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Troubleshooting

BLE Connection Issues: Diagnosis and Resolution Guide

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Fixing pairing failures, disconnections, and bonding problems

Updated Mar 23, 2026 | 4 min read

BLE Connection Issues: Diagnosis and Resolution Guide
Phase 1: Discovery Failures
Phase 2: Connection Establishment Failures
Phase 3: Random Disconnections
MTU Negotiation Issues

BLE Connection Issues: Diagnosis and Resolution Guide

Connection problems in Bluetooth Low Energy fall into three phases: discovery (device not visible), establishment (pairing/bonding failures), and maintenance (random disconnections). Each phase has distinct root causes and diagnostic approaches.

Phase 1: Discovery Failures

Device not appearing in scan results

Diagnostic flow:
1. Is the device advertising?
   → Check device-side LED/log: advertising state active?
   → nRF: sd_ble_gap_adv_start() returns NRF_SUCCESS?
2. Is the [advertising interval](/glossary/advertising-interval/) too long?
   → Minimum recommended: 100 ms (1000 ms = rare visibility)
   → Scanner window must overlap with adv event
3. Is the [advertising](/glossary/advertising/) non-connectable?
   → ADV_NONCONN_IND is filtered by many mobile OS scanners
   → Switch to ADV_IND for connectable + discoverable
4. Is there a [whitelist](/glossary/whitelist/) filtering active?
   → Disable filter temporarily for diagnostics
5. Is [LE Privacy](/glossary/le-privacy/) generating a rotating address?
   → Central must have bonded IRK to resolve; unregistered scanner sees unknown device

Symptom	Root Cause	Fix
Invisible to phone, visible to sniffer	OS-level filter active	Check app's scan filter ATT">UUID
Intermittent visibility	advertising-interval/" class="glossary-term-link" data-term="Advertising interval" data-definition="Time between BLE advertising events." data-category="GAP & Advertising">Advertising interval > 500 ms	Reduce to 100–200 ms
Never visible after reboot	Advertising not auto-restarted	Add GAP disconnected → restart adv handler
Different address each scan	LE Privacy resolvable address	Bond device to resolve IRK

Phase 2: Connection Establishment Failures

Pairing and bonding failures

Error codes and their meanings (Bluetooth Core Spec Vol 3, Part H):

Error Code	Meaning	Typical Fix
0x01 Passkey Entry Failed	User entered wrong PIN	Retry; check for UI timing issues
0x02 OOB Not Available	OOB requested but not supported	Change pairing IO capabilities
0x03 Authentication Requirements	Peer requires LESC, we only offer legacy	Enable LESC on both sides
0x05 Pairing Not Supported	Device rejected pairing	Check security manager initialized
0x08 Unspecified Reason	Catch-all	Check SMP logs; often MTU negotiation race
0x0E Invalid Parameters	Malformed pairing request	Check IO caps and OOB flags

iOS-specific: iOS caches bonding data. If the peripheral re-bonds with a new LTK (factory reset without informing iOS), iOS silently fails the encrypted connection. Resolution: un-pair the device in iOS Bluetooth settings before re-pairing.

Android-specific: Android 12+ requires BLUETOOTH_CONNECT and BLUETOOTH_SCAN runtime permissions. Missing permissions cause silent scan failures with no error callback.

Phase 3: Random Disconnections

Diagnostic flowchart:

Disconnection observed
        │
   ┌────┴────┐
HCI code    HCI code
0x08        0x13
(Timeout)  (Remote terminate)
   │              │
   ▼              ▼
Radio          Application
issue          layer issue
   │              │
Check:         Check:
- [RSSI](/glossary/rssi/) at time   - Peripheral side
  of disconnect    disconnect reason
- 2.4 GHz         - Supervision
  interference     timeout value
- [TX power](/glossary/tx-power/)        - keepalive logic

HCI Disconnect Reason	Code	Root Cause
Connection timeout	0x08	Radio link lost; supervision timeout expired
Remote user terminated	0x13	Peripheral app called disconnect
Remote device terminated (low resources)	0x14	Peripheral out of GATT buffers
Remote device terminated (power off)	0x15	Device lost power or rebooted
Unsupported LE feature	0x1A	Feature negotiation mismatch

MTU Negotiation Issues

Default ATT MTU is 23 bytes. Applications that assume larger MTU without negotiating it cause truncated writes:

# Android — MTU negotiation
gatt.requestMtu(247)  # Call after onConnectionStateChange(CONNECTED)
# Wait for onMtuChanged callback before writing large characteristics

# iOS — MTU is auto-negotiated; read maximumWriteValueLength(for:)
let mtu = peripheral.maximumWriteValueLength(for: .withoutResponse)

If both peers support DLE (Data Length Extension), negotiate a larger LL PDU first (up to 251 bytes data), then negotiate ATT MTU to match. MTU > 23 bytes requires EATT or a connection parameter update that both peers support.

Use the GATT Browser to verify MTU negotiation and inspect the live attribute table during debugging. For throughput-specific issues, see BLE Throughput Optimization.

Frequently Asked Questions

Yes, our guides range from beginner introductions to advanced topics. Each guide indicates its difficulty level and prerequisites so you can find the right starting point.