Sensor Error / ??
Dexcom Continuous Glucose Monitor (CGM)
Severity:What Does This Error Mean?
Dexcom G7 'Sensor Error' or '??' means the sensor cannot produce a reliable glucose reading at that moment. This is temporary in most cases — it usually clears within 3 hours. Do not make insulin or treatment decisions without a fingerstick blood glucose reading during this time.
Affected Models
- Dexcom G7
Common Causes
- Sensor still in warm-up or initialisation phase
- Rapid change in blood glucose — sensor cannot keep up
- Compression lowing — sensor being compressed (sleeping on the sensor side)
- Sensor placed in a site with low blood flow or significant movement (near a joint)
- Sensor nearing the end of its 10-day wear period
- Faulty sensor — manufacturing defect
How to Fix It
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Wait up to 3 hours.
Dexcom G7 Sensor Error or ?? displays are often temporary. The sensor may be recalibrating after a rapid glucose change, recovering from compression, or experiencing brief signal interference. Wait up to 3 hours before taking further action. Most sensor errors resolve on their own.
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Check for compression.
If you are lying down or wearing tight clothing, the sensor may be compressed against your body. This restricts interstitial fluid flow to the sensor filament. Change position, loosen clothing over the sensor, and wait 15–30 minutes to see if readings resume.
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Test blood glucose with a fingerstick meter.
During a Sensor Error or ?? display, do not estimate or guess your glucose level. Use a conventional blood glucose meter (fingerstick test) to check your glucose. Make all treatment decisions (insulin, food, activity) based on the fingerstick result until the sensor recovers.
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Check the sensor wear period.
The Dexcom G7 sensor lasts 10 days (with a 12-hour grace period). If the sensor is in its final 12 hours or has expired, sensor errors and ?? readings are common. Insert a new sensor — the new sensor warm-up takes 30 minutes.
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Replace the sensor if the error persists beyond 3 hours.
If the Sensor Error or ?? persists for more than 3 hours with no recovery, the sensor is defective. Remove and discard the sensor and insert a new one at a fresh site (abdomen or the back of the upper arm). Contact Dexcom at dexcom.com/support/ to request a replacement for the failed sensor — Dexcom typically replaces failed sensors under the warranty.
Frequently Asked Questions
What does '??' mean on Dexcom G7?
?? on Dexcom G7 means the sensor is detecting conditions it cannot interpret as a reliable glucose number. This happens most often in the first two hours after insertion (normal), during rapid glucose changes, or due to physical compression of the sensor. Wait up to 3 hours — most ?? displays resolve. Always use a fingerstick meter to check your actual glucose while the ?? is displayed.