diff --git a/MainWindow.IedBrowser.cs b/MainWindow.IedBrowser.cs
index e94cffacf..952e72d37 100644
--- a/MainWindow.IedBrowser.cs
+++ b/MainWindow.IedBrowser.cs
@@ -532,9 +532,12 @@ private void RefreshIedBrowserVisibleValues()
                 }
                 else
                 {
-                    row.Value = "Not read";
+                    row.Value = IedBrowserReadPlan.UnobservedValue(
+                        row.FunctionalConstraint);
                     row.ObservationSource = "";
-                    row.ObservationTooltip = "";
+                    row.ObservationTooltip = row.FunctionalConstraint == "CO"
+                        ? "FC CO: command parameters are not scalar MMS Read targets. Use the explicit controlled Signal Testing workflow."
+                        : "";
                 }
                 continue;
             }
diff --git a/Models/IedBrowserReadPlan.cs b/Models/IedBrowserReadPlan.cs
index 0624c2190..f783c2b34 100644
--- a/Models/IedBrowserReadPlan.cs
+++ b/Models/IedBrowserReadPlan.cs
@@ -31,6 +31,16 @@ public static IedBrowserReadSelectionKind SelectionKind(IedBrowserTreeRow? page)
     private static readonly HashSet<string> ReadableFc = new(
         ["ST", "MX", "CF", "DC", "SP", "SG", "SE", "EX"], StringComparer.OrdinalIgnoreCase);
 
+    /// <summary>
+    /// FC CO denotes command parameters, not scalar MMS Read targets.
+    /// Keeping that separate from a genuinely missing read is essential
+    /// during FAT; no control operation is implied by Browser navigation.
+    /// </summary>
+    public static string UnobservedValue(string? functionalConstraint)
+        => string.Equals(functionalConstraint, "CO", StringComparison.OrdinalIgnoreCase)
+            ? "Control only"
+            : "Not read";
+
     public static IReadOnlyList<IedBrowserReadTarget> Build(
         LiveIedModelDiscoveryDocument? model, IedBrowserTreeRow? page,
         bool readAll, string? selectedDoReference = null)
diff --git a/Models/IedBrowserStructuredDetails.cs b/Models/IedBrowserStructuredDetails.cs
index d20ceffaa..8e66e57ad 100644
--- a/Models/IedBrowserStructuredDetails.cs
+++ b/Models/IedBrowserStructuredDetails.cs
@@ -336,7 +336,8 @@ private static IReadOnlyList<IedBrowserDetailRow> BuildAt(
                     : TimeChildren(depth + 1, attr.MmsReference, attr.FunctionalConstraint);
 
             result.Add(new IedBrowserDetailRow("Data",
-                $"{name} [{attr.FunctionalConstraint}]", "Not read", depth,
+                $"{name} [{attr.FunctionalConstraint}]",
+                IedBrowserReadPlan.UnobservedValue(attr.FunctionalConstraint), depth,
                 $"IEC declaration · {attr.MmsReference} · {attr.TypeSource}")
             {
                 // A structured intermediate row may have its own observed
diff --git a/tests/ARSAS.Tests/M214IedBrowserControlOnlyReadStatusTests.cs b/tests/ARSAS.Tests/M214IedBrowserControlOnlyReadStatusTests.cs
new file mode 100644
index 000000000..4c8ed3fd3
--- /dev/null
+++ b/tests/ARSAS.Tests/M214IedBrowserControlOnlyReadStatusTests.cs
@@ -0,0 +1,48 @@
+using AR.Iec61850.Discovery;
+using ArIED61850Tester.Models;
+
+namespace ARSAS.Tests;
+
+public sealed class M214IedBrowserControlOnlyReadStatusTests
+{
+    [Fact]
+    public void CommandParameters_AreNotDisplayedAsMissingMmsReads()
+    {
+        const string objectRef = "LD/CSWI1.Pos";
+        var attributes = new[]
+        {
+            new LiveIedDataAttributeModel
+            {
+                ObjectReference = objectRef,
+                AttributePath = "Pos.Oper.ctlVal",
+                MmsReference = "LD/CSWI1$CO$Pos$Oper$ctlVal",
+                FunctionalConstraint = "CO"
+            },
+            new LiveIedDataAttributeModel
+            {
+                ObjectReference = objectRef,
+                AttributePath = "Pos.stVal",
+                MmsReference = "LD/CSWI1$ST$Pos$stVal",
+                FunctionalConstraint = "ST"
+            }
+        };
+        var roots = IedBrowserStructuredDetails.AttributeRows(attributes, "Pos");
+        var control = Assert.Single(roots, row =>
+            row.Name == "Oper [CO]");
+        Assert.Equal("Control only", control.Value);
+        Assert.True(control.HasChildren);
+        Assert.Equal("Control only", Assert.Single(control.LoadChildren!()).Value);
+        var status = Assert.Single(roots, row => row.Name == "stVal [ST]");
+        Assert.Equal("Not read", status.Value);
+    }
+
+    [Theory]
+    [InlineData("CO", "Control only")]
+    [InlineData("co", "Control only")]
+    [InlineData("ST", "Not read")]
+    [InlineData("SG", "Not read")]
+    [InlineData("CF", "Not read")]
+    [InlineData("", "Not read")]
+    public void UnobservedLabel_HasExactFcSemantics(string fc, string expected)
+        => Assert.Equal(expected, IedBrowserReadPlan.UnobservedValue(fc));
+}