Diesel NOx Sensors. What are they and what do they do?
Diesel NOx Sensors. What are they and what do they do?
If you are involved in the diesel repair, machinery, or trucking industries you've probably heard about NOx sensors on diesel exhaust systems. Whether good, bad, or curious let me explain their purpose and why they can be tricky when diagnosing a problem.
Diesel engines produce several products of emission that the Federal Government wants regulated and controlled. The four main pollutants are Carbon Monoxide (CO), Hydrocarbons (HC), Particulate Matter (PM), and Nitrogen Oxide (NOx). There are several components of a modern diesel exhaust system which all play a specific role in emissions control.
When dealing specifically with NOx gas, the main component responsible for emissions control is the Selective Catalyst Reduction (SCR) Filter. This is where the NOx portion of diesel exhaust is brought from an unacceptable level to an environmentally friendly level and then expelled from the tail pipe. This is done with the use of UREA fluid and a chemical reaction which takes place inside the filter.
UREA fluid is injected through the "Doser Injector" into a Decomposition Tube where it is mixed with the exhaust gas. As the gas and UREA mixture travel through the SCR filter, the NOx gas is converted into Nitrogen and Oxygen and then expelled out the tail pipe.
SCR filters are typically located last in the exhaust emission equipment and therefore are the last to make adjustments. When diagnosing a faulty NOx sensor, it is imperative that the rest of the emission equipment be in good operating condition so that no faulty symptoms are recorded. Although a fault code may tempt the technician to replace the inlet or outlet NOx sensor, thorough troubleshooting should be performed to confirm that another component of the emission system has not failed, causing faulty readings. NOx sensors are expensive and may times non-returnable parts.
After a repair is made to the emissions system on a diesel engine, a computer controlled and automated test should be performed, which tests all components of the exhaust system and confirms that all values are within good operating range. A road test should then be performed and data should be monitored by a second technician, or data logged and reviewed after the road test to ensure that the values are with spec.
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