If a DexCom CGM sensor falls off, it is important to take prompt action to ensure accurate continuous glucose monitoring. First, check the sensor site for any irritation or redness, and clean the area if necessary. If the sensor is still in its lifespan, you may be able to reapply it using an adhesive patch designed for this purpose. If this is not feasible, or if the sensor is no longer usable, you will need to replace it. Before applying a new sensor, ensure that the site is clean and dry to promote proper adhesion. It is crucial to follow the specific guidelines provided in the user manual for properly inserting a new sensor. For detailed instructions and additional tips, it may be helpful to refer to the information on the official DexCom website.