I had that message once on my last sensor (not on any others). When it started working again it was all haywire. The up and down arrows were ridiculous. For example, previous scan was in the 5s with a horizontal arrow. Next scan was in the 8s with a downward arrow, next was in the 6s with an upward arrow, all within 10 minutes. It took half an hour to return to normal. I also looked at the downloaded log file which showed all the 15 minute readings. There were 2 missing in this period. At the time I was doing nothing in particular, just sitting watching TV. Finger pricks showed 5s throughout.