diff --git a/custom_components/localthings/registry/capabilities/air_monitor.py b/custom_components/localthings/registry/capabilities/air_monitor.py
index 71c382b..7bc3cd4 100644
--- a/custom_components/localthings/registry/capabilities/air_monitor.py
+++ b/custom_components/localthings/registry/capabilities/air_monitor.py
@@ -7,8 +7,9 @@ only); this registry deliberately doesn't include common.POWER.
 air_purifier.AIR_QUALITY and range_hood.AIR_QUALITY already read via
 common.sensor_item_value -- reused here rather than re-decoded, including
 the same dust/fine_dust/super_fine_dust/odor/clean_level keys so this
-device shares those capabilities' catalog entries. This board additionally
-reports a CO2 reading the other two families don't.
+device shares those capabilities' catalog entries. This board reports a
+CO2 reading; air_purifier.AIR_QUALITY now models the same type when a
+purifier lists it (issue #387).
 
 A second `value` list element on the particulate-matter types (Dust's
 `['31', '2']`) is the device's own graded air-quality level for that
diff --git a/custom_components/localthings/registry/capabilities/air_purifier.py b/custom_components/localthings/registry/capabilities/air_purifier.py
index f479970..5b42fd4 100644
--- a/custom_components/localthings/registry/capabilities/air_purifier.py
+++ b/custom_components/localthings/registry/capabilities/air_purifier.py
@@ -51,6 +51,18 @@ def _has_top_level_modes(rep, resources):
     return isinstance(rep.get("x.com.samsung.da.supportedModes"), (list, tuple))
 
 
+def _has_sensor_type(type_):
+    """True when /sensors/vs/0's items[] lists an item of this type."""
+
+    def fn(rep, resources):
+        return any(
+            isinstance(i, dict) and i.get("x.com.samsung.da.type") == type_
+            for i in (rep.get("x.com.samsung.da.items") or [])
+        )
+
+    return fn
+
+
 # Columns: key, icon, device item type, state_class, device_class, unit.
 # state_class is what makes Home Assistant keep long-term statistics --
 # without one, a reading is only in the short-term recorder history and
@@ -106,17 +118,33 @@ _AIR_QUALITY_SENSORS = (
 AIR_QUALITY = Capability(
     href="/sensors/vs/0",
     poll_tier="warm",
-    entities=tuple(
+    entities=(
+        *(
+            SensorDesc(
+                key=key,
+                field="x.com.samsung.da.items",
+                icon=icon,
+                state_class=state_class,
+                device_class=device_class,
+                unit=unit,
+                value_fn=lambda items, t=sensor_type: sensor_item_value(items, t),
+            )
+            for key, icon, sensor_type, state_class, device_class, unit in _AIR_QUALITY_SENSORS
+        ),
+        # CO2 (issue #387) -- same field/shape air_monitor.SENSORS already
+        # models with device_class='carbon_dioxide'/unit='ppm'. Gated on the
+        # type being listed so boards that don't report it (every current
+        # fixture) don't grow an empty entity.
         SensorDesc(
-            key=key,
+            key="co2",
             field="x.com.samsung.da.items",
-            icon=icon,
-            state_class=state_class,
-            device_class=device_class,
-            unit=unit,
-            value_fn=lambda items, t=sensor_type: sensor_item_value(items, t),
-        )
-        for key, icon, sensor_type, state_class, device_class, unit in _AIR_QUALITY_SENSORS
+            icon="mdi:molecule-co2",
+            device_class="carbon_dioxide",
+            state_class="measurement",
+            unit="ppm",
+            exists_fn=_has_sensor_type("CO2"),
+            value_fn=lambda items: sensor_item_value(items, "CO2"),
+        ),
     ),
 )
 
diff --git a/tests/test_air_monitor_capabilities.py b/tests/test_air_monitor_capabilities.py
index cff6a59..8179f06 100644
--- a/tests/test_air_monitor_capabilities.py
+++ b/tests/test_air_monitor_capabilities.py
@@ -54,8 +54,8 @@ def test_no_unbound_hrefs():
 
