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 HRM-Dual Heart Rate Monitor and Chest Strap | REI Co-op
A solution to heart rate dropouts/spikes with Garmin HR Soft Straps | DC Rainmaker
Troubleshooting your heart rate monitor/strap HR spikes | DC Rainmaker
Troubleshooting your heart rate monitor/strap HR spikes | DC Rainmaker