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How to Reset a Nox Sensor

Oct 31, 2023      View: 6111


Under the current emission standards, automotive NOx sensors play a crucial role, and they are not unfamiliar to us. In the past, NOx sensors were solely used to measure the emissions of nitrogen oxides (NOx) in the exhaust. Nowadays, vehicles are equipped with two NOx sensors, each dedicated to monitoring the NOx emissions at the inlet and outlet of the aftertreatment system. The aim of this transformation is to comprehensively monitor and control vehicle tailpipe emissions.


As emissions laws become increasingly stringent, modern vehicles have indeed become more complex. NOx sensors, or Nitrogen Oxides sensors, are crucial components in this context. They play a pivotal role in monitoring and controlling emissions, especially in diesel engines. If NOx sensors fail, it's essential to understand what they are and explore your options. Keep reading to discover more!



What is a NOx Sensor 
How to Reset Nox Sensor
What Causes NOx Sensors to Fail
How do I know if my Nox sensor is bad?
Final Words


What is a NOx Sensor 

The Nitrogen and Oxygen (NOx) sensor serves the crucial function of measuring the concentrations of nitrogen and oxygen in the engine's exhaust gas. This data is essential for both the closed-loop control of the engine after-treatment system and the on-board OBD (On-Board Diagnostics) diagnostic system. Typically, this system includes two NOx sensors.


By employing both an inlet and outlet NOx sensor solution, the system can calculate real-time nitrogen oxide conversion efficiency and effectively regulate the engine's original nitrogen oxide emissions. This precise closed-loop control logic not only ensures compliance with emissions standards but also contributes to enhanced fuel economy.



  • Front NOx Sensor

This sensor is responsible for detecting the concentrations of NOx and oxygen in the engine's initial exhaust gas. It then transmits corresponding signals to the Engine Control Unit (ECU), which uses this data to calculate and regulate the amount of urea injection and initiate Diesel Particulate Filter (DPF) regeneration.


  • Rear NOx Sensor

The primary function of the rear NOx sensor is for OBD monitoring. It primarily detects the NOx concentration in the exhaust gas. When combined with the data from the front NOx sensor, it calculates the conversion efficiency. This calculation enables the closed-loop control of urea injection, contributing to more effective emissions control.

In summary, NOx sensors are integral to modern vehicle emissions control systems, facilitating compliance with emissions standards while also optimizing fuel efficiency through precise closed-loop control.



How to Reset Nox Sensor

The NOx (Nitrogen Oxides) sensor is an important component in modern vehicles, particularly those with diesel engines. It helps monitor and control the emission of nitrogen oxides, which are harmful pollutants. If you suspect an issue with your NOx sensor and want to reset it, here are the general steps you can follow: (Note that the steps might not work for all vehicles. )


  • Check for Error Codes

Before resetting the NOx sensor, it's a good idea to scan your vehicle for error codes using an OBD-II scanner. This will help you confirm whether the sensor is actually the problem and identify the specific error code related to it. The error code will give you more information about the issue.


  • Address the Underlying Issue

NOx sensor problems are often related to other issues in the exhaust or emission system, such as faulty exhaust gas recirculation (EGR) valves, clogged DPF (Diesel Particulate Filter), or problems with the selective catalytic reduction (SCR) system. Make sure to address these underlying issues before attempting to reset the sensor.


  • Disconnect the Battery

To reset the NOx sensor, you can start by disconnecting the vehicle's battery. This will clear the power to the sensor and reset the engine control unit (ECU). It's important to disconnect both the positive and negative terminals and wait for about 15 minutes before reattaching them. This should reset the ECU and, in turn, the NOx sensor.


  • Drive the Vehicle

After reconnecting the battery, take the vehicle for a test drive. This will allow the ECU to relearn and adapt to the new sensor status. Drive for a significant distance and in various conditions to ensure the sensor is functioning properly.


  • Clear Error Codes

Use your OBD-II scanner to clear any error codes related to the NOx sensor. This step should only be done after addressing the underlying issues and resetting the sensor.


