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NOx Sensors: What They Are and How They Work

mods With carbon neutrality being a serious issue, many governments worldwide insist that diesel vehicles should be installed with Selective Catalytic Reduction (SCR) technology to reduce diesel emissions.

What is a NOx sensor?

The NOx sensor is part of the SCR system made up of the sensor, SCR catalyst, DPF, turbocharger, and other components. The NOX sensor is located upstream of the SCR catalyst, where it gives the most accurate reading.

The sensor monitors the amount of nitrogen oxide being released into the air by the vehicle, and if it’s too much, the sensor feeds the information to the SCR system, which then adjusts the output accordingly so that the car releases nitrogen oxide that is within the emission regulations.

How does the NOx sensor work?

The workings of the sensor depend on the quality of the sensor. The high-quality sensors work by using an electrode that contains a catalyst made from activated carbon, titanium oxide, and other base metals.

The catalyst reacts with nitrogen oxides and sends a voltage to the SCR system. The voltage strength is directly proportional to the amount of NOx that the car is producing, so the stronger the voltage, the higher the amount of NOx produced by the car.

The cheaper, low-quality NOx sensors, on the other hand, work by analyzing the exhaust gas. They start with eliminating the amount of oxygen in the exhaust gas then judge the concentration of NOx present. Besides this method being slower, it’s also less accurate.

NOx sensor problems

Like other parts of the car, the NOx sensor has a limited lifespan, after which you have to replace it. When the sensor isn’t emitting a signal, it will flag up an error within the engine control unit, and it’s up to you to find out the cause of the problem and fix it right away.

It’s hard to tell whether the engine light is due to a faulty NOx sensor. In fact, you can’t even tell that the NOx sensor is the culprit even after removing it. The best way to tell the source of the problem is to scan the vehicle’s Engine Control Unit (ECU) for error codes.

If this is your first time scanning the codes, ensure that you interpret them correctly so you don't misdiagnose the problem.

If you have an engine code scanner, scan the codes, and tell the source of the problem. If you don’t have the scanner at home, visit your local garage, and they will scan the codes for you.

Besides the engine warning light illumination, a faulty NOx sensor will bring about other problems such as:

Increased fuel consumption

When the NOx sensor isn’t sending data and information to the ECU, the ECU can’t tell the right amount of fuel to send to the engine, and in most cases, it sends incorrect fuel mixture and air-to-fuel ratio which leads to increased fuel consumption.


Again, since the NOx sensor isn’t sending information to the ECU, the ECU can’t determine the amount of fuel that the fuel injectors should spray inside the chamber. As a result, the injectors surge or sporadically starve the engine.

When this arises, the car might attempt to “rev” the engine even without having your foot on the throttle.

In other cases, the ECU panics and activates the emergency mode, which sees your vehicle behaving erratically.

What should you do when the NOx sensor fails?

Good quality and durable NOx sensors don’t come cheap, so before you jump into replacing them, try fixing the old sensor.

Many car owners have successfully cleaned the sensors and restored them to proper working conditions, so you should try this before you do anything else.

Due to their location, soot buildup on the sensor is the leading cause of sensor failure, so when you clean the sensor and get rid of the buildup, you have a high chance of bringing the sensor back to life.

If cleaning the sensor doesn’t work, try repairing it.

You should note that the sensor is complex, and it’s hard to pinpoint the exact area where the problem lies, so you will need to have advanced skills to fix the problem. Thankfully, there are plenty of online guides you can use to your advantage.

If you aren’t a dab hand at DIY projects, the best way to go about it is to have the sensor looked at by an experienced professional.

Replace the NOx sensor

If cleaning or repairing the sensor doesn’t work, you should replace the sensor. As you are doing the replacement, you should note that there are many types of sensors, such as 4326870 and many others you can go for.

The sensors also go at different prices depending on the make and vehicle model. The price will also be determined by whether you go for a factory or after-market sensor.

Some car owners have reported spending up to four figures replacing their NOx sensors, so always take your time to research and make comparisons before settling on a given sensor.

If you are seriously cash-strapped, consider going for refurbished NOx sensors. These sensors are often 1.5 times cheaper than the new ones, so you save a lot of money.

While the refurbished sensors save you a lot of money, the downside is they can easily fail within a few days of installation. To protect yourself, ensure that the refurbished sensor comes with a warranty.

If you have the budget for it, go for new sensors. While they come with an eye-watering price tag, they have the best warranties, so even if they fail after installing them, as long as it's within the warranty period, you need to return them to the seller and get a new one.

Parting shot

NOx sensors are a must-have for diesel cars, so you should install them if you have a diesel car. When buying the units, ensure they are made by a reputable manufacturer. While these sensors will be expensive to buy, they will last for a long time, and you won’t have to deal with incessant sensor problems.


♦ “Why my engine came back on after I replace the sensor.”
Submitted on 26/03/2022 21:03 by a visitor
♦ “once new NOX sensor is installed how do you re=set it to turn engine light off”
Submitted on 12/10/2021 20:10 by a visitor
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