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 or CS655 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.