Hello Gummi,
The R300 water pan sensors, once used for a while, seem to become a problem once they dry out. There are two solutions, one is to have a cup of water available where the sensor is placed when not in use. The second solution, once the sensor failed, is to clean the electrode of the sensor. It looks like a pin about 2mm diameter. To reach this pin, use a small nail file to clean the electrode, then use a cup with your fountain solution to test the sensor.
Cleaning the water pan regularly helps as well, as the deposits can not accumulate and affect your sensor.
Can you change the sensitivity of the sensor? Yes, you can, however, I would not recommend. If set incorrectly, and the water level is to low it will destroy your rollers, or if to high, it will flood your machine without you realizing it.
Let me know if I can help any further.
Sven
