A Garmin Forerunner 935 that refuses to sync can derail your training routine. You finish a run, open Garmin Connect, and nothing appears — or the watch just spins endlessly. This frustration is common, and in nearly every case it has a straightforward fix.
This guide walks you through every layer of the problem: from the quickest Bluetooth toggle to a full factory reset. Whether you use an Android phone, an iPhone, or a computer, you’ll find the solution that works for your situation. By the end, your Forerunner 935 will be uploading activities reliably again.
Why Syncing Fails: Root Causes
Syncing between the Forerunner 935 and Garmin Connect depends on a chain of connections — any weak link can break it. Understanding the root cause helps you pick the right fix.
Bluetooth Connection Instability
The Forerunner 935 uses Bluetooth Low Energy (BLE) to communicate with your phone. BLE is reliable but sensitive to interference. Common culprits include:
- Physical distance — more than a few metres (or even walls) can drop the signal.
- Other wireless devices — Wi‑Fi routers, wireless headphones, or even a second smartwatch can cause interference.
- Phone battery optimisation — many phones kill background apps or turn off Bluetooth to save power when the battery is low.
Outdated Software
Garmin regularly releases firmware updates for the Forerunner 935 and updates for the Garmin Connect app. Running old versions is one of the most frequent causes of sync failures. As of 2026, the latest Forerunner 935 firmware supports the same BLE stack used in modern phones, but compatibility breaks if either side is outdated.
Corrupted Device Pairing
Pairing is more than a one‑time handshake — it establishes a “bond” between your watch and phone. A phone OS update, a reset of either device, or switching to a new phone can break that bond. The watch may still appear in your Bluetooth list but cannot establish a data connection.
Insufficient Storage
Your Forerunner 935 stores activity data locally. When its internal storage is nearly full, it may stop recording or refuse to sync. The same applies to your phone — if Garmin Connect cannot save incoming data because the phone’s storage is full, the sync stalls.
Account or Server Issues
Occasionally the problem is not on your devices but on Garmin’s side. A server outage or a problem with your Garmin account can prevent activities from uploading. Always check Garmin’s server status page if you suspect a service disruption.
Battery Saving Modes
Modern phones aggressively manage battery usage. If Garmin Connect is restricted from running in the background, the sync process cannot complete. This is especially common on Android devices with custom power‑saving features from manufacturers like Samsung, OnePlus, or Xiaomi.
Quick Fixes You Can Try in Minutes
Start here. These steps resolve about 80% of sync problems.
1. Force a Manual Sync
Open the Garmin Connect app on your phone and pull down on the main screen. A circular sync icon should appear. Wait until it finishes spinning — if it stops quickly without updating, the sync failed.
2. Toggle Bluetooth Off and On
On your phone, turn Bluetooth off, wait ten seconds, then turn it back on. This resets the radio and clears temporary glitches.
3. Restart Both Devices
Restart your Forerunner 935 (hold the light button, then select Power Off) and reboot your phone. This clears any stuck processes.
4. Charge to at Least 20%
If your watch battery is below 20%, Bluetooth may be partially disabled. Plug it in until it reaches at least 20% and try syncing again.
5. Update All Software
- Garmin Connect app: check your phone’s app store for the latest version.
- Watch firmware: on the watch, go to Settings > System > Software Update.
- Phone OS: install any pending system updates. Sync failures often appear after an OS update, but the fix is usually already included in a subsequent patch.
6. Check Your Internet Connection
Your phone needs a working internet connection (Wi‑Fi or mobile data) to upload activities to Garmin Connect. Open a browser or an app that requires internet to confirm connectivity.
Advanced Troubleshooting
If the quick fixes didn’t work, move to these deeper solutions.
Re‑pairing Your Forerunner 935
A fresh pairing clears corrupted bonds. Follow these steps carefully.
On your phone (Garmin Connect app):
- Go to Garmin Devices and select your Forerunner 935.
- Tap Remove Device.
- In your phone’s Bluetooth settings, find the Forerunner 935 and select Forget or Unpair.
On your watch:
- Go to Settings > Bluetooth > Pair Mobile Device. The watch will display a pairing code.
Back on your phone:
- Open Garmin Connect, tap Add Device, and follow the on‑screen prompts.
- Enter the code shown on your watch when prompted.
A pro tip: restart both your phone and watch before starting the pairing process. This ensures no stale Bluetooth handshake lingers.
Using Garmin Express via USB
If your phone’s Bluetooth is unreliable, Garmin Express on a PC or Mac is a rock‑solid alternative. It syncs through the USB charging cable and does not depend on wireless connections.
- Download and install Garmin Express from the official Garmin download page.
- Open Garmin Express and connect your Forerunner 935 using the included USB cable.
- Follow the prompts to add your device. Garmin Express will automatically sync all unsaved activities to your Garmin Connect account.
This method also lets you update the watch firmware even if Bluetooth is completely broken.
Clearing the Garmin Connect Cache
Corrupted cache files can cause the app to hang or fail to recognise your watch.
On Android:
- Go to Settings > Apps > Garmin Connect > Storage.
- Tap Clear Cache (not Clear Data — that logs you out and removes local data).
- If that doesn’t help, you can tap Clear Data as a last resort, but you will need to log in again.
On iPhone: iOS does not offer a per‑app cache clear. Instead, uninstall the Garmin Connect app, restart your phone, and reinstall it from the App Store. Sign in again and re‑pair your watch.
