diff --git a/homeassistant/components/kaco_modbus/diagnostics.py b/homeassistant/components/kaco_modbus/diagnostics.py
new file mode 100644
index 000000000000..f8f1ebc1dd28
--- /dev/null
+++ b/homeassistant/components/kaco_modbus/diagnostics.py
@@ -0,0 +1,46 @@
+"""Diagnostics for KACO Modbus."""
+
+from typing import Any
+
+from homeassistant.components.diagnostics import async_redact_data
+from homeassistant.core import HomeAssistant
+
+from .coordinator import KacoConfigEntry
+
+TO_REDACT = {"serial_number"}
+
+
+async def async_get_config_entry_diagnostics(
+    hass: HomeAssistant, entry: KacoConfigEntry
+) -> dict[str, Any]:
+    """Return diagnostics for a config entry.
+
+    The raw register map is included because it loads straight into
+    ``modbus_connection.mock``, which reproduces a reported problem without
+    the hardware. It covers the polled components only, so the serial number
+    model 1 holds in the clear is not in it.
+    """
+    coordinator = entry.runtime_data
+    device = coordinator.device
+    info = device.info
+    assert info is not None
+
+    return async_redact_data(
+        {
+            "manufacturer": info.manufacturer,
+            "model": info.model,
+            "firmware": info.firmware,
+            "options": info.options,
+            "serial_number": info.serial_number,
+            "base_address": device.base_address,
+            "models": sorted(device.models or ()),
+            "strings": len(device.strings),
+            "updated": sorted(coordinator.data.updated),
+            "failed": {
+                component: str(error)
+                for component, error in coordinator.data.failed.items()
+            },
+            "raw": await device.async_read_raw(),
+        },
+        TO_REDACT,
+    )
diff --git a/homeassistant/components/kaco_modbus/quality_scale.yaml b/homeassistant/components/kaco_modbus/quality_scale.yaml
index 5e4ff32ee2aa..a2e69a830d66 100644
--- a/homeassistant/components/kaco_modbus/quality_scale.yaml
+++ b/homeassistant/components/kaco_modbus/quality_scale.yaml
@@ -52,7 +52,7 @@ rules:
   test-coverage: done
   # Gold
   devices: done
-  diagnostics: todo
+  diagnostics: done
   discovery-update-info:
     status: exempt
     comment: There is no discovery to carry updated network information.
diff --git a/tests/components/kaco_modbus/snapshots/test_diagnostics.ambr b/tests/components/kaco_modbus/snapshots/test_diagnostics.ambr
new file mode 100644
index 000000000000..c73fb850d3c3
--- /dev/null
+++ b/tests/components/kaco_modbus/snapshots/test_diagnostics.ambr
@@ -0,0 +1,282 @@
+# serializer version: 1
+# name: test_diagnostics
+  dict({
+    'base_address': 40000,
+    'failed': dict({
+    }),
+    'firmware': 'V5.53',
+    'manufacturer': 'KACO new energy',
+    'model': 'blueplanet 8.6 TL3 INT',
+    'models': list([
+      1,
+      103,
+      113,
+      120,
+      121,
+      122,
+      123,
+      126,
+      129,
+      130,
+      132,
+      135,
+      136,
+      160,
+      64204,
+    ]),
+    'options': '086L32',
+    'raw': dict({
+      'holding': dict({
+        '40070': 103,
+        '40071': 50,
+        '40072': 484,
+        '40073': 164,
+        '40074': 162,
+        '40075': 157,
+        '40076': 65534,
+        '40077': 65535,
+        '40078': 65535,
+        '40079': 65535,
+        '40080': 2265,
+        '40081': 2282,
+        '40082': 2278,
+        '40083': 65535,
+        '40084': 100,
+        '40085': 1,
