Product review (5 stars): Six (6) blinks on the "call service" light after "reset"
As advertised.  You can find the diagnostic "blink" codes on the internet on a troubleshooting sheet or in the "Professional Installer" Guide for the humidifier.  If your unit is overhead, as mine was, you may be able to work with a mirror and not take the whole humidifier down.  There are only two screws, one for the top cover that's on the outside and a T27 torx to remove the clamp/bracket that holds the Water Level Sensor down.  There's a good probability both the sensor and gaskets are shot.

While you have the sensor out, make sure the two parts of the tank it covers have clean drain ports, just shine a light through the bottom.  You can clean them from the bottom outside, easy.