Medtronic Guardian Sensor Error — Calibration & No Signal Fix

Hello. If your Medtronic Guardian 4 or Guardian 3 sensor is displaying an error, a calibration alert, or a signal loss warning, this guide explains what each alert means and how to respond. Step 1: Understand Guardian sensor alerts. The Medtronic Guardian system uses a pump, transmitter, and sensor together. The most common alerts are: Calibrate Now — requiring a fingerstick entry; Sensor Updating — temporary gap in readings; Signal Loss — transmitter disconnected from pump or app; and Sensor Error — the sensor needs replacement. Step 2: The Guardian 4 and Guardian 3 have different calibration requirements. The Guardian 3 requires fingerstick calibrations every 12 hours to remain active. If you miss a calibration window, the sensor goes into a "Cal Error" state and you must enter a new fingerstick value to resume readings. The Guardian 4 sensor does not require routine calibrations but still accepts up to 4 per day if readings seem off. Step 3: Respond to Calibrate Now alerts within 15 minutes. When the Guardian 3 displays "Calibrate Now," enter your fingerstick glucose into the pump or Guardian Connect app within 15 minutes. If you miss the window, the sensor will suspend glucose readings until you enter a new calibration. Always calibrate when glucose is stable — not during rapid changes. Step 4: For Guardian 3 Cal Error — do not remove the sensor. A Cal Error does not mean the sensor has failed. Enter a fingerstick calibration value in the pump menu. If the reading is between 40 and 400 mg/dL and entered within the calibration window, the sensor should resume normal operation. Step 5: For Signal Loss — check transmitter attachment. The Medtronic transmitter clicks into the sensor base. Press firmly until you feel a distinct click. If the transmitter is not fully seated, the signal will drop. Signal loss can also occur if the pump or phone is more than 6 feet from the transmitter. Step 6: Wait through the warm-up period. Guardian 3 sensors have a 2-hour warm-up period. Guardian 4 sensors also require approximately 2 hours before readings begin. During this time, a "Sensor Warmup" message displays. This is normal. Step 7: For Sensor Error or "Replace Sensor" alert — the sensor has failed. Medtronic will replace sensors that fail before the end of their approved wear period. For Guardian 3, the approved wear is 7 days. For Guardian 4, it is up to 7 days. Contact Medtronic at 1-800-646-4633 to report the failure and request a replacement. Step 8: Bridge with fingersticks during any sensor error. Never make insulin dosing decisions based on a sensor that is displaying an error or calibration alert. Use a fingerstick meter until the sensor is calibrated and showing stable readings. Still need help? Find a CGM provider near you →