Nox Sensor Failure Symptoms – An Inclusive Guide

Atmospheric pollution caused by several forms of gases like methane, carbon dioxide, and nitrogen oxide was once the order of the day. Some gases even cause ozone depletion. As a result, the authorities keep setting stricter emission regulations, and diesel vehicles’ purchase cost keeps scaling through the roof. 

Automakers now design a few Nox sensors in cars that notify the driver when emissions are not as clean as they should be. That brings us to this mind-boggling question, ‘what happens when a Nox sensor fails?’

Here, we’ll discuss Nox sensor failure symptoms, causes, diagnoses, and fixes. But first, let’s look at what it is.

Nox Sensor Failure Symptoms

What Are Nox Sensors?

As the name implies, Nox sensors are components used in measuring and calculating the amount of nitrogen oxides in the exhaust gas. The amount of nitrogen oxides in the exhaust gas shows whether there is adequate oxygen during the combustion process. This aids the catalytic converter is functioning properly. 

Their primary function is to reduce harmful exhaust byproducts and ensure a safe environment. You may wonder, ‘what happens if a Nox sensor fails’? This sensor will cause the powertrain control module to display the check engine light on the dashboard. You may also experience low engine power output, poor acceleration, and bad gas mileage. 

Nox sensors come in various designs, and it varies from vehicle to vehicle. Old vehicles may include DFP, turbocharger, SCR catalyst, and upstream Nox sensor from the exhaust catalyst. 

Modern vehicles come with upstream and downstream Nox sensors. They measure the exhaust emissions and send the data to the engine control module. The ECM calculates how much fuel to send into the combustion chamber. 

Both the upstream and downstream Nox sensors are mounted on the exhaust pipes. They perform differently and offer varying benefits. The upstream sensor measures the engine-output Nox, and the downstream sensor calculates the Nox emissions from the SCR catalyst.

How Does A NOx Sensor Work?

By answering the question, ‘what does a Nox sensor do in diesel’ vehicles, we’ll have a clear picture of how it works.

A Nox sensor is an SCR system component that measures the amount of Nox in an exhaust system. This helps the SCR to know the right level of ammonia or urea to provide better fuel economy and less harmful emissions to the atmosphere.

Aside from measuring the amount of nitrogen oxides in the exhaust gas, some Nox sensors also monitor the mass of exhaust flow. 

A high-end Nox sensor on modern cars features some compounds that react to nitrogen oxides. These compounds are known as catalysts and are usually manufactured with activated carbon, titanium oxide, and other base metals.

These compounds send a voltage correlated to the level of nitrogen oxide. The higher the nitrogen in the exhaust gas, the stronger the voltage.

Nox sensors with less sophisticated compounds work monitoring the exhaust gas and erasing oxygen, and judging the Nox concentration. So, how do I know if my Nox sensor is bad?

NOx Sensor Failure Symptoms 

Like other car sensors, Nox sensors can fail. Whether you’re searching for Nox sensor failure symptoms on DD15 engines or BMW Nox sensor failure symptoms, here are the key signs you should watch out for.

High fuel consumption 

If the Nox sensor goes bad and cannot deliver accurate information to the engine management software regarding the air-fuel mixture ratio, the car computer may not know the proper amount of air fuel to mix.

In some scenarios, this will cause the powertrain control module to compensate by mixing more fuel to ensure optimal engine performance. This is known as a rich fuel mixture.

If you have bad gas mileage on your Cummins, this could be a Cummins Nox sensor problem. It’s important to note that other factors can lead to bad gas mileage. 

Rough idling 

One common Nox sensor failure on Mercedes and other vehicles is rough or erratic idling. The fuel injectors inject a pre-defined amount of gas into the combustion chamber when the engine is running. The DME oversees and electronically manages this task. But when the Nox sensor malfunctions, it causes unstable, rough, or erratic idling. 

Erratic acceleration 

Uneven or erratic acceleration is a common sign of a lousy Nox sensor. Here’s why; the engine control unit will compensate for the inappropriate reading by the sensor by using more fuel. In return, this will cause the vehicle to jerk when you depress the gas pedal.

Engine misfire and soot buildup 

Several factors can cause an engine to misfire. However, a sudden engine misfire may mean the Nox sensor is malfunctioning. This happens when there’s little oxygen during the combustion process.

Nox sensors are one of the most costly exhaust system components because they contain platinum, which helps to reduce toxic nitrogen from the exhaust byproducts. 

A lousy Nox sensor will cause higher emissions, affecting the catalytic converter. It damages the catalyst, making it unable to manage emissions. This can also lead to soot buildup. Of course, you know what this means. It’ll lead to a failed emission test.

Check engine light 

While the Nox sensor is not on under the hood, it can trigger an engine check light when it goes bad. However, several reasons can cause the car computer to trigger a check engine light. Therefore, do not conclude that an engine warning light shows a lousy Nox sensor. 