 def test_air_quality_sensors_read_from_shared_items_decode():
     """Same /sensors/vs/0 {type, value} shape and common.sensor_item_value
-    decode air_purifier.AIR_QUALITY already uses -- this board adds CO2,
-    which neither air_purifier nor range_hood report."""
+    decode air_purifier.AIR_QUALITY already uses -- this board lists CO2
+    unconditionally, unlike the exists_fn-gated purifier descriptor."""
     state = _state()
     assert state["dust"] == 31
     assert state["fine_dust"] == 23
diff --git a/tests/test_air_purifier_air_quality_statistics.py b/tests/test_air_purifier_air_quality_statistics.py
index 3bec9cb..bfa1cb1 100644
--- a/tests/test_air_purifier_air_quality_statistics.py
+++ b/tests/test_air_purifier_air_quality_statistics.py
@@ -96,6 +96,25 @@ def test_air_monitor_takes_the_pm_labels_but_not_the_state_class():
         assert (desc.device_class, desc.unit) == (None, None), key
 
 
+def test_co2_is_not_in_the_shared_tuple():
+    """air_monitor already has its own CO2 SensorDesc. Putting CO2 in
+    _AIR_QUALITY_SENSORS would create a second entity with the same key."""
+    assert all(row[0] != "co2" for row in air_purifier._AIR_QUALITY_SENSORS)
+
+
+def test_co2_matches_air_monitor_mapping():
+    """ppm / carbon_dioxide is the same contract air_monitor.SENSORS already
+    ships for this field (issue #387), not a unit guess."""
+    desc = _desc("co2")
+    assert desc.device_class == "carbon_dioxide"
+    assert desc.unit == "ppm"
+    assert desc.state_class == "measurement"
+    assert desc.exists_fn is not None
+    assert desc.value_fn(
+        [{"x.com.samsung.da.type": "CO2", "x.com.samsung.da.value": ["498"]}]
+    ) == 498
+
+
 def test_every_air_quality_sensor_still_reads_a_plain_int():
     """A state_class is only honoured for a numeric state, so the value
     contract this depends on is asserted here too."""
diff --git a/tests/test_air_purifier_capabilities.py b/tests/test_air_purifier_capabilities.py
index 5669364..4ca6a54 100644
--- a/tests/test_air_purifier_capabilities.py
+++ b/tests/test_air_purifier_capabilities.py
@@ -69,6 +69,34 @@ def test_air_quality_sensor_values():
     assert state["super_fine_dust"] == 5
     assert state["odor"] == 0
     assert state["clean_level"] == 0
+    assert "co2" not in state
+
+
+def test_co2_reads_when_the_items_list_includes_the_type():
+    """Issue #387 -- same {type, value} shape air_monitor.SENSORS already
+    models. Gated so boards that don't list CO2 don't grow an empty entity."""
+    reg, resources = _purifier()
+    resources = {
+        **resources,
+        "/sensors/vs/0": {
+            **resources["/sensors/vs/0"],
+            "x.com.samsung.da.items": [
+                *resources["/sensors/vs/0"]["x.com.samsung.da.items"],
+                {"x.com.samsung.da.type": "CO2", "x.com.samsung.da.value": ["612"]},
+            ],
+        },
+    }
+    bound = discover(resources, reg.capabilities, reg.pattern_capabilities)
+    assert flatten(bound, resources)["co2"] == 612
+
+    desc = next(e for e in air_purifier.AIR_QUALITY.entities if e.key == "co2")
+    assert desc.exists_fn({"x.com.samsung.da.items": []}, {}) is False
+    assert (
+        desc.exists_fn(
+            {"x.com.samsung.da.items": [{"x.com.samsung.da.type": "CO2"}]}, {}
+        )
+        is True
+    )
 
 
 def test_filter_progress_reads_named_consumable_item():