The internal temperature has changed more than 10 degrrees C since the last User Cal, or more than 1 week has passed. Either re-do the User Cal using the YBAC1 or YBAC3 calibration kit (or some other known-good calibration standards) or select the Factory Calibration. This will be less accurate than a recently-done User Cal, but it will still be within +/-1 dB typically. A User Calibration can improve accuracy to twice that of the Factory Calibration (at 17 dB Return Loss).