

Challenges

Harsh environment

Due to the high temperature of the combustion environment, only certain types of material can operate in situ. The majority of NO_x sensors developed have been made out of ceramic type metal oxides, with the most common being yttria-stabilized zirconia (YSZ), which is currently used in the decades-old oxygen sensor. The YSZ is compacted into a dense ceramic and conducts oxygen ions (O²⁻) at the high temperatures of a tailpipe such at 400 °C and above. To get a signal from the sensor a pair of high-temperature electrodes such as noble metals (platinum, gold, or palladium) or other metal oxides are placed onto the surface and an electrical signal such as the change in voltage or current is measured as a function of NO_x concentration.

High sensitivity and durability required

The levels of NO are around 50–1000 ppm (parts per million) and NO₂ 10–100 ppm in a range of 1–10% O₂. The sensor has to be very sensitive to pick up these levels.

The main challenges in the sensor development are selectivity, sensitivity, stability, reproducibility, response time, limit of detection, and cost. In addition due to the harsh environment of combustion the high gas flow rate can cool the sensor which alters the signal or it can delaminate the electrodes over time and soot particles can degrade the materials.

One of the major challenges faced by such gas sensors is humidity. The relative effect on signal response is highly subjective to the sensor type. Electrochemical sensors are mostly immune from humidity effect as water molecules help regulate the electrolyte concentration but long term exposure to dry gas can reduce the solvent concentration of the electrolyte. High amount of cross sensitivity has been observed in gas sensors due to similarity in electron exchange mechanism between target gases and water molecules.

What does a NOx sensor do?

The NOx sensor not only monitors the level of the oxides of nitrogen being released into the air, but it also plays an essential role in controlling it through the Selective Catalytic Reduction (SCR) system in diesel vehicles.

The NOx sensor is a key part of this system, which also includes the SCR catalyst, Diesel Particulate Filter (DPF), and turbocharger amongst others. If the NOx sensor detects too much NOx gases being emitted, it sends that information to the SCR system, which then adjusts the output to a level that’s compliant with emission regulations.

How does a NOx sensor work?

The sensor is made up of a Nernst cell with a current flowing through. This corrects the air/fuel ratio to λ=1. A second cell, usually made of ceramic with a layer of rhodium, breaks down the oxides of nitrogen into nitrogen and oxygen. The NOx controller – usually closely positioned to the NOx sensor – calculates the level of oxides of nitrogen present in the exhaust gas, and sends that data to the SCR controller. The SCR controller then adjusts the amount of diesel exhaust fluid injected into the SCR catalyst, which in turn transforms excess oxides of nitrogen into water and nitrogen.

Where is the NOx sensor located?

The NOx sensor is most commonly found after the SCR catalyst. This allows the SCR controller to check that NOx levels have dropped to the correct amounts. Some vehicles have pre- and post-SCR sensors, although just the one sensor is more common.

Do you need a NOx sensor?

Yes, NOx sensors are a necessity, especially for modern diesel vehicles.

With oxides of nitrogen causing harmful effects on human health and indirectly damaging the environment, emissions controls are tightly regulated.

NOx sensors have been fitted in vehicles since the early 2000s. To be Euro 6 compliant, a modern diesel vehicle cannot emit more than 80mg/km of NOx gases, which would be impossible without a NOx sensor to accurately monitor the levels.

You can find a handy breakdown of the regulations here.

Are NOx sensors only found in diesel engines?

Most modern vehicles are fitted with NOx sensors – both petrol and diesel.

Even though petrol engines produce significantly less NOx than diesel engines, emission levels are still highly regulated. To pass Euro 6 compliance tests, modern vehicles have at least one NOx sensor to control the vehicle’s output.

Common NOx sensor faults

Precision components such as NOx sensors have limited lifespans, so it is fairly common to have to replace them at some point during vehicle ownership.

One of the main causes of a NOx sensor fault is buildup of soot from the combustion process. Soot is not only abrasive, meaning that it can damage the sensor over time, it can also coat the sensor, inhibiting it from measuring the gas accurately.

For an in-depth look at common causes of NOx sensor issues, check out our guide.

Can you test a NOx sensor?

