The data logger manual provides this answer: “Indicates a problem with the analog measurement hardware during the self-calibration. An invalid external sensor signal applying a voltage beyond the internal ±8 V supplies on a voltage input can induce this error. Removing the offending signal and powering up the logger will initiate a new self-calibration. If the error does not occur on power-up, the problem is corrected. If no invalid external signals are present and/or self-calibration fails again on power-up, the data logger should be repaired by a qualified technician.”