Resetting Network Settings on Your Phone
A corrupted Wi‑Fi or Bluetooth configuration can block syncing. Resetting network settings clears all saved networks and pairings — you will need to reconnect to Wi‑Fi and re‑pair Bluetooth devices afterwards.
On Android: Settings > General Management > Reset > Reset Network Settings.
On iPhone: Settings > General > Transfer or Reset iPhone > Reset > Reset Network Settings.
Factory Reset (Last Resort)
A factory reset erases all data and settings on your Forerunner 935. Use this only after backing up your activities (sync via Garmin Express if needed).
- On the watch, go to Settings > System > Reset.
- Select Delete Data and Reset Settings.
- Confirm. The watch will reboot as if new.
- Set up the watch again from scratch and re‑pair with Garmin Connect.
Special Scenarios
After a Major OS Update
Phone OS updates (iOS 19, Android 15, etc.) often change Bluetooth stack behaviour. If your Forerunner 935 stopped syncing right after an update, re‑pairing the device (see above) almost always fixes it. There is no need to wait for a watch firmware update — Garmin usually keeps the Forerunner 935 compatible.
Switching to a New Phone
Always unpair your watch from the old phone before setting it up on the new one. On the old phone, remove the device from Garmin Connect and forget it in Bluetooth settings. Then on the new phone, add it fresh. Skipping this step leaves the watch in a “bonded but orphaned” state that prevents pairing.
Bluetooth Interference from Other Devices
Wireless headphones, smart speakers, and even certain USB 3.0 cables can emit signals that disrupt BLE. Try turning off nearby Bluetooth devices temporarily. Also, keep your watch and phone within a metre of each other with no metal obstacles in between.
Garmin Connect Server Outages
Rarely, the problem is not your gear but Garmin’s servers. You can check service status on Garmin’s support page. If the servers are down, wait a few hours and try again — no local troubleshooting will help.
Common Mistakes
- Relying on auto‑sync only — Garmin Connect may not trigger auto‑sync if the app is in the background. Always open the app and pull down to force a sync after each activity.
- Forgetting to “forget” the device before re‑pairing — simply deleting from Garmin Connect without also forgetting in Bluetooth settings can leave a residual bond.
- Ignoring low‑power mode on the phone — Battery Saver, Low Power Mode, or similar features can disable background Bluetooth. Disable these while syncing.
- Using the wrong USB cable — only the original Garmin charging cable (or a certified replacement) carries data. A generic cable may charge but not sync.
Frequently Asked Questions
Why does my Forerunner 935 keep disconnecting mid‑sync?
This usually points to Bluetooth interference or a phone battery optimisation setting. Turn off power‑saving modes for Garmin Connect, and keep the watch within a few feet of the phone. If the problem persists, try syncing with Garmin Express on a computer.
Can I sync without a smartphone?
Yes. Use Garmin Express on a PC or Mac via USB. All your activities will upload to Garmin Connect when the computer is online. This is also a good fallback if your phone is being repaired or replaced.
Will a factory reset wipe my activity data?
A factory reset erases everything stored on the watch — activities, settings, and personal records. Sync with Garmin Connect first (or use Garmin Express) to back up your data. After the reset, you can restore past activities from Garmin Connect’s history.
Why do my runs show in the watch but not in the app?
The activities are stored on the watch but were not transferred. Open Garmin Connect and force a sync. If nothing happens, the watch may think it has already synced (a known bug). Re‑pairing the device usually forces a full transfer.
My watch syncs but my heart rate data is missing
Some daily health metrics (heart rate, steps, sleep) sync only when “All‑day Sync” is enabled. In Garmin Connect, go to Settings > Data Sharing > All‑day Sync and turn it on. Also ensure your watch is set to record heart rate continuously (Settings > Heart Rate > Status > On).
When to Contact Garmin Support
Try all the steps above before calling support. If you’ve exhausted them and still cannot sync, contact Garmin when:
- The watch does not power on or is stuck on the boot logo.
- The USB port on the watch is physically damaged or shows corrosion.
- You get a consistent error code (e.g., “Sync Failed – Error 20”) that does not resolve.
- The watch fails to sync across multiple phones and a computer.
Garmin’s support team can run remote diagnostics, process warranty claims, or set up a repair. Visit the Garmin Support website to start a case.
Preventive Tips for Reliable Syncing
- Sync after every activity — don’t let multiple workouts pile up. A single sync failure then loses everything.
- Keep Garmin Connect in your phone’s “unrestricted” battery list — on Android, allow the app to run in the background without restrictions.
- Update firmware regularly — even if your watch is working fine, check for updates every few months.
- Clear old activities from the watch — delete completed runs from the watch’s history to free up storage. The data is already on Garmin Connect.
- Avoid syncing near strong Wi‑Fi routers or microwaves — those frequencies can step on BLE signals.
Conclusion
A Garmin Forerunner 935 that won’t sync is almost never a hardware failure. Most of the time, the fix takes under five minutes: a manual sync, a Bluetooth toggle, or a quick software update. For stubborn problems, re‑pairing the device or using Garmin Express on a computer will get you back on track.
By following this guide — from the basic checks to the advanced resets — you should be able to restore syncing without needing support. Once your data flows again, you can focus on your training, not on troubleshooting.
If this article helped you, share it with a fellow runner or bookmark it for your next syncing hiccup. Reliable syncing means less frustration and more time enjoying your runs.
Example of a sync failure notification in the Garmin Connect app.
The Forerunner 935 displays a small arrow icon when actively syncing.
The Garmin Forerunner 935 – a multisport GPS watch known for its reliable performance.