If the vehicle is exhibiting NOx sensor fail symptoms such as:

• engine warning light illumination
• increased fuel consumption
• reduced power
• going into limp mode
• reduced acceleration
• reduced performance

…a diagnostic tool such as Delphi's BlueTech VCI can scan the vehicle’s ECU for error codes.

However, these codes may not tell the full story as they could be caused by a number of issues. If a NOx sensor error is suspected, a further test can be performed on the sensors themselves using a multimeter to see if they’re operating correctly. Take a look at our guide for a run through.

How do you clean a NOx sensor?

Cleaning a NOx sensor is not recommended. NOx sensors are actually self-cleaning, so if the sensor is displaying signs that it isn’t working properly, it should be replaced rather than cleaned manually to solve the issue.

Can you repair a NOx sensor controller?

A NOx sensor replacement can be costly, so the repair option may be tempting. However, for such a precise component, a repair may only be a short-term solution before it fails again, meaning you’d have to do the job twice.

Can you replace a NOx sensor?

Absolutely, and we have a handy how to guide that tells you how to replace a NOx sensor, step by step.

For cost reasons, it may be tempting to replace a faulty NOx sensor with a refurbished one. We would recommend only using brand new, quality components to achieve the longest lifespan and accurate performance.

NOx Sensor Applications

The development of exhaust gas NOx sensors started in the 1990s. Commercial sensors were first introduced in the early 2000s on lean-burn, stratified charge gasoline passenger cars with NOx adsorbers, followed by diesel cars with NOx adsorbers and light- and heavy-duty diesel engines with urea-SCR aftertreatment.

The first generation of NOx sensors was developed by NTK, also known as NGK/NTK or NGK Spark Plug (not to be confused with NGK Ceramics) in Japan, and was first used in 2001 in the Volkswagen Lupo 1.4 FSI. Eventually, all stratified charge gasoline engines in the Volkswagen Group (1.4, 1.6 and 2.0 L) were equipped with NOx sensors. Other OEMs, including Daimler and BMW, also put large numbers of gasoline engines with charge stratification onto the roads. After a few years, however, the use of stratified charge engines and the associated market for NOx sensors started to decline, due to the lower than expected CO2 emission benefits and the high cost of NOx adsorber aftertreatment. Volkswagen bid farewell to stratified charge engines in 2006, and BMW followed suit five years later. Only Daimler has continued to use spray-guided stratified charging in their M270/M274 engine family.

Another area of NOx sensor application has opened with the introduction of NOx adsorber catalysts on light-duty diesel engines. Some of the first applications included the Toyota DPNR system, launched in 2003, and the diesel engine Renault Espace model. The technology was widely adopted on diesel cars—primarily in Europe, but also in the US and other markets—including models from Volkswagen, BMW, and Daimler. These vehicles were typically equipped with a NOx sensor after the NOx storage catalytic converter.

The most recent area of NOx sensor application are urea-SCR systems for light- and heavy-duty diesel engines. To satisfy various OBD (on-board diagnostics) requirements, SCR systems typically use a NOx sensor downstream of the SCR catalyst. If excessive NOx or ammonia concentrations exist at the SCR outlet, an OBD malfunction will be triggered, as NOx sensors are sensitive to both gases. Depending on the SCR control strategy, another NOx sensor may be installed in front of the SCR catalytic converter. If two sensors are installed, the conversion rate of the SCR catalytic converter can be easily determined.

Further development of NOx sensors is driven by future heavy-duty engine emission standards such as those being proposed by CARB and the US EPA for 2027. The NOx limits may be lowered to values as low as 0.015 g/bhp-hr, while the durability and useful life requirements could be extended up to 850,000 miles (1,360,000 km) and 18 years. Improved sensor performance would not only be required for potential changes to OBD thresholds but also for in-use emissions monitoring that is being proposed as an alternative to the more conventional durability demonstrations. NOx sensor technology would need to develop further to be able to monitor emissions at low NOx levels, over the whole duty cycle of heavy-duty vehicle operations, and over their entire useful life.

The most common in-situ NOx measurement technology relies on yttrium-stabilized ZrO2 (YSZ) electrochemical sensors , similar in construction and operating principle to broadband oxygen sensors. Commercial sensors are available from Continental/NGK  and Bosch , while others such as Denso have sensor development programs . The YSZ sensors are discussed in detail in the following sections.

