Fitbit Sense, the company’s wearable with advanced health-tracking features like ECG and SpO2, is the subject of a new email advising some owners to return their watches for a replacement. The reason, according to Fitbit, is a hardware issue that may affect the device’s functionality, necessitating a replacement with a fully-functional unit.

The issue came to light when some Fitbit users posted on the company’s community forum seeking confirmation about the emails they’d received. 
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https://www.slashgear.com/fitbit-tells-some-sense-owners-to-return-watch-over-ecg-issue-04649875/