When to change Dexcom sensor?

I know that you can restart your sensor after the seven day warning comes up, but how do you know when it needs to be changed? Does it just stop working? I restarted my sensor and am on day 8 right now.

I haven’t tried to restart it after 7 days. When it says change sensor soon I go ahead and change it and so far so good. Mine have all lasted the full 7 days. Except for 2 sensor failures which they say were caused because I took tylenol.

Rickst29 can answer this question far (FAR) better than me. I have the Dexcom Seven Plus system.

I change mine when the readings aren’t as good at being on track as it was the first week. For example you have a reading of 100 and it says 150+, etc. I think I’ve had some of mine last for 14 days but I’ve heard of some pretty amazing record living sensors! If a sensor causes me problems in the second week I pitch it (which you already know) and don’t call Dexcom to complain about sensor. I only call and have Dexcom replace sensors if they act up within the first week.

Rickst29 gave me some great advice about the ???.. I hope he doesn’t mind that I copy and paste this as it is very helpful info…

??? means that Dexcom (the receiver) finds the data it’s getting to be “outlandish, not to be trusted”. The OLD Seven did it a lot more than the 7Plus does, and that declaration was often a bad mistake: I’ve reviewed a few graphs of raw readings (Dexcom’s ISIG) versus reported data and non-reported “???” situations with support; we concluded that the readings would have been just about perfect if only Dexcom had been willing to show them. (Those discussions may have played a role in the Development of software changes in that area.)

So first thing: if you’re still on the old Seven, upgrade immediately.

I feel that you should NEVER allow “???” to continue for more than about 1/2 hour of normal wake-time activity. Rather than feeding it vast numbers of calibration attempts (over several hours), I do this:

First: remove the Transmitter from the clip housing and wipe it clean. Although “water tight” from outside, I’ve seen mine get just a bit grungy over time. (Probably from sweat underneath.) Sweaty “grunge” can create a bit of a short circuit between the contacts, causing data readings of “zero”. It can also create insulation resistance between the contacts, reducing the current capability.

Second: Stop the Sensor. All of those calibrations which you entered during the period of “???”, trying to get it going, WILL be used (with raw data of “???” quality) to try and create a valid calibration. It won’t work, because those numbers are no good!

To get rid of calibration attempts entered during “???”, you have to RESTART the Sensor and wait the full two hours. Do the double-finger pokes, and if it’s still good, it should come up within a few minutes. (Maybe just one "extra poke at +15 minutes). If it doesn’t come up within about 20 minutes, immediately declare it “gone bad” and throw it away.

A Sensor which has already had plenty of time to “warm up” in the body should be given just ONE chance to start working right after a restart. If it’s not showing a graph right after that 3rd poke, it’s done.


Good Luck!

RE: Does it just stop working?

It just stops working less well… more ??? signs… more readings that don’t synch up as well with your meter. When you don’t have faith in the readings or are annoyed by the ???'s, swap out.

Thanks for the responses.


And thanks for the Kudos, Mommy-of-3 :)) But that posting was mostly about repeated “???” incidents, and that’s usually not the first, or most likely, symptom of “worn out, unreliable” Sensor data. Here’s the indication which usually happens instead, quite a bit before the extended “???” stuff begins:

The line of the graph becomes “jumpy” on the one-hour and 3-hour displays. And, on the 6 and 12 hour displays, it looks kind of “fuzzy”.

Game over, that Sensor is done.

Why? well, glucose in your blood (and in your ISF, too) constitutes what your high school math teachers called a “Continuous Function”. There are no sudden leaps of +20 mg/dL up, followed by another leap -10 mg/dL below the one BEFORE “+20 higher”. The line needs to be smooth-- If you’re in a change from rising bG to dropping bG, it will show the uphill flattening off, then becoming level, then dropping slowly, then dropping quicker. No jagged cliffs!

