I was using N8485A sensor with N1913A for over 3 years. From last week the sensor calibration process encounters error "-231:data questionable". In fact when I connect the sensor to the internal 50 MHz 1mW source, the meter reads -31 dB instead of near 0dBm, but the reading value is totally stable. I also directly connected the sensor to the Signal Generator and the output varies linearly with input power (for example 10dBm input to the sensor is read -21dBm).
What could be the possible problem causing this error? Is the problem with sensor or the meter?
In addition, the meter self-test is passed successfully, the firmware is latest one and the sensor is recognized by meter. The only error appear on meter is "-231:data questionable".