The two final sections of this article cover, respectively, new NOx sensor developments and ammonia sensors. The latter technology, based on the same YSZ electrochemical system, has been commercialized in some SCR applications, but its use remains limited

Principle of Operation

Overview

Commercial NOx sensors for automotive applications are primarily YSZ electrochemical sensors of the amperometric type. Figure 1 illustrates the basic operating principle. The sensor uses two or three electrochemical cells in adjacent chambers. The first cell electrochemically pumps O2 out of the sample so it does not interfere with the NOx measurement in the second cell. The need to remove O2 allows this type of NOx sensor to serve a dual purpose; it can also detect exhaust O2 level.

Figure 1. Schematic representation of an amperometric NOx sensor

The O2 in the first cell is reduced and the resulting O ions are pumped through the zirconia electrolyte by applying a bias of approximately -200 mV to -400 mV. The pumping current is proportional to the O2 concentration. The remaining gases diffuse into the second cell where a reducing catalyst causes NOx to decompose into N2 and O2. As with the first cell, a bias of -400 mV applied to the electrode dissociates the resulting O2 which is then pumped out of the cell; the pumping current of the second cell is proportional to the amount of oxygen from the NOx decomposition. An additional electrochemical cell can be used as a Nernstian lambda sensor to help control the NOx sensing cell.

All HC and CO in the exhaust gas should be oxidized before the NOx sensing cell to avoid interference. Also, any NO2 in the sample should be converted to NO prior to NOx sensing to ensure the sensor output is proportional to the amount of NOx.

Solid Zirconia Electrolyte

A number of zirconia formulations doped with metal oxides have been investigated for use in oxygen (λ, lambda), as well as NOx sensors. Materials that have been tested include Fe2O3, Co3O4, NiO, CuO, ZnO, CeO2, La2O3, Y2O3, as well as mixtures of zeolites, aluminum and silicates . Several chemical elements were also selected as potential electrode materials, including platinum, rhodium and palladium.

The system that has been most widely adopted and used in almost all commercial NOx and lambda sensors is based on solid state yttrium-stabilized zirconia electrolyte (the same material that was used in the Nernst lamp). A key property of the YSZ ceramics is its high conductivity for O2 ions at elevated temperatures. The stabilization with yttrium has two benefits: (1) it impedes ZrO2 phase transformation, which increases the mechanical strength of the material, and (2) it enhances the oxygen ion conductivity of zirconia.

Zirconium oxide ceramics can have one of three crystalline phases, depending on the temperature :

• Monoclinic crystal structure at room temperatures
• Tetragonal crystal structure from 1,170°C
• Cubic crystal structure from 2,370°C

The cubic crystal structure displays a particularly regular arrangement of elements, and is characterized by high oxygen ion conductivity. Through the addition of metal oxides, the high temperature crystal structures can remain stable at lower temperatures. By adding sufficient quantities of yttrium oxide (Y2O3) in a sintering process at approximately 1,000°C, it is possible to cubically stabilize zirconium oxide.

If the yttrium oxide quantities are too low, mixed crystals form, consisting of the monoclinic and cubic phase. These partially stabilized zirconium oxide (PSZ) materials feature a pronounced resistance to thermal fluctuations.

Two types of YSZ ceramics, 4YSZ and 8YSZ, are the basis of almost all lambda and nitrogen oxide sensors. These designations indicate the level of doping with yttrium oxide, as follows:

• 4YSZ—partially stabilized ZrO2 doped with 4 mol% of Y2O3
• 8YSZ—fully stabilized ZrO2 doped with 8 mol% of Y2O3

When zirconia is stabilized with yttrium oxide, the Y3+ ions replace Zr4+ in the atomic lattice. This way, two Y3+ ions generate one oxygen gap. These gaps are utilized for the transport of oxygen.

The maximum oxygen ion conductivity is observed within the temperature range from 800°C to 1,200°C. Unfortunately, at these temperatures a separation also occurs into Y-lean and Y-rich areas. This process is irreversible and results in a severe reduction in oxygen conductivity. At 950°C, O2 conductivity can be reduced by as much as 40% after 2,500 hours . This is the reason why lambda and NOx probes may not be subjected to temperatures above approximately 930°C. Nitrogen oxide sensors by Continental, for example, are operated at 800°C.

