Numbers off after restarting G6 sensor

Anyone experienced numbers being off after restarting a G6 sensor? I’ve had this before and if I try to calibrate, either the numbers will go back to what they were before extremely quickly (<5 mins), or it will give me the hourglass and say that the sensor failed.

For example, right now Dexcom is saying 4.9, Contour Next One showing 7.5.

Any tips/tricks?

I always do a calibration after a restart because they are always way off. Usually within 15-20 minutes its fine.

My numbers are WAY off after a restart, around +/- 100 mg/dl, and even worse the second time (though my current sensor is a freak, it was only off by 20 mg/dl on day 20.)

I’ve learned that if I’m right around 100 (5.6 mmol), though, I can drop the difference to 50-60 mg/dl instead.

There are calibration rules to abide by. You should be in range (you get into extreme numbers that confused the system if you’re not, and both systems are less accurate out of range anyway), you should be well- hydrated, and you should have a flat trend arrow. It’s only under those conditions that you can EXPECT Dexcom to match your fingerstick. On top of that, you have to make conservative changes. Right around 30% different than the Dexcom value should always be safe, but never more than 40 mg/dl (2.2 mmol). I don’t know if the buffer converts as well to mmol, so you might want to be more conservative. 2 mmol maybe?

If I’m only 50-60 mg/dl off, because my glucose was low enough to start with, then 1 calibration will usually get me close enough for my comfort. I tend to restart my sensors in the evening after I’ve come down from the dinner spike, so I’m pretty happy to go to bed still being that little bit off. I run so flat through the night on sleep mode that it doesn’t matter if I’m actually a little lower. I’ll do one more calibration in the morning, and it’s perfect for the next ten days.

Now day 20 is a lot more stressful. (I pay cash for CGM, so run a sensor for a full month to offset cost.). I’ve got a 50/50 chance of the calibration rejecting and falling into the dreaded calibration loop. Usually within 4-6 requests for a new calibration, though, it does finally accept it. It’s annoying, but it’s not “endless” as people often call it. 2 hours worth of calibrations is a small exchange for what I save on the sensors, though.

You’re lucky to get that long out of a sensor! I also pay out of pocket and they barely last 10 days for me.

Trust me, I know! I have no idea what the magic is, but I eat well, drink like a fish, and try really hard to keep my BGs good. I suppose some of it has to be body chemistry though.

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Mine seem to average about 25 days, although lately it feels like I’ve had a few more that have only lasted about 16 days. I try to time a restart when I am around 4.4 (80). If it restarts saying I am at 8.3 (150) or below I will try to wait and see what it says in a couple of hours to do a calibration, if I it says more I will immediately do a calibration and wait. By 24 hours my goal is to be within 5 points of accuracy around the 4.4-6.7 (80-120) range and that usually takes 1-2 calibrations the first day and 1-2 the next day. The sensors that required a bunch of calibrations when I started them are bound to need more as a restart too.

I am lucky that my insurance completely covers my sensors, but I like restarted sensors better. A new sensor requires more work and more calibrations and I just can’t trust it as much. That’s me though and my 5 point accuracy desire.

Yup! Who knows.

Nice…25 days. I wish. I understand the method of using an “offset” if it is constantly off by the same amount. The problem is that a lot of the time it gives false low readings, to the point where I’m getting notified by my pump multiple times per hour and no matter what you do it will go low again…extremely annoying.

What do you guys do for example if Dexcom is reading 3.1, but you are actually 6.5? If I calibrate, I know it will give me a sensor failed error…but it is annoying getting alerts every 15 minutes. I have my Tandem alerts for 1 hour, but they STILL alert every roughly 10-15 mins. It is extremely annoying.

Sometimes you can do a series of calibrations, to gradually get it back on track. Or wait it out without calibration until it starts to move in right direction.

It is more tricky with CIQ, since it may also adjust insulin based on bad cgm data, so turning that off may help too.

Except for the initial I know it’s going to be off, I always calibrate it if it’s more than .3 mmol (5 points) off. I do not have my intergrated with a pump so I don’t know what part that plays in you getting sensor failure? I do not get sensor failure error when I calibrate. When they try to trend low, I calibrate to fix it.

It has an algorithm it wants to follow. So you do have to give the calibrations a little time to settle. I’m just not sure why you are getting sensor failed error.

Right. Do you typically do calibrations 2 at a time? I believe I read the calibration guide once on another website and the author said that 2 were better as the Dexcom then “resets” the calibration and has a “trajectory path” instead of just 1 calibration point where it was kind of lost.


In my case, I rarely calibrate at all. But I do a meter bg when I see cgm bg that is unexpected. Then I decide whether to use meter bg to calibrate. If cgm bg is changing quickly, I would just keep eye on cgm and not do calibration, but recheck when levels out.

If the difference was big, I would repeat a bg check about 20-30 minutes later.

Two quick calibrations with the same number makes it read the number you have entered. This is usually a way to stop it from asking repeated calibration requests. Or to force a number on it.

But it still has it’s algorithm it wants to follow so if it is trying to adjust say lower already and you enter a the low number twice it will then end up trending lower than that number and be too low. Whereas if you enter the number once, it usually slowly gets closer to your desired number in a few hours on its own? But like I said restarts are more predictable than new. I spend more time calibrating a brand new sensor to make sure it reads within my .3 mole (5 points) desired.

