diff --git a/bmspy/ecoflow_bt.py b/bmspy/ecoflow_bt.py
index af56bf0..86511d9 100644
--- a/bmspy/ecoflow_bt.py
+++ b/bmspy/ecoflow_bt.py
@@ -413,6 +413,8 @@ class EcoFlowBTDevice:
         self._auth_fallback_task: asyncio.Task | None = None
 
         self._pkt_count     = 0
+        # Reason for the last failed connect(), for callers that log failures.
+        self.last_error: str | None = None
         self._authenticated = asyncio.Event()
         self._on_update: list[Callable[[], None]] = []
         self._on_packet: list[Callable[[EcoFlowPacket], None]] = []
@@ -466,6 +468,7 @@ class EcoFlowBTDevice:
         self._priv_key = None
         self._set_state(_State.CONNECTING)
         self._authenticated.clear()
+        self.last_error = None
 
         try:
             self._client = BleakClient(
@@ -474,6 +477,7 @@ class EcoFlowBTDevice:
             )
             await self._client.connect()
         except Exception as exc:
+            self.last_error = f"connect failed: {type(exc).__name__}: {exc}"
             self._log(1, "connect failed: %s", exc)
             await self.disconnect()
             self._set_state(_State.ERROR)
@@ -492,6 +496,7 @@ class EcoFlowBTDevice:
             else:
                 await self._auth_type7_init()
         except Exception as exc:
+            self.last_error = f"auth error: {type(exc).__name__}: {exc}"
             self._log(1, "auth error: %s", exc)
             await self.disconnect()
             self._set_state(_State.ERROR)
@@ -1057,9 +1062,16 @@ def collect_data(cfg: EcoFlowBTConfig, debug: int = 0) -> EcoFlowSnapshot | bool
     *listen_timeout* seconds in total. Then it disconnects.
 
     Returns the populated :class:`EcoFlowSnapshot`, or ``False`` on failure.
+    Each failure is logged with its reason, at every debug level.
     A snapshot from a collection that reached the limit is partial, and its
     ``ef_missing_measurements`` field names the reports that did not arrive.
     """
+    def failed(reason: str) -> bool:
+        # Always logged: without this line, a device that is out of reach
+        # only shows as missing data.
+        debugger(f"ecoflow_bt [{cfg.address}]: read failed: {reason}")
+        return False
+
     async def _run() -> EcoFlowSnapshot | bool:
         device = EcoFlowBTDevice(
             address=cfg.address,
@@ -1071,13 +1083,13 @@ def collect_data(cfg: EcoFlowBTConfig, debug: int = 0) -> EcoFlowSnapshot | bool
         )
         try:
             if not await device.connect():
-                return False
+                return failed(device.last_error or "connect failed")
             if not await device.wait_authenticated(timeout=30.0):
-                return False
+                return failed("authentication did not complete in 30 seconds")
             missing = await device.wait_for_measurements(cfg.listen_duration, cfg.listen_timeout)
             # Metadata alone is not a successful telemetry collection.
             if not device._pkt_count:
-                return False
+                return failed("no telemetry packets received")
             _record_missing(cfg.address, device.snapshot, missing)
             return device.snapshot
         finally:
@@ -1086,6 +1098,4 @@ def collect_data(cfg: EcoFlowBTConfig, debug: int = 0) -> EcoFlowSnapshot | bool
     try:
         return asyncio.run(_run())
     except Exception as exc:
-        if debug > 0:
-            debugger(f"ecoflow_bt [{cfg.address}]: collect_data error: {exc}")
-        return False
+        return failed(f"{type(exc).__name__}: {exc}")
diff --git a/tests/test_ecoflow_bt.py b/tests/test_ecoflow_bt.py
index 20641f5..d93e0f6 100644
--- a/tests/test_ecoflow_bt.py
+++ b/tests/test_ecoflow_bt.py
@@ -495,7 +495,7 @@ def test_collection_always_disconnects(monkeypatch, connect_ok, authenticated, p
         connect=AsyncMock(return_value=connect_ok),
         wait_authenticated=AsyncMock(return_value=authenticated),
         wait_for_measurements=AsyncMock(return_value=set()),
-        disconnect=AsyncMock(), _pkt_count=packets,
+        disconnect=AsyncMock(), _pkt_count=packets, last_error=None,
     )
     monkeypatch.setattr("bmspy.ecoflow_bt.EcoFlowBTDevice", Mock(return_value=fake))
     result = collect_data(initialise_device(ADDRESS, SERIAL, "test-user", listen_duration=0))
@@ -503,6 +503,53 @@ def test_collection_always_disconnects(monkeypatch, connect_ok, authenticated, p
     fake.disconnect.assert_awaited_once()
 