Oxygen Pump Cells

If a dividing wall made of YSZ ceramics is placed between two chambers with different oxygen partial pressure, nothing will happen at room temperature. However, when the temperature of the ceramic wall is increased to approximately 600°C, oxygen ions can move through the gaps in the crystal lattice. An alignment takes place, where the chamber with the higher partial pressure pushes oxygen ions through the wall to the chamber with the lower pressure.

If both surfaces of the dividing wall are fitted with an electrode, it is possible to verify the movement of ions through voltage measurement. And this is precisely what happens in the binary (switching) lambda sensor. The reduction of oxygen to O2- that occurs in the chamber of a higher O2 pressure is described by Equation (1):

${\ce {O2+4e=^2}}$

and the sensor voltage is given by the Nernst equation:

Us = (RT/4F) ln(pref / pexh)(2)

where:

Us - sensor signal, V

T - temperature, K

p - partial pressure of oxygen

R - gas constant = 8.314 J/mol

F - Faraday constant = 96,485 sA/mol

The diagram in Figure 2 presents the chamber with high oxygen partial pressure as the blue-colored area, and the chamber with low oxygen partial pressure as the gray area. If the brown-colored ceramic is heated to 600°C, the micro-porous platinum electrodes presented in yellow will generate approximately 1V.

Figure 2. Schematic of a solid zirconia electrolyte cell

Passive Cells. The chamber with the high partial pressure of oxygen is the reference air duct. Rich exhaust gas (λ < 1) has a low oxygen content. If the zirconium oxide ceramics is heated using a heating element to approximately 600°C, oxygen ions move from the reference air duct through the ceramic wall onto the exhaust gas side and almost one volt signal voltage is generated. In the case of lean exhaust gas (λ > 1), the oxygen partial pressure difference relative to the reference air is low and a signal of only 0.1V or less is measured. At λ = 1, the signal voltage is approximately 0.4-0.5V, depending on the manufacturer and probe model. The voltage-lambda characteristic is almost stepwise, allowing the sensor to distinguish between two lambda values—rich and lean—hence the term “binary” lambda sensor.

In such operation—representative of a binary lambda probe—the generated voltage correlates with the drop in oxygen partial pressure. The passive YSZ ceramics cell is also called the potentiometric or Nernst cell.

Active Cells. It is also possible to actively operate the probes, as is the case in broadband (linear) oxygen sensors and in the amperometric cells in NOx sensors. In active operation, no voltage is picked up on the electrodes, but rather the electrodes are connected to a power source. In such active cells—referred to as “pump cells”—it is possible to “pump” oxygen ions from the oxygen-lean to the oxygen-rich side by reversing the polarity. The pumping current provides a measure of oxygen concentration. The current-lambda characteristic is linear, which makes it possible to measure O2 concentrations at various air-to-fuel ratios.

NOx sensors include at least two oxygen pump cells (Figure 1)—one to remove excess oxygen from the exhaust gas, and another to measure the concentration of oxygen released from the decomposition of NOx.

Technical data

Very precise and real-time readings from the exhaust gas system are required for efficient exhaust-gas treatment. In diesel vehicles, the nitrogen oxide (NOx) sensor is used to provide the signals for evaluation of quantity of injected (urea solution) AdBlue® in systems for selective catalytic reduction (SCR) and NOx reduction. In petrol as well as in diesel applications the signals of a NOx sensor can be used to monitor the vehicle (on-board monitoring of emissions and on-board diagnostics).

High accuracy: +/- 5 ppm at low NOx level (0 ppm)

High lifetime: 15,000 hours with robust sensing element und molded sensor control unit

Univolt sensor for12V and 24V applications

Very fast light-off time < 60s for full tolerance

0–3,000 ppm

Measuring range

±5 ppm

NOx accuracy at 0 ppm

140 °C

Max. permissible temperature (sensor control unit)

~ 1,000 ms

Response time NOx

For use in passenger cars and commercial vehicles

NOx sensor

NOx sensors can be used for 12 V and 24 V applications in passenger vehicles (petrol and diesel) as well as light or heavy commercial vehicles (on- and off-highway).

The sensors consist of a probe and a sensor control unit (SCU) connected to one another via a wiring harness. The probe is quick and easy to install thanks to a retaining screw.