So a lot of “fuzziness” in the line, where it went up 5 points, then DOWN 10 points, the UP 8 points, then up 2 points, then DOWN 7 points – it’s physically impossible for ISF Glucose to behave that way, and the raw readings from your Sensor are going bad (unreliable) if Dex shows this on the graphs. The line on the graphs should be smooth, razor sharp, with no more than just a couple of outlying bogus numbers from time to time.

How do you acctually restart the sensor after the 7 days? just hit start sensor again?

Yes. If you would prefer to have the two-hour wait occur at a different time, or dislike Dexcom’s nagging countdown, you can perform the “Stop Sensor” at an earlier time. You don’t need to remove the Transmitter or touch the Sensor at all-- just “Start Sensor” on the receiver, wait for the two-hour warm-up to complete, and enter your startup bG readings.

Rick, I am now on day 13 and the readings are still good. Do you use a sensor until the readings are off or do you have an upper limit for the number of days?

No upper limit for me. If the line of the graphs is razor sharp (per above), and moderately high/moderately low glucose levels are still being reported correctly, I keep going.

On the original STS, I frequently got over 20 days. The original ‘Seven’ never got anywhere near that. The calibration logic kept trying to use really old data (obtained from near-new Sensors) as factor, even when Sensors where actually getting quite old and showing much lower raw data. So, they just wouldn’t calibrate decently, even when the line of the graph WAS sharp. The ‘Seven+’ solves that problem, I often get 15-18 days, hardly ever less than 12-13.

In fact, I’m waiting for my two hours on a 14th day restart to complete right now, as I’m typing this. Still working great. :))

Thanks. The line of the graph became “jumpy” today. I used the sensor for 14 days but did not continue with a restart. I am so glad that I now use the sensors past 7 days. On day 7 my sensors were always really good and I was dreading the not so good readings of day 1.

i am on my 10day.all of my readings are good . the only problem i having is it try to come off.so how to you kept on for 15 days

Daniel, this issue has been recently discussed in


Search in the middle of the second page.

I"m on day 15 on sensor 2. On day 6 I had to add some tape around the perimeter, and on day 14 i took it off and put on some more fresh. I’m not going to change the sensor till the readings start being unreliable.

When you put on your next sensor use the back of your finger nail to apply localized pressure to the tape. I am on day 7 and the tape looks the same as on day 1.

End of my first week with the Dexcom and just did my first sensor restart.

I am so glad to have found this group as a resource. Looking forward to seeing how far I get before the line fuzzies kick in.

I didn’t press “start sensor” right away because I wanted to see what it would do if I let it go to the “alleged” end of the road. But I will definitely restart on the first alarm next time. I keep it in my pocket and still jump every time it goes off!

I used to change or restart my sensor exactly at the end of the magic 7 days. It took me 3 months to figure out that absolutely nothing is magic about 7 days. Now I restart my sensor close to the end of 7 days during a time period when I expect my bg to be stable. This affords me a good first calibration which has a very positive effect on initial accuracy. On day 6 I figure out when my last stable bg time will be before the 7 day forced shutoff and I schedule the sensor restart for this time.

It’s day 8 and I’m giving up. Too many ??? . Next time, I’ll try Helmut’s suggestion to restart during a stable period, which I did not do the last time. I think I confused the sensor to death.

Etta, I was about to say “I never saw ???” but I just did. I was on day 11 with this sensor and the graph had some ‘escapes’. Data points that were clearly wrong. Then I got ??? for an hour, a reading in between and ??? again. I am not desperate enough to deal with this. A new sensor entered the race. BTW, I don’t expect that restarting during a period of fast moving bg can cause ???. When the bg is moving the 7+ has to guess the delay between bg and ISF. If the 7+ guesses the delay wrong the readings will be off. But that should be it. I interpret ??? as “consecutive readings don’t make sense, something is wrong”. The mild form of ??? is a fuzzy line.

That makes sense. Thanks Helmut.