This is for Medtronics Brand. Not sure how the others work.
Whenever you get a Cal Error, it will tell you to Meter BG. You will normally get two chances to get a good calibration. If, you get a second Cal Error you will get a Bad Sensor reading and you will need to insert another sensor. This is what I found to help avoid getting a bad sensor.
Turn off the sensor, do everything that is needed to get a good stable glucose reading. This may take some time. Restart the CCM. You can choose the new sensor or lost sensor button and it should start working again.
I should add that I also, look at the ISIG number to give me an idea if the sensor is ending. Another words, if it's low like around 5 that's usually a sign that it probably needs changing so, trying to restart it would be a waste of time.