But I never get a sensor failure message, but I have gotten the repeated calibration request and the double entry stops that.

Calibrations need to be at least 5 min apart. You are calibrating a time interval. So if you calibrate and then again in one min, you just changed the value in the same interval, so the first one is overwritten.
Dexcom has a weird stepping up or down calibration so 2 calibrations is better than one.
If you get a 200 and you calibrate to 100, you will see your reading reads 150. So another cal in 5 min of 100 gets you to 125. You are in the ball park so check again in a half hour.

I wish there was a way for it to just accept the calibration you give it the way the g5 did. But there is an internal calibration that you need to fight against. And if it’s too far off the money it will give you an error.

So,if you have a sensor error, calibrate,and again in 5 min.
If you still have a sensor error, it’s time to dump the sensor.


Just to add my experience. I am a never calibrater. I don’t advocate this for everyone, but it’s the policy I’ve more or less followed and it more or less works.
Yesterday I had a new G6 session that started off pretty bad as they unfortunately often do. (Maybe one out of three times for me…not sure) It was OK for the first hour or so and then bad for about three hours after that. During that three-hour period I frequently checked my BG with a Contour meter and all my readings were between 81 and 121 while my G6 showed readings between LOW and 79 with a lot of readings in the forties and fifties. I knew it was wrong and felt fine but I checked A LOT to be sure I was actually still conscious :slight_smile:
Anyway, no calibrations because I was afraid that if I calibrated when it was jumping around and showing LOW it would just give me more problems. After about three hours it just started being more or less correct (maybe 20 points off) and has been more or less correct ever since. (Last time I checked my meter said 96 and Dexcom 97)
The point being that not doing anything can work as well as working hard to get the thing calibrated. Which isn’t to claim that three hours of being told you’re about to lose consciousness is any kind of picnic. Also, don’t actually know what would have happened if I had calibrated. I might have gotten on track much faster. But, anyway, in my experience, it’s certainly possible for the Dexcom to get on track all by itself. It’s also my experience that once that initial 12 to 24 hour period is over, it will be good for the next nine days. I can’t remember it ever returning to a wacko state after it got over it’s problems other than an occasional compression low or something.

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I’ve read reports like yours, @Tnyc, many times over the last few years. I don’t doubt the accuracy of the reports.

I am not patient enough to live with that scenario, however. If I know my CGM numbers are seriously inaccurate, I’m faced with taking my automated insulin dosing system offline until things sync up.

I’d rather do my process of pre-soaking (about 20 hours) and a few well-timed early calibrations, if needed, and then enjoy good accuracy within 30 minutes of the session start.

Your hands-off preference would be a great exercise for me to hone my patience. I’ll take a pass and strengthen my patience muscles in other ways!


Your experience is mine as well and I never calibrate. I find if my BG’g mostly flatline for the 6 or so hours before inserting the transmitter into the new sensor my BG numbers are even better than you show, however, to guarantee the flatline, I need to be on an LFHC diet for those 6 hours. With a more varied diet, comes more variability and my numbers after inserting a transmitter into a new sensor are worse than yours but self-correct in about the same time frame.

I don’t think I could stand the repeated low alarms for 6 hours. Since I can’t turn off the 55-Low alarm on either my pump or phone, it drives me crazy. Sometimes I turn off my phone but I can’t turn off my pump. Pre-soaking helps me some but it doesn’t eliminate the problem.

When I think about the mental health challenges of diabetes, I think my biggest stressor is the first 24 hours of every new G6 sensor.

When I used G4 and G5, I usually restarted my sensor once or twice. I didn’t need to do it to save money. I did it to avoid Day 1’s. Unfortunately for me, a G6 restart doesn’t pick up the accuracy from the previous session. It reapplies the algorithm which makes my restarts read high so I am back to calibrating.

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I also prized the performance improvement in the second week after the first restart in the G4. It was easier to live with.


I too have crazy low readings in the first hours if I don’t do a pre-soak. If I do a pre-soak (usually 6 to 12 hours) then I never have crazy false low readings. Strongly recommend pre-soak!

Before I discovered pre-soaking, I tried to “calibrate out” the fake lows with calibrations during the first few hours. It’s pointless. The most common scenario was a seemingly infinite loop of calibrate, “sensor error”, “recalibrate after”, etc. The behavior in the first few hours has nothing to do with where the sensor will settle down after 6 hours or so. So I too fell into the “hardly ever calibrate” camp and never would calibrate until the sensor had settled down.

With the presoak I do not have the crazy false lows and usually don’t have to calibrate at all. I will occasionally cross-check with finger stick and occasionally after a couople consecutive cross-checks that are running high or low by more than 20 (all the same direction!) I will do a calibrate.

For example today, my pump has been showing near low all morning, 4.2-4.6 range. I am actually 10.2 right now. I try to calibrate and put in 10.2, it freaks out and tells me to calibrate again in 15 mins. I do that again, put in 10.2 again, it just tells me over and over to calibrate again in 15 mins.

This always happens! I can never seem to calibrate these sensors without them failing. I am literally calling Dexcom for almost every sensor I have going bad, soon they will probably think I’m trying to scam the system.