Your Garmin Fenix 8 is showing 50 bpm during a hill sprint, or it's reading 180 while you're sitting on the couch. Or it just won't register a reading at all. The Fenix 8's Elevate optical heart rate sensor is one of the better wrist-based units on the market, but it still depends on a few specific conditions to work right.
The single biggest cause of bad readings is wrist fit. Garmin's manual asks owners to wear the case roughly two finger widths up from the wrist bone for activities, with the strap firm enough that the case doesn't shift when you bend your wrist. A loose fit is responsible for the bulk of Fenix 8 heart rate complaints.
Reposition the Watch and Tighten the Strap
Slide the Fenix 8 up your forearm so the case clears the wrist bone, the skin further up is flatter and moves less during running or lifting. This keeps the Elevate sensor's green and infrared LEDs lined up with the same patch of capillary bed. For workouts, take up one extra hole on the strap beyond your daily fit.
Test by gently rotating the watch on your wrist. It should resist twisting; if it slides freely, the LEDs are getting partial readings during impact. Loosen back to your normal fit after the workout to avoid skin irritation.
Clean the Sensor Lens on the Back
The Fenix 8's optical sensor uses green and infrared LEDs to read blood flow through the skin. Once a layer of sweat salt, sunscreen, or moisturizer dries onto the lens, the light scatters before it gets back to the photodiodes and the watch reports nonsense numbers. This is especially common on the AMOLED model if you're wearing it for all-day smartwatch use.
Garmin's official guidance is to rinse the back of the watch with fresh water after every workout and dry it with a clean cloth. For dried-on residue between cleanings, a damp lint-free cloth is enough. Skip household cleaners and skin-prep alcohol pads, which can damage the sensor seal over time.
Check for Wrist Tattoos or Dense Hair
Dark wrist tattoos absorb the green and infrared light from the optical sensor before it can reflect back. The Fenix 8 may show a flatlined heart rate or just no reading at all if the sensor is over a tattoo. Dense forearm hair under the sensor also scatters the light and can cause erratic readings.
If you have a tattoo on the underside of your wrist, try wearing the Fenix 8 on the opposite arm. Or pair an external chest strap, the Fenix 8 supports any ANT+ or Bluetooth heart rate strap and will use it instead of the wrist sensor automatically.
Use a Chest Strap for Accuracy-Demanding Workouts
Optical wrist sensors lag behind chest straps for sudden heart rate changes (HIIT, intervals, lifting). If you're training to specific zones or following a structured plan, pair a Garmin HRM-Pro Plus or any compatible Bluetooth chest strap. The Fenix 8's multi-band GPS handles trail accuracy fine, but the wrist sensor still has the same optical limitations as every other watch.
Hold the upper-right button to open the menu, then go to Settings > Sensors and Accessories > Add New. Choose Heart Rate and follow the pairing prompts. Once paired, the watch automatically uses the strap during workouts.
Force Restart the Watch
If readings were fine and suddenly went off, the heart rate service in Garmin OS may have crashed. Hold the power button (top-left) for 15 seconds until the display turns off. Wait 5 seconds, then press the power button to turn the watch back on.
This clears any background process that was reading bad data. After the restart, check your resting heart rate (it should match your normal range) and try a quick walk to verify accuracy. The Fenix 8's 15-second force restart is the same across both AMOLED and Solar models.
Update the Firmware Through Garmin Connect
Garmin has shipped Fenix 8 firmware patches that addressed heart rate accuracy issues during workouts. Some early AMOLED models had a known issue where the sensor would temporarily drop readings during GPS recording. Make sure you're current.
Open the Garmin Connect app on your phone, then go to your device, scroll to Software Updates, and install if available. The watch needs at least 30% battery and to be within Bluetooth range of the phone during the install. Updates take 15-45 minutes depending on size. The Fenix 8 charges via the proprietary 4-pin cable, so keep that handy if battery is low.
Other Factors That Mess With Heart Rate Readings
If you've checked fit and cleaned the sensor and readings still drift, work through these:
- Cold skin: when arms are cold, blood flow at the wrist is reduced and the sensor struggles. Warm up before your run.
- High-impact activity: lifting weights or rowing shakes the watch loose and causes spikes in the reading.
- Watch worn over a sleeve: fabric blocks the sensor entirely. Fenix 8 voice commands are great, but the sensor still needs skin contact.
- Battery Saver mode active: Garmin throttles HR sampling to a lower rate when the watch is in Battery Saver, so spikes get smoothed. The AMOLED model hits Battery Saver faster than the Solar variant.
- Per-activity HR set to Off: check Settings > Activities and Apps > [activity] > Heart Rate and confirm it's set to Wrist.
- Recent firmware update: occasional regressions hit specific Fenix 8 variants. Check forums to see if others report the same issue.
- Connect IQ app interference: some third-party watch faces or data fields conflict with the heart rate service. Remove any recently installed Connect IQ apps and test.
Reset the Heart Rate Sensor From the Watch
Garmin OS includes a sensor reset that recalibrates the optical heart rate baseline. Hold the upper-right button to open the menu, then Settings > Sensors and Accessories > Wrist Heart Rate > Status > Off, wait 30 seconds, then toggle back to On.
The sensor recalibrates and the next 10-15 minutes of readings rebuild your baseline. If readings are still off after this, factory reset the watch via Settings > System > Reset > Restore Defaults. Re-pair to Garmin Connect after, your activity history syncs back from Garmin's cloud.
