Bad Sensor!


For you minimed cgm users, how often do you find that your sensors give you a “cal error”? Mine seem to be doing it more and more lately. This one I just took out at one point alarmed at me and said I was falling below 70, and when I checked my level was in the 130’s. So I calibrated and it told me it was a bad sensor. Aren’t these a little expensive to be going bad so often?
Seriously though, it’s only happened to about 3 of them in this first box I got. Anyone else experiencing this problem?


i get lots of sensors that dont last the 3 or 7 days they are supposed too…mostly due to ‘noise’ (a dexcom term for reading jumping around) i have had several ‘early shutoffs’ but i can generally restart without issue as long as the early shut off is not preceeded by alot of noise. I have had only one truly BAD SENSOR.

all sensors that i mentioned above were immediatly replaced by Dexcom without issue.


Hi Dara,
I get these from time to time, but not so much as I did in the beginning. I have the MM CGM also, and I had a terrible time with my first few sensors… I called Medtronic and they were very helpful, they WILL give you a credit for the sensor if it is truly bad but will do everything in their power to help you get it going again. One really useful piece of information they gave me is their “calibration formula” - basically you take the ISIG value from your pump and divide your current BG by that number. if you end up with anything above 20, you will throw a cal error if you try to calibrate. They told me just to clear the “meter BG NOW” alarm and wait until it’s safe to calibrate. You can also restart the sensor (as a “new” sensor) if you get a bad sensor error.
One thing that may be helpful - I had really low ISIG values with my first couple sensors, which I put in my abdomen. I have had excellent luck inserting them in my upper butt cheek instead and can usually keep them going for several days with no cal errors - they seem to be much more accurate in fleshier areas. If you have any questions let me know!


Hi Dara, I get these errors also, but I don’t think that it necessarily means that the sensor itself is bad. More like based on what you are reporting as your actual blood sugar, it isn’t getting a good reading of the interstitial fluid. That’s how I have rationalized it, anyway. There are some times that I am surprised by the error but other times I had suspected I might get an error. Sometimes in those cases I will restart the sensor so that I don’t have to deal with it… I’ve read a lot of posts on forums on this and it seems to be a popular way to deal with an off-track sensor rather than waiting for the calibrations to bring it back in line. Does that make sense?


I believe that any time you enter a calibration value that is more than 40 away from what the CGMS is currently reporting, it displays that error. So if it was reporting 70 and you entered 130 for a new calibration, that difference of 60 would cause it to report a problem. The sensor may have a lot of life left, so you shouldn’t remove the sensor just because you get that error.

What I do in a similar situation is to either:

  1. Enter a made up calibration value that is close to the meter, but still within 40 of the CGMS (something like 105 in your case). Then I’ll try to calibrate again in an hour.
  2. Wait another hour for my BG to stabilize and hopefully the CGMS & meter values are within 40. I’d then calibrate.



I have been using the CGMS for about a month now. I haven’t had any problems with mine. I would give Medtronic a call and see what they tell you. Have you checked with the nurse that initially worked with you? S/he may be able to guide you in this as well.