If the NOx sensor problem persists, or if you are not comfortable performing these steps, it's best to seek professional help. A mechanic or technician can diagnose and replace the sensor if necessary.



What Causes NOx Sensors to Fail

NOx sensor failure can stem from a variety of factors, and having a grasp of these root causes is vital for effective troubleshooting and maintenance. Here, we explore the common reasons for NOx sensor malfunction.


  • Aging and Wear

NOx sensors are not immune to the effects of time. Over the years, they can degrade, particularly when subjected to extreme temperatures or challenging operating conditions. This gradual aging process can lead to a decrease in sensor accuracy, resulting in unreliable readings.


  • Contamination

External contaminants, such as sulfur, oil, or other substances found in exhaust gases, can accumulate on the sensor's surface. This contamination interferes with the sensor's ability to accurately detect NOx levels. Regular maintenance practices, coupled with clean exhaust systems, can go a long way in alleviating this issue.


  • Electrical Problems

NOx sensors depend on precise electrical signals to communicate with the Engine Control Module (ECM). Any glitches in the sensor's wiring, connectors, or even within the ECM itself can disrupt this communication, causing sensor malfunctions. Therefore, meticulous examination and testing of the sensor's electrical connections are indispensable during troubleshooting.


  • Component Failure

The overall performance of the NOx sensor can be adversely impacted by the condition of other components in the vehicle's emission control system. For example, a malfunctioning exhaust gas recirculation (EGR) valve or a blocked diesel particulate filter (DPF) can lead to erroneous NOx sensor readings. Hence, rectifying issues with these interconnected components is pivotal for ensuring precise sensor operation.


  • Physical Damage

NOx sensors often find themselves in vulnerable locations within the vehicle's exhaust system. Physical damage resulting from road debris, collisions, or accidents can compromise the sensor's functionality, ultimately leading to sensor failure. Implementing protective measures or relocating the sensor to a less exposed position can help safeguard it from physical harm.


  • Corrosion

Exposure to corrosive compounds in the exhaust, such as acidic elements, can cause the sensor to corrode over time. Regular inspections to detect early signs of corrosion and timely replacement are essential to preserve the sensor's performance.


  • Software or Calibration Issues

On occasion, NOx sensor issues may not be attributed to hardware defects but instead to software or calibration discrepancies. Resolving such problems may involve updating or reprogramming the sensor or its associated control software.


How do I know if my Nox sensor is bad?

Identifying whether your NOx sensor is experiencing issues holds paramount importance in safeguarding your vehicle's performance and emissions control. Here's a guide on recognizing potential problems with your NOx sensor.


  • Illumination of the Check Engine Light (CEL)

An unmistakable sign of NOx sensor woes is the activation of the Check Engine Light (CEL) on your dashboard. While this light signals concerns related to the NOx sensor, it can also point to troubles within the broader emission control system. When the CEL illuminates, it is prudent to initiate a diagnostic scan to pinpoint the specific problem.

Expanded: The Check Engine Light serves as an invaluable early warning system, serving notice of potential issues within your vehicle's intricate systems. When this light comes on, it is advisable to have your vehicle examined promptly, regardless of whether the problem pertains to the NOx sensor or another pivotal component.


  • Diminished Engine Performance

NOx sensor malfunction can lead to a decline in engine performance. This may manifest as reduced power, diminished acceleration, and an overall drop in engine efficiency. Should your vehicle exhibit a lackluster and unresponsive demeanor, it warrants a thorough assessment of the NOx sensor's condition.


  • Deteriorating Fuel Efficiency

An ailing NOx sensor can contribute to compromised fuel economy. When the sensor falters in accurately monitoring emissions, the engine may toil needlessly, leading to escalated fuel consumption. This inefficiency exacts a direct toll on your finances.


  • Failed Emissions Test

Failing an emissions test serves as an unequivocal indicator of an issue within the vehicle's emission control system, which encompasses the NOx sensor. If your vehicle falls short of the mandated emissions standards during testing, it serves as an unmistakable call to action, demanding swift investigation and resolution of the issue.



