Vehicles these days are a lot smarter than they used to be, with plenty of electronics and interconnected systems that improve a car’s performance and efficiency. A smart car uses a huge amount of data fed to its systems by various sensors, one of which is the NOx or nitrogen oxides sensor.
NOx sensors are the part of an exhaust system that measures the NOx levels in exhaust gases. When a NOx sensor fails, you can expect performance issues in your vehicle, such as rough idling (the engine runs rough), difficulty accelerating, and increased fuel consumption.
This guide will help you understand everything there is to know about NOx sensors, what happens when they fail, and what symptoms you can look out for.
What Does a NOx Sensor Do?
These days, vehicles have all sorts of sensors, but the NOx sensors aren’t often discussed. So, you’re not alone if you’re wondering, “What is a NOx sensor?”.
Here’s what you need to know:
NOx sensors are those that sense nitrogen oxides (plural), which are emissions produced by combustion engines. More specifically, the sensors are designed to pick up on the following:
- Nitric Oxide (NO)
- Nitrogen Dioxide (NO2)
NOx sensors work alongside Selective Catalytic Reduction (SCR) systems which reduce the NOx levels before the exhaust system releases these gases into the air. That way, the vehicle remains compliant with emissions rules and regulations.
You’ll often find NOx sensors in the exhaust systems of diesel vehicles, and typically, there are two of them, namely:
- The upstream sensor - measures NOx levels as exhaust gases come out of the engine
- The downstream sensor - measures NOx levels after the SCR system has reduced them
Suppose we’re talking about the NOx sensors in a Mercedes truck. The upstream sensor should show higher NOx readings than the one downstream, assuming that everything is working correctly.
How Do I Know If My NOx Sensor Is Bad?
The NOx sensor location, whether your vehicle has one or two, is concealed in the exhaust system. So, there aren't any visual signs you can look for to know if the sensor has gone wrong.
Instead, you’ll have to rely on indicators inside your vehicle and performance changes to know if something is wrong.
Here are 5 ways to know if your NOx sensor is bad:
#1 Check Engine Light and Active Error Codes
What you need to do first is to look at the indicators inside your vehicle. The most common one that will likely turn on is the Check Engine light on your instrument panel.
Although that could be a sign that your NOx sensor is failing, it’s not a particularly reliable one. As you probably know, the Check Engine light can be triggered by hundreds of different issues.
A more reliable indicator is the onboard diagnostics system (OBD) and its error codes. If you’ve got the code reader/scanner, you can check for any active error codes in your vehicle related to its NOx sensors.
#2 Rough Idling
Aside from the indicators mentioned above, you’ll most likely learn about NOx sensor failures from performance issues.
The first of them is when your vehicle idles roughly. Under normal conditions, your car can remain stable while idling.
However, anything that affects the engine’s exhaust system (like a problematic NOx sensor or two) will cause erratic behaviours, like the RPMs going up and down for no reason.
#3 Engine Runs Rough
Unfortunately, engine problems caused by bad NOx sensors don’t just happen while the vehicle is idling. You’ll also experience the engine running rough and having issues like misfiring and a lot more.
#4 Unstable Acceleration
All combustion engines can only function well when their input (e.g., fuel and air) and output (e.g., exhaust gases) move smoothly. NOx sensors are on the output side, and when they turn bad, they’ll also affect the engine’s ability to accelerate smoothly.
#5 Fuel Consumption Increases
Last but not least, you’ll notice a drop in fuel efficiency. Of course, you’ll only notice this over an extended period when you see that you’re paying a lot more than usual for fuel.
As you can see, NOx sensor failures don't just cost you money directly (i.e., parts and repairs), but they'll also cost you indirectly, especially if you let the problem continue for too long.
What Causes NOx Sensor Failure?
By now, it should be clear to you that NOx sensors are kept out of the way in the exhaust system. So, you’ll typically experience NOx sensor failure symptoms due to excess wear, mishandling, or other faults that only happen on rare occasions.
For example, NOx sensors fail for reasons like:
- Soot buildup around the sensor: Soot is a carbon byproduct of the combustion process. When soot builds up around one or both NOx sensors, they won't be able to measure nitric oxide levels accurately.
- Loose or damaged wires: Like any other sensor in your vehicle, NOx sensors are connected using wires. As such, your NOx sensors can fail if those wires become loose or experience damage.
- Electrical faults: Short circuits, power surges, and other electrical faults can damage a NOx sensor directly or burn out the wires. In both cases, the sensor won’t be able to send correct signals to your vehicle.
- ECU water intrusion: lastly, the problem might not even be with the NOx sensor at all. Instead, water that gets into the Engine Control Unit (ECU), which also controls NOx sensors, can cause all sorts of problems.
Can You Drive with a Bad NOx Sensor?
You might be able to drive with a bad NOx sensor, depending on how well your vehicle can still perform. As you saw above, engine misfires, rough acceleration, and other issues will likely occur secondary to NOx sensor failures.
So, if you have any reason to believe you’ve got a problem, you should get to a mechanic for NOx sensor repairs as soon as possible.
How Much Does It Cost to Replace a NOx Sensor?
The cost of a NOx sensor replacement can vary greatly depending on your vehicle type, make, and model.
For example, you can get a brand-new NOx sensor for some car brands for under $100, but a pair of Mercedes NOx sensors can set you back as much as $700 instead.
If you’re looking for a good deal on new or used parts like NOx sensors, an excellent place to check out is CarpartAU. The Marketplace shows deals on used parts, or you can use the Directory to find local suppliers who can get you new NOx sensors instead.
By Ray Hasbollah