The laser diode in the Klotz particle counting sensor will deteriorate over time. As a result the noise level will continually increase until the sensor is not longer able to accurately resolve 4µm counts from the background noise. The time it takes for this to happen is highly dependent on the type of samples that are being routinely analysed.
Resolution
There are three options availabe, 1) repair the Klotz sensor, 2) replace the sensor with a new Klotz unit, or 3) request a service exchange sensor. Prior to replacing the sensor the noise level should be checked first to confirm this is the issue.