Three of the values returned by the M3! command are never changed to NAN by logical tests. If non-zero readings for soil temperature, period average, and voltage ratio are displayed, the sensor is working.
A simple test in air and water will also show whether the CS650-L or CS655-L is functioning. When performing this test, look at the permittivity value and not the water content value. In air, the sensor will read a permittivity value of 1. In room-temperature water, the value should be close to 80. The water container should have a diameter of at least 20 cm (8 in.) and a depth of 33 cm (13 in.). A smaller container will result in a lower permittivity value because the sensor will be measuring some of the air outside the container as well as the water. Tap water is acceptable for this test.