Diagnose the vehicle with a diagnostics scan tool to track the actual cause of the light. If you don’t have this tool, drive down to a mechanic garage for a professional inspection. 

Having seen the symptoms, what can cause a NOx sensor to fail?

What Causes NOx Sensors To Fail?

Some factors can cause a Nox sensor to fail. Here are the probable causes of Nox sensor failures. 

Soot buildup: From the location of these sensors, it is no surprise that soot buildup can damage them. While soot buildup is a sign of a lousy Nox sensor, it is also a common cause. 

Burnt or damaged cables: A burnt or damaged wiring harness can be the root cause of Nox sensor malfunction. In such a case, a virtual inspection will pinpoint the culprit.

It may not mean the Nox sensor itself is bad. It could be that internal connectors that read the Nox level are defective.

There may be other causes, but these are the common ones you should note. A visual inspection could be all you need to track the cause of the Nox sensor failure. 

How Do You Fix NOx Sensors?

 If you notice two or more bad Nox signs outlined above, diagnose the vehicle with a diagnostics scan tool. This will help you determine if the sensor is bad or the symptoms project other system malfunction. 

You may have a bad probe or a broken/damaged sensor. First, locate the sensor on the exhaust pipe and carry out a visual inspection. If the probe is the cause, follow these steps to fix it. 

Uninstall the Nox sensor: Unscrew the Nox sensor from the exhaust pipes. You may visit your owner’s booklet for a clear picture of the sensor location on your specific model. The booklet will also explain how to remove the Nox sensor.

Get your necessary tools

You’ll need some common tools to carry out this repair job. Here are the tools you’ll need.

  • Scissors
  • Knife/cutter
  • Electrical tape
  • Soldering iron

Shift the dust cover: Shift back the dust cover to gain access to the Nox sensor. Hold the dust cover with electrical tape so it won’t disturb when repairing the sensor.

Split the probes: Split the probes with your cutter or knife. Do not cut all the wires from the same spot. Ensure they are not of the same length.

Fix the new wires/probes: Get the new probes and connect them appropriately. Connect the probes according to the colors. And now wrap and solder the wires properly.

Diagnose the Nox sensor again: Now that you have fixed the probe issues, diagnose the vehicle again and ensure everything is working properly. If the diagnosis detects any issues, replace the Nox sensor or contact your mechanic.


Q: Can I drive with a faulty NOx sensor?

Of course, you can drive with a bad Nox sensor. However, I do not recommend it. Driving with a faulty Nox sensor for an extended period will cause several drivability issues.

It may cause poor engine power output, erratic idling, sudden engine misfire, and bad gas mileage. Before the engine starts projecting any of these problems, the ECU will trigger an engine warning light on the dashboard.

Q: How do you clean a NOx diesel sensor?

A basic Nox sensor cleaning requires soaking it in a cleaning solvent for a long period. You can soak them in a fuel injector cleaner for 3-4 hours. After that, get a carburetor or throttle body cleaner and spray it on the Nox sensor. I recommend using carb and choke cleaner since we use it in getting rid of carbons on throttle bodies and carburetors.

Q: How much does it cost to replace a NOx sensor?

The replacement cost of a Nox sensor depends on various parameters. When budgeting how much to spend, ask yourself whether you want brand new, aftermarket, or original OEM parts.

The cost also depends on the make and model of the car. However, the average cost of Nox is between $200 to $250. Though, high-end sensors can be as high as $350 to $700.

Q: Can you repair a NOx sensor?

Like every component in a car, Nox sensors can fail and need fixing or replacement. Once the sensor fails, it can cause several issues, like poor engine performance, erratic acceleration, bad gas mileage, and the vehicle going into limp mode.

If you ignore these kinds of problems, things will go south quickly. Therefore, you need to repair the sensor as soon as you can. Most times, it is not the sensor that goes bad, but the lambda probes that monitor the nitrogen oxides in the exhaust gas.

Q: Do petrol cars have NOx sensors?

Automakers have seen the need to develop Nox sensors since the 1990s because of emission regulations. The development took off in the 1990s, but in the early 2000s, manufacturers built commercial sensors on stratified charge, lean-burn gasoline vehicles with nitrogen oxides absorbers. Manufacturers extend the development of Nox sensors to both light and heavyweight diesel engines with a urea SCR system.

Q: How long do NOx sensors last?

Nox sensor warranty on cars differs. However, on average, a Nox sensor can last between 50,000 to 60,000 miles. With proper maintenance, the sensor can last for more than that. In the same manner, it can fail long before covering 50k miles.

Final words

You’ll agree that modern-day vehicles are pretty complex and expensive. No one wants to part with his hard-earned money when problems like lousy Nox sensors spring up.

That’s why we highlighted Nox sensor failure symptoms, how to fix them, and other crucial factors concerned. Before proceeding with any repair jobs in your vehicle, ensure you’re confident enough to do it. If you are not, seek professional help.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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