The NOx Sensor (p/n 000121) gradually loses sensitivity by about 10% per year, and should be replaced every 3 years.
This sensor must be recalibrated using standard calibration gas containing NOx gas. The customer either has to return the analyzer to Bridge Analyzers for calibration with the new NOx sensor or field calibrate it using reference gas when the sensor is installed. There is no way to determine NO sensor accuracy in the field without the use of calibration gas.
There is no replacement prompt on the analyzer display for this sensor.