The best way to test whether the sensor is faulty is to connect an A205 interface to the 4-pin connector of the sensor and use DevConfig software to communicate directly with the sensor through a computer. This is described in the Wireless Sensor Network Instruction Manual. Select Connect in DevConfig, and press the Setup button on the back of the sensor.
Select the Settings Editor tab and scroll down to Measurements. This will query the sensor every 5 seconds and display readings for volumetric water content (VWC), bulk electrical conductivity (EC), temperature (Ts), apparent bulk permittivity (Pe), voltage ratio (AR), period, internal sensor temperature (Ti), and battery voltage (BV). Certain soil conditions may cause NAN values in the VWC, EC, and/or Pe fields, but a working sensor always reports Ts, AR, period, Ti, and BV. If any of those fields report NAN, the sensor is damaged. If only one, two, or three of the values are NAN, the NAN values are caused by soil conditions.