Monitoring and control according to amperometric principle

The NOx sensor is installed in the exhaust gas flow downstream as well as upstream of the SCR catalytic converter or three way catalyst depending on the application, and measures the nitrogen oxide and oxygen content in the exhaust system. It is connected to the engine management system via the CAN bus.

The core component of the NOx sensor is a ceramic sensor element that operates according to the amperometric double chamber principle. The closed-loop controller results in further benefits, such as the reduction of NOx emissions over the entire lifecycle.

Design and Function

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.

Interesting facts

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.

Interesting facts

Integrated heating element in the NOx sensor

The integrated heating element enables a constant and optimum operating temperature to be maintained in the sensor. This allows the sensor to be heated to the predefined operating temperature regardless of the ambient temperature and engine temperature. This ensures that the NOx sensor can react optimally even at low temperatures. The temperature of the heating element is usually regulated by the engine control. The engine control unit adapts the heating output to the ambient conditions. This not only improves the accuracy of the nitrogen oxide measurement, but also has a positive effect on the service life of the sensor.

Important information

Causes of failure and symptoms

Due to the installation position in the exhaust system and the ambient conditions there, the functional life of the sensor is not unlimited.

A malfunction or failure can be caused by the following reasons

• Sensor function loses efficiency
  • Wear due to ageing. Like a lambda sensor, the NOx sensor unit can also age
  • Due to operating conditions such as exhaust gas composition, temperatures and vibrations
• Sensor head sooted
  • Short-distance operation, incorrect mixture composition or high oil consumption
• Environmental influences
  • Moisture, water or road salt
• Mechanical damage
  • Incorrect installation, accident or marten bite
• Faulty power supply
  • Cable interruptions
  • External or internal short circuits
• SCR system faulty
  • Defective components – incorrect dosing of AdBlue® can lead to deposits. These can damage the sensor and cause it to fail

The following symptoms may occur if the NOx sensor fails.

• Engine warning light comes on
• SCR system warning in the instrument cluster display
• Saving an error code in the control unit
• Malfunction or emergency operation of the SCR system
• Increased fuel consumption or poor engine performance

5. Practical tips

Troubleshooting the NOx sensor

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.

Nox sensors are used to detect nitrogen gases in hot exhaust gases. The principle of the operation of the NOX sensor consists in dividing the exhaust gas into nitrogen and oxygen by a catalytically active electrode. The amount of oxygen is measured in the same way like at a broadband or linear lambda sensor.

The NOX sensor has two chambers. In the first chamber (which resembles a lambda sensor), the oxygen content of the exhaust gas is maintained at approximately 10 parts per million by means of a so-called flow pump. The power required is inversely proportional to the air to fuel ratio and can be used to correct the NOX signal depending on the measured ratio of these two quantities. In the second chamber there is a NOX reaction, a separation of nitrogen and oxygen. The electrical current that is required to maintain an oxygen-free environment in the platinum-rhodium electrode is proportional to the NOX concentration and thus provides a measuring signal.

What is NOX SENSOR and how does it work ?

The NOx sensor is typically located in the exhaust pipe, downstream of the catalytic converter. It works by measuring the amount of NOx present in the exhaust gases using a specialized ceramic material that reacts with the nitrogen oxides. This reaction generates an electrical current, which is then used to calculate the concentration of NOx in the exhaust.

The NOx sensor is an important part of the engine's emissions control system. The sensor sends the information about the NOx level to the engine control unit (ECU), which then makes adjustments to the engine's performance to reduce the NOx emissions. This can be achieved by adjusting the fuel injection timing, the air-to-fuel ratio, or by activating the catalytic converter.

It is important to note that the NOx sensor can become damaged or worn over time, which can lead to inaccurate readings and reduced engine performance. Symptoms of a faulty NOx sensor include decreased fuel efficiency, increased emissions, and a check engine light. If you suspect that your NOx sensor is not working properly, it is important to have it inspected and replaced if necessary.

In conclusion, NOx sensors play a crucial role in reducing emissions from diesel engines. They work by measuring the amount of NOx present in the exhaust gases and communicating this information to the engine control unit, which then makes adjustments to the engine's performance to reduce the NOx emissions. Ensuring that your NOx sensor is in good working condition is crucial for maintaining the performance and efficiency of your diesel engine, as well as for complying with emissions regulations.

See also

• List of sensors