diff --git a/OWNd/message.py b/OWNd/message.py
index 69a4fe4..75ec75e 100755
--- a/OWNd/message.py
+++ b/OWNd/message.py
@@ -430,6 +430,7 @@ def __init__(self, data: str) -> None:
 
         self._type: str | None = None
         self._state: int | None = None
+        self._unknown_state: int | None = None
         self._brightness: int | None = None
         self._brightness_preset: int | None = None
         self._transition: int | None = None
@@ -492,10 +493,20 @@ def __init__(self, data: str) -> None:
             elif self._state >= 20 and self._state <= 29:  # Light blinking
                 self._blinker = 0.5 * (self._state - 19)
                 self._human_readable_log = f"Light {self._where}{self._interface_log_text} is blinking every {self._blinker}s."
+            elif self._state == 30 or self._state == 31:  # One level up/down
+                direction = "up" if self._state == 30 else "down"
+                self._human_readable_log = f"Light {self._where}{self._interface_log_text} is dimmed {direction} one level."  # pylint: disable=line-too-long
             elif self._state == 34:  # Motion detected
                 self._type = MESSAGE_TYPE_MOTION
                 self._motion = True
                 self._human_readable_log = f"Light/motion sensor {self._where}{self._interface_log_text} detected motion"
+            else:
+                # Not in the WHO 1 WHAT table (e.g. 19, seen from an MH200
+                # actuator next to a WHO 1001 autodiagnostic mask): the on/off
+                # state is unknown, not "on".
+                self._unknown_state = self._state
+                self._state = None
+                self._human_readable_log = f"Light {self._where}{self._interface_log_text} reports unknown lighting WHAT {self._unknown_state}."  # pylint: disable=line-too-long
 
         if self._dimension is not None and self._dimension_value:
             if self._dimension == 1 or self._dimension == 4:  # Brightness value
@@ -593,11 +604,21 @@ def transition(self) -> int | None:
     @property
     def is_on(self) -> bool | None:
         """True/False when the on/off state is known, None otherwise (e.g. a
-        reply carrying only a dimension such as illuminance or a timer)."""
+        reply carrying only a dimension such as illuminance or a timer, or a
+        WHAT outside the published table; see unknown_state)."""
         if self._state is None:
             return None
         return 0 < self._state < 32
 
+    @property
+    def unknown_state(self) -> int | None:
+        """The raw WHAT when it is not in the published WHO 1 WHAT table, else None.
+
+        19 has been seen from an MH200 actuator next to a WHO 1001 DIMENSION 11
+        autodiagnostic mask; the mask itself is not interpreted here.
+        """
+        return self._unknown_state
+
     @property
     def is_sensor(self) -> bool:
         return (
diff --git a/tests/test_message_protocol.py b/tests/test_message_protocol.py
index 68545e4..0916afd 100644
--- a/tests/test_message_protocol.py
+++ b/tests/test_message_protocol.py
@@ -23,6 +23,7 @@
     OWNHeatingCommand,
     OWNHeatingEvent,
     OWNLightingCommand,
+    OWNLightingEvent,
     MESSAGE_TYPE_FAN_SPEED,
     OWNMessage,
     OWNSoundCommand,
@@ -212,6 +213,74 @@ def test_shutter_level_outside_0_100_is_an_unknown_position() -> None:
     assert closed.is_position_unknown is False
 
 
+def test_lighting_what_19_is_an_unknown_state() -> None:
+    # MH200, 2026-09-24: *#1*74## answers *1*19*74## plus a WHO 1001
+    # DIMENSION 11 fault mask. 19 is not in the WHO 1 WHAT table.
+    msg = OWNEvent.parse("*1*19*74##")
+
+    assert isinstance(msg, OWNLightingEvent)
+    assert msg.is_on is None
+    assert msg.unknown_state == 19
+    assert msg.human_readable_log == "Light 74 reports unknown lighting WHAT 19."
+
+
+@pytest.mark.parametrize("what", [19, 32, 33, 35, 39, 99])
+def test_lighting_what_outside_the_table_is_an_unknown_state(what: int) -> None:
+    msg = OWNLightingEvent(f"*1*{what}*21##")
+
+    assert msg.is_on is None
+    assert msg.unknown_state == what
+    assert msg.is_sensor is False
+    assert f"unknown lighting WHAT {what}" in msg.human_readable_log
+
+
+@pytest.mark.parametrize(
+    ("what", "is_on"),
+    [(0, False), *((w, True) for w in range(1, 19)), *((w, True) for w in range(20, 32))],
+)
+def test_lighting_what_in_the_table_keeps_its_state(what: int, is_on: bool) -> None:
+    msg = OWNLightingEvent(f"*1*{what}*21##")
+
+    assert msg.is_on is is_on
+    assert msg.unknown_state is None
+    assert "unknown" not in msg.human_readable_log
+
+
+def test_lighting_motion_what_34_is_not_an_unknown_state() -> None:
+    # 34 is not in WHO_1.pdf; OWNd decodes it as motion (ZigBee variant).
+    msg = OWNLightingEvent("*1*34*21##")
+
+    assert msg.motion is True
+    assert msg.is_on is False
+    assert msg.unknown_state is None
+
+
+@pytest.mark.parametrize("frame", ["*1*1000#1*74##", "*1*1000#19*74##"])
+def test_lighting_translation_wrapper_is_not_an_unknown_state(frame: str) -> None:
+    msg = OWNLightingEvent(frame)
+
+    assert msg.is_translation is True
+    assert msg.is_on is None
+    assert msg.unknown_state is None
+
+
+@pytest.mark.parametrize(
+    ("frame", "log"),
+    [
+        ("*1*30*21##", "Light 21 is dimmed up one level."),
+        ("*1*31*21##", "Light 21 is dimmed down one level."),
+        ("*1*30#2#3*21##", "Light 21 is dimmed up one level."),
+        ("*1*31#2#3*21##", "Light 21 is dimmed down one level."),
+    ],
+)
+def test_lighting_step_what_has_a_log(frame: str, log: str) -> None:
+    msg = OWNLightingEvent(frame)
+
+    assert msg.is_on is True
+    assert msg.unknown_state is None
+    assert msg.human_readable_log == log
+
+
 def test_sound_volume_down_uses_documented_step_what() -> None:
     # Encyclopedia who-16-sound-system: up = 1001..1015, down = 1101..1115.
     assert str(OWNSoundCommand.volume_up("21")) == "*16*1001*21##"