+        '40086': 49944,
+        '40087': 65533,
+        '40088': 99,
+        '40089': 1,
+        '40090': 0,
+        '40091': 1,
+        '40092': 1000,
+        '40093': 65533,
+        '40094': 185,
+        '40095': 63009,
+        '40096': 0,
+        '40097': 230,
+        '40098': 65534,
+        '40099': 4437,
+        '40100': 65535,
+        '40101': 102,
+        '40102': 1,
+        '40103': 469,
+        '40104': 32768,
+        '40105': 32768,
+        '40106': 32768,
+        '40107': 65535,
+        '40108': 4,
+        '40109': 4,
+        '40110': 0,
+        '40111': 0,
+        '40112': 0,
+        '40113': 0,
+        '40114': 65535,
+        '40115': 65535,
+        '40116': 65535,
+        '40117': 65535,
+        '40118': 65535,
+        '40119': 65535,
+        '40120': 65535,
+        '40121': 65535,
+        '40184': 120,
+        '40185': 26,
+        '40186': 4,
+        '40187': 860,
+        '40188': 1,
+        '40189': 860,
+        '40190': 1,
+        '40191': 860,
+        '40192': 32768,
+        '40193': 32768,
+        '40194': 860,
+        '40195': 1,
+        '40196': 65535,
+        '40197': 32768,
+        '40198': 50880,
+        '40199': 32768,
+        '40200': 32768,
+        '40201': 50880,
+        '40202': 65533,
+        '40203': 65535,
+        '40204': 32768,
+        '40205': 65535,
+        '40206': 32768,
+        '40207': 65535,
+        '40208': 32768,
+        '40209': 65535,
+        '40210': 32768,
+        '40212': 121,
+        '40213': 30,
+        '40214': 860,
+        '40215': 2300,
+        '40216': 32768,
+        '40217': 65535,
+        '40218': 65535,
+        '40219': 860,
+        '40220': 32768,
+        '40221': 32768,
+        '40222': 32768,
+        '40223': 32768,
+        '40224': 65535,
+        '40225': 32768,
+        '40226': 32768,
+        '40227': 32768,
+        '40228': 32768,
+        '40229': 65535,
+        '40230': 65535,
+        '40231': 65535,
+        '40232': 65535,
+        '40233': 65535,
+        '40234': 1,
+        '40235': 65535,
+        '40236': 32768,
+        '40237': 32768,
+        '40238': 1,
+        '40239': 32768,
+        '40240': 32768,
+        '40241': 32768,
+        '40242': 32768,
+        '40243': 32768,
+        '40244': 122,
+        '40245': 44,
+        '40246': 7,
+        '40247': 65535,
+        '40248': 1,
+        '40249': 0,
+        '40250': 0,
+        '40251': 0,
+        '40252': 0,
+        '40253': 0,
+        '40254': 0,
+        '40255': 0,
+        '40256': 0,
+        '40257': 0,
+        '40258': 0,
+        '40259': 0,
+        '40260': 0,
+        '40261': 0,
+        '40262': 0,
+        '40263': 0,
+        '40264': 0,
+        '40265': 0,
+        '40266': 0,
+        '40267': 0,
+        '40268': 0,
+        '40269': 0,
+        '40270': 0,
+        '40271': 0,
+        '40272': 0,
+        '40273': 32768,
+        '40274': 32768,
+        '40275': 65535,
+        '40276': 32768,
+        '40277': 65535,
+        '40278': 65535,
+        '40279': 65535,
+        '40280': 65535,
+        '40281': 19265,
+        '40282': 17231,
+        '40283': 8302,
+        '40284': 101,
+        '40285': 12830,
+        '40286': 1609,
+        '40287': 65535,
+        '40288': 65535,
+        '40289': 32768,
+        '40290': 123,
+        '40291': 24,
+        '40292': 65535,
+        '40293': 65535,
+        '40294': 1,
+        '40295': 1000,
+        '40296': 65535,
+        '40297': 300,
