Dexcom G7 sensors fail for a handful of common reasons: the sensor wire shifted or pulled out, the insertion site wasn’t ideal, something was pressing on the sensor, or the hardware itself was defective. Most failures fall into one of these categories, and understanding which one caused yours can help you prevent it next time and get a free replacement from Dexcom.
How a Sensor Error Becomes a Sensor Failure
When your G7 can’t measure glucose properly, it first shows a “Sensor Error” alert and tries to fix itself. The system gives itself up to three hours to recover. If it can’t, the alert changes to “Sensor Failed,” meaning the sensor is permanently done and won’t produce readings again. At that point, you need to remove it and start a new one.
This is different from signal loss, which happens when your phone or receiver temporarily loses the Bluetooth connection to the sensor. Signal loss means the sensor is still working but your device can’t hear it, usually because your phone is too far away, a wall or body of water is blocking the signal, or a battery optimization setting on your phone is interfering. The sensor transmits glucose data via Bluetooth in short bursts every five minutes, so even after you fix a signal loss issue, it can take a few minutes for readings to reappear. If signal loss continues for more than 30 minutes, that’s worth a call to Dexcom support. But signal loss alone doesn’t mean your sensor has failed.
The Most Common Reasons for Failure
The Sensor Wire Moved
The G7 works by inserting a tiny filament wire just under your skin to measure glucose in interstitial fluid. If that wire shifts, partially pulls out, or gets kinked during insertion, readings become unreliable and the sensor fails. This often happens when the adhesive patch starts peeling up, which lets the sensor move. Using the Dexcom overlay patch and keeping the sensor firmly stuck to your skin is the single most important thing you can do to prevent this.
Poor Insertion Site
Where you place the sensor matters more than most people realize. The G7 is approved for the back of the upper arm for anyone age 2 and older. Children ages 2 to 6 can also use the upper buttocks. The 15-day version is approved for the back of the upper arm in adults 18 and older. Placing it anywhere else increases the risk of inaccurate readings and early failure.
Even within approved areas, certain spots cause problems. Avoid areas with loose skin or not enough fat, since the wire can hit muscle or bone. Stay at least 3 inches away from any insulin pump infusion set or injection site. Scarring, tattoos, irritation, and bony areas all interfere with readings. If you tend to sleep on one side, don’t put the sensor on that arm.
Bleeding During Insertion
Sometimes the insertion needle nicks a small blood vessel. A little blood at the site can interfere with the sensor’s ability to read glucose accurately, leading to erratic readings or outright failure. You can’t always prevent this, but choosing a site with a decent layer of subcutaneous fat helps. If you see blood pooling under the adhesive right after insertion, the sensor may not survive the session.
Skin Prep Problems
Oils, sweat, lotion, or residue on the skin can prevent the adhesive from sticking properly and can also interfere with the sensor wire’s readings. Clean the site with soap first, then an alcohol wipe, and let it dry completely before inserting. Skipping this step is one of the most common causes of sensors that fail in the first day or two.
Compression Lows vs. Actual Failures
If your sensor showed a sudden low reading (especially at night) that didn’t match how you felt, you may have experienced a compression low rather than a true failure. This happens when you lie on the sensor or something presses against it, compressing the tissue around the wire. The pressure temporarily changes the fluid around the sensor and produces a falsely low reading.
The key difference: compression lows resolve on their own once the pressure is removed, and your readings return to normal. A true sensor failure doesn’t recover. If your sensor gave you alarming low readings overnight but was fine by morning, compression was likely the culprit. Placing the sensor in a spot you’re less likely to sleep on solves this for most people.
Medications That Interfere With Readings
Two medications are known to affect G7 accuracy. Hydroxyurea, used for conditions like sickle cell disease, causes the sensor to read higher than your actual glucose. This isn’t a temporary glitch; it’s a persistent interference for as long as you’re taking the medication.
Acetaminophen (Tylenol) was a major problem for older CGM models, but the G7 handles it much better. Taking normal doses generally won’t affect your readings. Only doses above the maximum recommended amount (more than 1,000 mg every six hours in adults) may push sensor readings higher than actual glucose. At standard doses, the G7 meets accuracy requirements even with acetaminophen on board.
Environmental and Storage Causes
Sensors should be stored between 36°F and 86°F. Leaving a sensor in a hot car, in direct sunlight, or in freezing temperatures before you apply it can damage the chemistry inside and cause it to fail once inserted. If you received sensors by mail during extreme weather and they sat on a porch for hours, that exposure may have compromised them before you ever opened the box.
Applicator Malfunctions
Occasionally, the applicator itself doesn’t work correctly. The most common issue is the button not activating. This happens when you haven’t pressed the applicator firmly enough against your skin. The clear safety guard needs to be fully pushed in before the button will release. Press more firmly and try again. If it still won’t activate after firm pressure, the applicator is defective. Don’t try to force it. Contact Dexcom for a replacement.
How to Get a Free Replacement
Dexcom replaces any sensor that fails before its intended session ends, with no limit on how many replacements you can receive. Their policy covers the G6, G7, G7 15 Day, and ONE+ systems. If the failure is confirmed as a product defect, the replacement is automatic. Even if the failure turns out to be user-related (wrong site, removal for a medical procedure), Dexcom may still issue a courtesy replacement.
Before you call, find your sensor’s serial number. This speeds up the process significantly. Dexcom’s global technical support line is 1-844-607-8398 and is available 24/7. You can also submit a product support request through their website. Contact them as soon as possible after the failure, since they’ll want details about what happened while it’s fresh.
Reducing Your Risk of Future Failures
Most sensor failures come down to a few preventable factors. Clean and dry the site thoroughly before insertion. Use only approved locations on the back of the upper arm (or upper buttocks for young children). Apply the overlay patch to keep the sensor secure for the full session. Avoid placing the sensor anywhere it’ll get bumped, compressed during sleep, or rubbed by clothing and waistbands. Keep unused sensors stored within the recommended temperature range. And if you’re using an insulin pump, maintain at least 3 inches of distance between the pump’s infusion site and your sensor.
If you trim body hair at the site with electric clippers before applying, the adhesive grips better and lasts longer. Don’t shave with a razor, since that can cause irritation that affects readings.