What do NOx sensors do?
Highway Automotive looks at what NOx sensors do, why they’re important and tips when fitting them.
Reducing harmful greenhouse gas emissions is a priority for legislators globally, which means that it’s also a central issue for those responsible for servicing and repairing commercial vehicles and for the fleet operators and logistics businesses running them.
Although no engine systems are designed to work in isolation, efficient fuel combustion is a necessity for the diesel engines that still make up the vast majority of Europe’s CVs. They generally achieve this by operating under high pressure and using forced induction, which increases oxygen intake and improves combustion, but also produces more heat.
Unfortunately, the greater the temperature, the more nitrogen oxides (NOx) are produced, and NOx is bad for people and the wider environment, which is where the exhaust gas recirculation (EGR) system steps in. However, without reliable monitoring of the NOx levels in the exhaust gases, the EGR system cannot perform efficiently, and this is the role of the NOx sensor.
Alongside the EGR system, many modern engines also use a diesel exhaust fluid such as AdBlue, which, when injected into the exhaust system, helps to reduce pollutants by breaking down the harmful NOx into benign nitrogen and water and can achieve reduction in NOx emissions of up to 80 per cent.
However, for the exhaust fluid to work effectively, the engine, and its related systems, need to analyse the amount of NOx being emitted, the exhaust gas temperature, the turbo boost pressure and the ambient temperature, as well as the status of the EGR system.
This data is collected and sent to the engine control unit (ECU), which processes the information to determine the required amount of exhaust fluid that needs to be introduced into the exhaust stream before it reaches the diesel particulate filter (DPF), as well as the fuel and air mixture for the combustion chambers.
As a result, a fast and accurately performing NOx sensor is a crucial element within the system, but because it is exposed to hot exhaust gases and operates in an extremely hostile environment, it has a limited lifespan and will eventually need to be replaced.
A faulty or failing NOx sensor can cause several problems including poor engine performance, increased fuel consumption, excessive wear on catalytic converters and damage to the DPF system.
If the engine displays any of these symptoms, it is likely that the NOx sensor will need to be replaced. However, replacing and installing a NOx sensor requires careful attention and should only be undertaken by trained technicians using the correct diagnostic tools.
Best practice tips
To ensure the replacement NOx sensor performs correctly and returns a long service life, these key principles must be followed:
- Do not clean the sensor’s measuring tip with any tools or chemicals.
- Once installed, the tip must face perpendicular to the exhaust stream.
- Mount the sensor on a straight part of the exhaust pipe where the gases are hot.
- To protect it from damage, install the sensor on the upper side of the pipe, which will prevent water condensation during engine warm-up.
- After installation, clear all errors and old data from the ECU and perform a sensor adaptation procedure.
It is important to be aware that incorrect installation can delay the sensor’s response time, disrupt data sent to the ECU and reduce the sensor’s lifespan.
When an original factory exhaust system is being repaired, it must be noted that ‘universal’ NOX sensors do not exist, rather they are tailored to a specific vehicle and engine variant. The selection of a replacement sensor must therefore be based on detailed parameters and guidelines specified by the vehicle manufacturer.
Accordingly, Highway Automotive only supplies replacement NOx sensors that are designed and produced following the vehicle manufacturers’ specifications, and through its product catalogues, it indicates precisely which type of sensor should be used in the vehicle being repaired. In addition, each is delivered individually calibrated and protected against accidental contamination or damage.
Highway Automotive also conducts NOx sensor testing with specialised equipment that simulates real-world conditions. This testing measures the sensor’s electrical parameters, response times, and readings. It helps determine if the sensor works appropriately or identifies any issues causing its failure.