+        '40298': 65535,
+        '40299': 0,
+        '40300': 1000,
+        '40301': 65535,
+        '40302': 300,
+        '40303': 65535,
+        '40304': 0,
+        '40305': 0,
+        '40306': 0,
+        '40307': 32768,
+        '40308': 65535,
+        '40309': 300,
+        '40310': 65535,
+        '40311': 1,
+        '40312': 0,
+        '40313': 65535,
+        '40314': 65533,
+        '40315': 65535,
+        '40316': 126,
+        '40317': 226,
+        '40318': 1,
+        '40319': 0,
+        '40320': 65535,
+        '40321': 65535,
+        '40322': 65535,
+        '40323': 4,
+        '40324': 10,
+        '40858': 160,
+        '40859': 48,
+        '40860': 65534,
+        '40861': 65535,
+        '40862': 1,
+        '40863': 32768,
+        '40864': 0,
+        '40865': 0,
+        '40866': 2,
+        '40867': 65535,
+        '40868': 0,
+        '40869': 19792,
+        '40870': 20564,
+        '40871': 8240,
+        '40872': 0,
+        '40873': 0,
+        '40874': 0,
+        '40875': 0,
+        '40876': 0,
+        '40877': 102,
+        '40878': 3551,
+        '40879': 36,
+        '40880': 0,
+        '40881': 0,
+        '40882': 27275,
+        '40883': 15290,
+        '40884': 45,
+        '40885': 65535,
+        '40886': 65535,
+        '40887': 65535,
+        '40888': 1,
+        '40889': 19792,
+        '40890': 20564,
+        '40891': 8241,
+        '40892': 0,
+        '40893': 0,
+        '40894': 0,
+        '40895': 0,
+        '40896': 0,
+        '40897': 122,
+        '40898': 5330,
+        '40899': 65,
+        '40900': 0,
+        '40901': 0,
+        '40902': 27275,
+        '40903': 15290,
+        '40904': 0,
+        '40905': 65535,
+        '40906': 65535,
+        '40907': 65535,
+      }),
+    }),
+    'serial_number': '**REDACTED**',
+    'strings': 2,
+    'updated': list([
+      'inverter',
+      'mppt',
+      'status',
+    ]),
+  })
+# ---
diff --git a/tests/components/kaco_modbus/test_diagnostics.py b/tests/components/kaco_modbus/test_diagnostics.py
new file mode 100644
index 000000000000..b43a90b2b9f6
--- /dev/null
+++ b/tests/components/kaco_modbus/test_diagnostics.py
@@ -0,0 +1,41 @@
+"""Test the KACO Modbus diagnostics."""
+
+import json
+
+from syrupy.assertion import SnapshotAssertion
+
+from homeassistant.core import HomeAssistant
+
+from . import MOCK_SERIAL
+
+from tests.common import MockConfigEntry
+from tests.components.diagnostics import get_diagnostics_for_config_entry
+from tests.typing import ClientSessionGenerator
+
+
+async def test_diagnostics(
+    hass: HomeAssistant,
+    hass_client: ClientSessionGenerator,
+    init_integration: MockConfigEntry,
+    snapshot: SnapshotAssertion,
+) -> None:
+    """Test generating diagnostics for a config entry."""
+    diag = await get_diagnostics_for_config_entry(hass, hass_client, init_integration)
+
+    assert diag == snapshot
+
+
+async def test_diagnostics_does_not_leak_the_serial_number(
+    hass: HomeAssistant,
+    hass_client: ClientSessionGenerator,
+    init_integration: MockConfigEntry,
+) -> None:
+    """Test the serial is redacted, and absent from the raw registers.
+
+    Model 1 carries it in the clear, so a raw map that reached back over that
+    block would hand it to anyone who decoded the dump.
+    """
+    diag = await get_diagnostics_for_config_entry(hass, hass_client, init_integration)
+
+    assert diag["serial_number"] == "**REDACTED**"
+    assert MOCK_SERIAL not in json.dumps(diag)