Most vehicles emit polluting gasses through the exhaust pipe, damaging air quality and human health. However, several upgraded car models now come with NOX sensors which are also known as upstream and downstream sensors. The NOx sensors warn the driver if the emission is as it should be. If the NOx sensor is defective, the system switches to an emergency operation. This also results in increased fuel consumption, engine misfire and uneven acceleration.
Here, we’ll discuss what happens when a NOx sensor fails, its diagnoses and fixes. But first, let’s look at what a NOx sensor is and how it works.
What is NOx Senso
There are different types of sensors present in the car. The NOx sensors measure the amount of nitrogen oxide in your car exhaust and send the information to the ECU (Engine Control Unit). However, the level of NOx in the exhaust gasses indicates if there is enough oxygen present while performing combustion. It also helps the catalytic converter to work efficiently.
How Does NOx Sensor Work
The NOx sensor sends warning lights on the car dashboard when the emissions are too high or too low. The sensor works according to the functional principle of the broadband lambda probe and measures the lambda value after the catalytic converter and the nitrogen oxides on the other.
Since the NOx sensor only has a limited service life, it will cease to function sooner or later. If the NOX sensor fails, the engine computer will register a fault and the engine starts stalling slightly. The check engine light will also come on. In many cases, the NOX sensor is broken. Moreover, the instrument cluster also causes the engine to be put into an emergency running program in which fuel consumption increases significantly (0.5 to 1.5 litres / 100 km).
Now let’s find out what causes a NOx sensor to go bad and what are the symptoms of a problematic NOx sensor.
What Happens When a NOx Sensor Fails
Just like other car parts and sensors, NOx can also fail. The sensor is located on the exhaust system right before the catalytic converter. Once the NOx sensor starts failing, here are some faulty NOx sensor symptoms you must watch out for.
High Fuel Consumption
As we have mentioned earlier, high fuel consumption is one of the symptoms of NOx sensor failure. The bad NOx sensor is unable to deliver accurate information to the car’s ECU system regarding the air-fuel mixture. This will also cause the powertrain control module to ensure efficient engine performance.
The bad NOx sensor sends wrong information to the car computer which can result in more fuel consumption being injected into the cylinder. Due to this, your vehicle may start using more fuel to keep the engine running smoothly and limit its RMP.
Engine Misfire and Soot Buildup
There could be several reasons that can cause an engine to misfire; however, a sudden engine misfire may also cause when the NOx sensor fails. This happens when the oxygen level in the exhaust is lower – below the threshold level that is required for proper fuel combustion. You can also clean the car’s oxygen sensor to extend its life.
Moreover, a malfunctioned NOx sensor also causes higher gas emissions which ultimately damages the catalysts and reduces its ability to manage the emission of harmful gasses. This also results in soot buildup which will lead to a failed emission test.
Check Engine Light Turns on
Check engine light can come on your dashboard due to several reasons; a bad NOx sensor is one of them. The check engine light usually turns on when there is something wrong with the electronic parts of your car such as emission control components. Check other reasons why your check engine light is on.
Rough Idling
One of the common engine problems caused by bad NOx sensors is rough idling. The car engine starts and then dies immediately, this problem is typically related to the internal engine issues such as carbon buildup in the car’s combustion chamber. Since a malfunctioned NOx sensor can’t measure the amount of oxygen in the vehicle’s exhaust system, the NOx sensor interferes with a car and causes unstable, rough, or erratic idling.
Uneven Acceleration
A jerking car or uneven acceleration can be another faulty NOx sensor symptom. The car jerks when accelerating because the engine uses additional fuel to compensate for insufficient sensor readings. This is why the car accelerates unevenly and causes jerking.
Faulty NOx Sensor Codes
You can also identify the NOx sensor failure with the help of these car diagnostic codes:
- SPN 3227 FMI 3 (SCR outlet NOx sensor circuit, failed high)
- SPN 3227 FMI 5 (SCR outlet NOx sensor circuit, failed open)
- SPN 3217 FMI 3 (SCR inlet NOx sensor circuit, failed high)
- SPN 3217 FMI 5 (SCR inlet NOx sensor circuit, failed open)
That’s the wrap on the symptoms that happen when a NOx sensor failures. A damaged wiring harness can be the root cause of a bad NOx sensor. Therefore, it is important to keep your car maintenance checklist up-to-date to avoid expensive car damage.
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