diff --git a/OWNd/message/heating.py b/OWNd/message/heating.py
index a9fda25..22b5539 100644
--- a/OWNd/message/heating.py
+++ b/OWNd/message/heating.py
@@ -250,6 +250,22 @@ def __init__(self, data: str) -> None:
                 self._human_readable_log = (
                     f"Zone {self._zone}'s remote control is enabled"
                 )
+            elif self._mode in (22, 23, 24, 30, 31):
+                # Central unit system status (Legrand WHO 4 p. 5, p. 64):
+                # 22 at least one probe OFF, 23 at least one probe in
+                # protection, 24 at least one probe in manual, 30 failure
+                # discovered, 31 central unit battery KO.
+                self._mode_name = None
+                _status_text = {
+                    22: "at least one probe is OFF",
+                    23: "at least one probe is in protection",
+                    24: "at least one probe is in manual mode",
+                    30: "a failure was discovered",
+                    31: "the central unit battery is KO",
+                }[self._mode]
+                self._human_readable_log = (
+                    f"Zone {self._zone}'s central unit reports {_status_text}"
+                )
             else:
                 self._mode_name = None
                 self._human_readable_log = f"Zone {self._zone}'s mode is unknown"
diff --git a/tests/test_who4_central_status_whats.py b/tests/test_who4_central_status_whats.py
new file mode 100644
index 0000000..e1b9a22
--- /dev/null
+++ b/tests/test_who4_central_status_whats.py
@@ -0,0 +1,32 @@
+"""WHO 4 WHAT 22/23/24/30/31 are central-unit status reports (Legrand WHO 4
+v2.0.0 p. 5, p. 24, p. 64; libqtdevices TS10_1_0_23 thermal_device.cpp:47-48
+MALFUNCTIONING_FOUND = 30, BATTERY_KO = 31)."""
+
+import pytest
+
+from OWNd.message import OWNEvent, OWNHeatingEvent
+
+
+@pytest.mark.parametrize(
+    ("what", "text"),
+    [
+        (22, "at least one probe is OFF"),
+        (23, "at least one probe is in protection"),
+        (24, "at least one probe is in manual mode"),
+        (30, "a failure was discovered"),
+        (31, "the central unit battery is KO"),
+    ],
+)
+def test_central_unit_status_whats_are_named(what: int, text: str):
+    msg = OWNEvent.parse(f"*4*{what}*#0##")
+    assert isinstance(msg, OWNHeatingEvent)
+    assert msg.mode is None
+    assert msg.message_type != "hvac_mode"
+    assert text in msg.human_readable_log
+    assert "unknown" not in msg.human_readable_log
+
+
+def test_other_unknown_whats_still_log_unknown():
+    msg = OWNEvent.parse("*4*999*1##")
+    assert msg.mode is None
+    assert "unknown" in msg.human_readable_log