 
+@pytest.mark.parametrize("connect,authenticated,packets,reason", [
+    (AsyncMock(return_value=False), False, 0, "connect failed: BleakError: device not found"),
+    (AsyncMock(return_value=True), False, 0, "authentication did not complete in 30 seconds"),
+    (AsyncMock(return_value=True), True, 0, "no telemetry packets received"),
+    (AsyncMock(side_effect=RuntimeError("adapter gone")), False, 0, "RuntimeError: adapter gone"),
+], ids=["connect", "authentication", "no-packets", "exception"])
+def test_failed_collection_is_logged_without_a_debug_option(monkeypatch, capsys, connect, authenticated, packets, reason):
+    """A device that is out of reach must show in the server log, not only as missing data."""
+    fake = Mock(
+        connect=connect, wait_authenticated=AsyncMock(return_value=authenticated),
+        wait_for_measurements=AsyncMock(return_value=set()),
+        disconnect=AsyncMock(), _pkt_count=packets,
+        last_error="connect failed: BleakError: device not found",
+    )
+    monkeypatch.setattr("bmspy.ecoflow_bt.EcoFlowBTDevice", Mock(return_value=fake))
+    config = initialise_device(ADDRESS, SERIAL, "test-user", listen_duration=0)
+    assert collect_data(config, debug=0) is False
+    assert f"ecoflow_bt [{ADDRESS}]: read failed: {reason}" in capsys.readouterr().out
+
+
+def test_successful_collection_logs_no_failure(monkeypatch, capsys):
+    from bmspy.ecoflow_snapshot import EcoFlowSnapshot
+
+    monkeypatch.setattr("bmspy.ecoflow_bt._last_missing", {})
+    fake = Mock(
+        connect=AsyncMock(return_value=True), wait_authenticated=AsyncMock(return_value=True),
+        wait_for_measurements=AsyncMock(return_value=set()),
+        disconnect=AsyncMock(), _pkt_count=3, last_error=None, snapshot=EcoFlowSnapshot(),
+    )
+    monkeypatch.setattr("bmspy.ecoflow_bt.EcoFlowBTDevice", Mock(return_value=fake))
+    config = initialise_device(ADDRESS, SERIAL, "test-user", listen_duration=0)
+    assert collect_data(config) is fake.snapshot
+    assert capsys.readouterr().out == ""
+
+
+def test_connect_records_the_reason_for_a_failure(monkeypatch):
+    client = Mock(connect=AsyncMock(side_effect=TimeoutError("no answer")), disconnect=AsyncMock(), is_connected=False)
+    monkeypatch.setitem(sys.modules, "bleak", SimpleNamespace(BleakClient=Mock(return_value=client)))
+
+    async def run():
+        dev = device()
+        assert not await dev.connect()
+        return dev.last_error
+
+    assert asyncio.run(run()) == "connect failed: TimeoutError: no answer"
+
+
 def test_wait_ends_when_the_required_heartbeats_arrive(monkeypatch):
     monkeypatch.setattr("bmspy.ecoflow_bt._COMPLETION_POLL_SECONDS", 0.01)