Dexcom - Putting in a New Sensor Without a Restart

By now everyone has read the trick on restarting a sensor without losing your readings for 2 hours. (microwave trick)

This is a slightly different trick. Maybe everyone already knows this, it was posted once before on FUD a while ago, but since it happened to me last night, I wanted to re-post it for anyone who had not seen it yet.

You can put in a new sensor without a restart.

Last night my sensor fell off at 2am. I didn’t want to be woken up at 4am for the calibration entries, so I just put in a new sensor, without telling the receiver it was a new sensor. It worked fine. When I woke up in the morning and looked at my receiver, it no longer had the hour glass or the ??? symbol. It was working with the new sensor.

There are a few caveats:

  • You have to put in the new sensor quickly. If it takes too long, eventually Dexcom will give you a failed sensor message, at which point you would have to do a sensor restart.

  • I got the hourglass for a while, followed by the ??? symbol for a while. But eventually the readings from the new sensor were picked up, without me ever telling the receiver that it was a new sensor.

  • Looking at my receiver after all of this was over, it appears that the time it took for the new sensor to start giving readings was 1 1/2 hours. Relatively close to the normal 2 hour restart.

  • It would definitely be a good idea to do a calibration on the new sensor as soon as it is convenient.

The early ??? symbol was because the new sensor had not been sufficiently immersed in interstitial fluid yet. It makes sense after all - Dexcom is doing the 2 hour warm-up for a reason!

So the reason you might want to do this if you have a similar situation - if the sensor falls off at a time that is inconvenient to do a normal sensor restart. It also shortens the “warm-up” time by a little bit (about 30 minutes shorter for me, but that can possibly vary for everyone).

Hope all of this makes sense. Let me know if you have ever done this.


We did that once. The reason was a little silly. Somewhat hard to believe.

We forgot to hit stop/start sensor.


The sensor was already in its second week which for us is when the data starts to get wonky/scattershot/shotgun (whatever you want to call it). (And no - nothing to do with not being taped down.)
Point being, it was not the seven day timer forcing the sensor change (or more accurately forcing the stop/start) but rather us proactively changing out the sensor due to bad cgm data.

(Routinely we get 10~12 days of good data from a sensor.)

Changed the sensor. Forgot to hit stop/start. Appeared to work fine.

Once I realized this however, I was concerned about the past 5 days (or however long) of calibrations that had been done using data from the older sensor in the different body location which were now being applied to the new sensor in this new location. The only 100% guaranteed method I know of clearing old calibrations is to do a stop/start sensor. (I know the other methods as discussed elsewhere but do not consider them 100%). So we just did the stop/start right then.

Putting the two methods together (physically replace sensor without stop/start) and skipping the two-hour delay (receiver out of range such as in vehicle outside in garage) then it certainly provides the ability to physically change out the sensor, clear all calibrations and start fresh without skipping a beat.

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That makes sense. I suspect that given enough calibrations, the old values will eventually be cleared out. But who knows how many calibrations it would actually take?

Just to be certain, I went ahead and did a restart now. Which is fine, because it isn’t 2am, it’s a much more convenient time than it was when the sensor fell out! :grinning:


I had read that it was six calibrations.
The first calibration is a factory calibration required to kickstart the process.
The second calibration is the double fingerstick which averages the two fingersticks into a single calibration.
It then takes another four calibrations (typically 12 hours apart at that point) to flush out the stack of calibrations used in the algorithm.
At that point, additional calibrations will drop the oldest one (starting with the factory calibration) off the stack.

But then I tried to go back and find that source again and just couldn’t find the reference. So now I just don’t know as well as am unsure if that could have been referring to the G4 or the G5 which appear to have slightly differing approaches to how calibrations are used.

It certainly made sense however as well as explaining why performing a bunch of extra calibrations (in a relatively short time period) would cause problems.

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If you get vastly volitle numbers, you will get the dreaded ???. This could be from a bad sensor, pressure on the sensor, or removal of the sensor. When you are doing a hotswap, timing is key. For best results having an acclimated sensor will yield the lowest risk of ???. @Eric is totally correct. Timing the swap just after a reading will give you 5 min to swap out the transmitter or else it will error. Nice post!

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