Final Words

In conclusion, resetting a NOx sensor is not a task to be taken lightly. The NOx sensor is a critical component in a vehicle's emissions control system, and its proper operation is vital for maintaining compliance with environmental regulations and optimizing fuel efficiency. If you suspect an issue with your NOx sensor, it's essential to first diagnose the problem accurately and address any underlying issues that may be affecting its performance. Proper diagnosis, regular maintenance, and professional guidance are key to ensuring that your vehicle complies with emissions standards, operates efficiently, and remains in good working condition.


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  • Will a bad NOx sensor cause misfires?
  • A defective NOx sensor can lead to various issues within the engine and its control systems. When the NOx sensor is malfunctioning, the engine management system may switch to emergency or limp mode. In this mode, the engine operates with limited performance to prevent further damage. Limp mode and the associated reduction in engine performance can cause engine misfires, which manifest as rough and uneven running.
  • Where is the NOx sensor located?
  • The NOx sensor is typically located in the exhaust system of a vehicle. It is commonly positioned both before and after the Selective Catalyst Reducer (SCR) in the exhaust system of diesel engines. The sensor before the SCR monitors the level of nitrogen oxides emitted by the diesel engine, while the sensor after the SCR helps ensure that the reduction of nitrogen oxides is effectively achieved.
  • How many NOx sensors does a car have?
  • In most vehicles, there are two NOx sensors. These sensors are commonly referred to as the upstream and downstream sensors. The upstream sensor is situated closer to the engine, typically before the catalytic converter, while the downstream sensor is positioned after the catalytic converter.
  • What is the life expectancy of a NOx sensor?
  • The life expectancy of a NOx sensor can vary, but on average, it's expected to last for approximately 50,000 to 60,000 miles. However, it's worth noting that many NOx sensors experience malfunctions well before reaching this mileage range.
  • How much does it cost to fix a NOx sensor?
  • The cost of replacing a NOx sensor can vary based on several factors, including the original equipment manufacturer (OEM), the specific truck model, and the source of purchase. Typically, a new OEM NOx sensor falls within the price range of $450 to $750 USD.
  • Can a car run without NOx sensor?
  • While it's technically feasible to drive for a limited period with a malfunctioning NOx sensor, it's not advisable. Operating your vehicle with a faulty NOx sensor can have detrimental effects on your engine and aftertreatment system. Initially, the engine may run in a rich fuel mixture, and the Diesel Exhaust Fluid (DEF) system might be overutilized. However, this is not a sustainable solution. Eventually, the Engine Control Unit (ECU) will detect the ongoing issue, leading to your vehicle entering limp mode and experiencing engine derate. In this mode, engine performance is intentionally limited to prevent further damage and to ensure compliance with emissions regulations.
  • What happens when a NOx sensor goes bad?
  • When a NOx sensor malfunctions, particularly in a diesel engine truck, a chain of unfavorable events unfolds. The engine's performance may start to deteriorate, causing it to run unevenly. This can result in a rough and less responsive engine operation. A failing NOx sensor can lead to inaccurate readings, causing the engine to work harder than necessary. This inefficiency results in a significant increase in fuel consumption, leading to higher operating costs. As the issue persists, the engine may enter a state known as derate or limp mode. In this mode, the engine's power and performance are deliberately restricted to prevent further damage or non-compliance with emissions standards. If the NOx sensor issue is not addressed promptly, it can lead to more severe problems. The truck may eventually break down and cease running altogether, necessitating extensive repairs.
  • Can I drive with a bad NOx sensor?
  • No, it's not advisable to continue driving with a faulty NOx sensor. When the NOx sensor is compromised, it can lead to engine performance issues and trigger multiple warning lights on your dashboard. The Engine Control Unit (ECU) will also log the fault, which can further affect your vehicle's operation.
  • Can a NOx sensor be cleaned?
  • As time passes, NOx sensors may accumulate dirt and become encrusted with soot or various impurities, potentially compromising their accuracy. It's worth attempting to clean the sensor using a gentle, non-abrasive cleaner and a soft brush.