Dexcom G5 question about ? ? ? on receiver

How often have you gotten ??? and how often has it resolved itself.

Also, how often does your receiver miss a reading (no backfill on G5 receiver)?

My first day, I had 4 missed data points.

Yesterday, I got my first “???” for about 15 minutes, got some accurate numbers for a short time, and then permanent “???”.

Sensor wasn’t loose, and had given very accurate readings for many days prior to the ???

How long have you been wearing the sensor? I find those oddball readings and dropouts often mean the sensor is starting to fail.

ETA: Very rarely does it not backfill, e.g., when I have left my phone sonewhere else. I had more dropouts with the receiver on occasions I’ve had to rely on it.

i can’t use a phone, thanks to Medicare rules. I’m using the Dex receiver (no backfill function)

1st day on the dropouts (1st ever dex sensor for me).

??? day 3

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Maybe one of our more tech-oriented members will have some advice but I suggest calling tech support at Dexcom. I have always found them helpful.

I spoke with them already. Just wanted to find out from users how often these issues crop up for them.

2 posts were split to a new topic: Medicare Dexcom restrictions

I have had this before, but this is due to the transmitter being unable to communicate with the receiver for whatever reason (laying on it, for instance.) The official Dexcom answer is below:

The question marks mean that the transmitter does not understand the sensor readings for the moment. You will want to wait at least three (3) hours for them to clear. If they do not clear within that time frame, please contact our Technical Support for further assistance.

If it sticks around longer than 3 hours, then they just need to replace the transmitter - hopefully it’s under warranty! I’ve had to buy a new one outright before because the one that failed was outside the warranty window.

Hope this gets resolved for you soon! CGM’s are crazy valuable…WHEN THEY WORK!

Dave, we find issues like this in week three, but never week one. The receiver is the most robust device we have. The phone often has issues, but the receiver never does for us.

Did you have any trouble with the insertion? There is a bit of a learning curve there, but not much really considering how thin the sensor wire is.

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3 posts were merged into an existing topic: Medicare Dexcom restrictions

@ClaudnDaye, that is not the case! The ??? error always means that the transmitter has a problem understanding (or getting) the sensor data. It never indicates a communication issue with the receiver: that kind of a problem shows up with a different error message on the receiver.

The step that Dexcom always recommends first is to change the sensor, not the transmitter. It is only after you had 2-3 sensors fail in the same way that Dexcom will swap the transmitter. We have had to swap sensors several times because of a ???, but never a transmitter (so far).

Dexcom always replaces the bad sensors by the way.


You are right. Thank you! I tend to think of these two as the same thing sometimes, but they are not! Fortunately I linked the actual Dexcom verbiage so that people can see the actual description of the problem!


@Dave, we get this error a couple of times a month I’d say. Most of the time it resolves itself within an hour or two, then the sensor is fine for the rest of its life. In the last 12 months, we probably had to change 3 sensors because of that issue. So it is not that common a problem – but it does happen.

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@dave - We get the ??? under two situations:

  1. The sensor is at the end of its life and starting to give bad numbers. For us, the sensor quite regularly lasts 12~13 days. So if we get the ??? on day 12, we don’t bother (at this point having done this for some time now) to troubleshoot but rather we just shutdown the sensor, peel it off and get a fresh sensor in place.
  2. After a shower on Day #1 ~ Day #11. In such a situation, we carefully pop the transmitter out (being careful not to dislodge the sensor). We wipe the transmitter down with an alcohol wipe (the surface that contacts the sensor), let it air dry and then put the transmitter back in place.

NOTE: Not ALL showers. Occasionally. I don’t remember this happening within the last month. But certainly it happens from time to time.

EDIT: In terms of missed readings, we use the app on the SmartPhone so if there are missed readings, they are usually backfilled. So unless we are watching closely at the time, we would not be aware of missed readings looking back in time unless it was a gap larger than 3 hours. The Dexcom Receiver of course does not have the backfill functionality.
I would say it is periodic that we happen to look and see a gap of no current readings but them it would connect and backfill and the gap is gone like it was never there. Hard to say how often but enough that it does not seem an unusual occurrence.


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@dave is this a G5 or G4? I am curious about how they manage to prevent a user from using their phone.

Times when I see “???”

  1. Pressure. If pressure is put on the sensor, it will wildly go low then back up. Usually when sleeping (laying on it). This screws up the predictive algorithm and it can’t give you a confident prediction.

  2. Water. Sometimes after a shower or bath, moisture can become trapped. You can carefully remove the transmitter, blot dry it, then put it back on. This sometimes works.

  3. Old sensor. Sounds like at 3 days, this is not the case for you.

  4. Bad sensors. I once received a bad batch of sensors. After a bit of trial and error, we suspect the package was overheated in delivery. No proof though.

  5. Bad transmitter. This is rare but could happen. As others say, try different sensors from different lots if you can. Dexcom is very good at replacing bad sensors. If none of them work, maybe it’s the transmitter.

  6. Stay hydrated. Since the whole point is that it is measuring off of the interstitial fluid, if you aren’t hydrated–it won’t read correctly.


If Dexcom wanted to prevent anybody from using the Dexcom App it would likely be trivial on the part of Dexcom as it requires a Dexcom server account. Whether they choose to block or not, Dexcom is certainly aware of who uses the Dexcom App and almost certainly would be obligated to provide this information to Medicare upon request.

On the other hand if somebody were to use xdrip or some other 3rd party software that does not hit the Dexcom servers then it would most likely be impossible for either Dexcom or Medicare to know about this. However these solutions are obviously not appropriate for everybody. As well, it is reported that Medicare requires people to agree not to use anything other than the Dexcom Receiver for the data collection so people may feel obligated to keep their agreement.


Lots of great info on the causes. Thank you so much, everyone. I thought the ??? had resolved itself after 15 minutes, because I started getting normal readings again (and accurate), but then later on, kapow, it reverted back to ???.

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Just to clarify – this trick works when the problem has to do with the contact prongs being wet or not making good contact – but there are other causes for the ??? error.

For us, the bulk of the ??? errors appear to be that the fiber is not in proper position anymore, in part because my son is very lean, and keeping it in the fat layer is not easy. So, the large majority of the time, cleaning the prongs doesn’t help for us.