This alarm automatically comes on every 2000 hours of sensor use to act as a reminder to service the H2O probe. The H2O sensor needs to be periodically cleaned and coated with phosphoric acid. This should typically be done about every 3-6 months. Refer to the instruction manual for directions on how to clean the probe. The alarm can be reset by navigating to the 'Common Parameters' screen, selecting 'Service H2O Probe', resetting the counter to zero and acknowledging the alarm. It is important to note that cleaning the probe is different than calibrating the sensor.