This NOx sensor measures the concentration of nitrogen oxides in the exhaust stream downstream of the selective catalytic reduction (SCR) system. It sends a voltage signal to the engine control unit, which uses the reading to adjust the diesel exhaust fluid (DEF) dosing rate for optimal catalyst efficiency. A failed or degraded sensor can cause the ECU to enter a derated power mode or trigger a diagnostic trouble code related to emissions compliance. The sensor is installed directly into the exhaust pipe and uses a threaded connection with a sealing washer that should be replaced on reinstallation.