ERR_SYNC_FAILED on Salesforce: Data sync failed. Root cause: The bidirectional synchronisation between two connected systems failed to complete. This can result from conflicting updates made simultaneously in both systems, a network interruption during the sync process, or a schema mismatch that prevents records from being mapped correctly between the two platforms. Step 1: Check the sync error log for the specific field causing the failure. In Salesforce, go to Setup → Integration → Connected Apps or your third-party sync tool (e.g., HubSpot-Salesforce sync, Zapier, MuleSoft). Open the error log and find the exact field name in the error message. The log will say something like "Field [Account.CustomField__c] does not exist or is not accessible". This tells you exactly where to look. Step 2: Verify the field exists in both systems with matching API names. In Salesforce, go to Setup → Object Manager → [Object] → Fields & Relationships. Confirm the field API name (e.g., Custom_Field__c) matches exactly what your sync tool is referencing. In the destination system, confirm the field exists and is mapped to the correct Salesforce field. API names are case-sensitive and must include the __c suffix for custom fields. Step 3: Check field-level security for the integration user. Salesforce sync failures caused by permissions are the most common and most misdiagnosed. Go to Setup → Users → find your integration user (the account your sync tool authenticates as). Click their profile → Field-Level Security → select the object. Confirm the field is set to Visible. If you use Permission Sets, check those too — field-level security in a Permission Set overrides the profile. Step 4: Check for conflicting update timestamps (the "last write wins" problem). Bidirectional sync conflicts occur when both systems update the same record within the same sync window. Most sync tools use a "last write wins" strategy, but some silently discard one update. Check your sync tool's conflict resolution settings. In HubSpot-Salesforce sync: go to HubSpot → Settings → Integrations → Salesforce → Sync Settings → Conflict Resolution. Set it to "Salesforce wins" or "HubSpot wins" depending on which system is your source of truth. Step 5: Run a manual sync on a single test record. Before re-enabling the full sync, pick one affected record and trigger a manual sync. In Salesforce, open the record → click the sync tool's button (e.g., "Sync to HubSpot"). Watch the sync log in real time. If it fails, the error message on a single record is far more detailed than the bulk error log and will tell you the exact cause. Step 6: Re-enable sync and set up an error notification. Once the root cause is fixed, re-enable the sync. Configure your sync tool to email you when sync errors exceed a threshold (most tools support this under Notifications or Alerts settings). Do not rely on manually checking logs — sync failures are silent until you look.