PSA: Look at the heart rate sensor of your fenix 5. Has it cracks like mine? If yes, it's a common problem. Send it to Garmin and get a free replacement. Do
Garmin fēnix® 5X | Multisport GPS Watch
GARMIN(ガーミン) Fenix 5 Sapphire (フェニックス 5 サファイア) Black Band 47mm [並行輸入品] : Electronics - Amazon.co.jp
Garmin Fenix 5 Sapphire Review | Running Shoes Guru