diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Runtime/StatusCodeMap.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Runtime/StatusCodeMap.cs
index 0fe9b7b3a..52f11e472 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Runtime/StatusCodeMap.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Runtime/StatusCodeMap.cs
@@ -133,6 +133,9 @@ internal static class StatusCodeMap
         // the belief that a non-zero source meant a pre-MXAccess transport failure — but every detected_by value
         // names an MXAccess layer (0 requesting LMX, 1 responding LMX, 2/3 NMX, 4/5 automation object; the
         // gateway's own failures arrive as ProtocolStatus, never as a status row), so the category decides (#872).
+        // The numbering is mxaccessgw's MxStatusProxyConverter.MapSource; NMX (2/3) names the inter-node message
+        // layer that noticed the failure, not a transport fault — a real comms loss is category CommunicationError
+        // or detail 2/5, which FromCategory already maps to BadCommunicationError.
         return FromCategory(status, logger);
     }
 
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Runtime/StatusCodeMapTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Runtime/StatusCodeMapTests.cs
index 64f99ced1..9ed5b850c 100644
--- a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Runtime/StatusCodeMapTests.cs
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Runtime/StatusCodeMapTests.cs
@@ -140,18 +140,72 @@ public sealed class StatusCodeMapTests
         logger.Entries.Count.ShouldBe(1);
     }
 
+    // ===== #872: detected_by never decides the code — the category does =====
+    // raw_detected_by is MXAccess's MxStatusSource, decoded by mxaccessgw's MxStatusProxyConverter.MapSource:
+    // -1 Unknown, 0 RequestingLmx, 1 RespondingLmx, 2 RequestingNmx, 3 RespondingNmx,
+    // 4 RequestingAutomationObject, 5 RespondingAutomationObject, anything else Unknown. Every value names the
+    // MXAccess layer that noticed the failure, not the kind of failure — the live write-refusal capture in
+    // mxaccessgw docs/AlarmProbeFindings.md is an OperationalError detected by RespondingAutomationObject (5).
+    // A transport/network failure shows up as category CommunicationError (or detail 2/5), and the gateway's own
+    // failures arrive as ProtocolStatus, never as a status row. The old rule (detail 0 and raw_detected_by != 0
+    // => BadCommunicationError) turned every one of those object/LMX-detected failures into a comms error.
+
     /// <summary>
-    ///     #872: detected_by names an MXAccess layer (3 = responding NMX), not a transport failure, so a detail-0
-    ///     failure row routes on its category — a communication-category row is still BadCommunicationError.
+    ///     #872: a detail-0 failure row maps on its category for every detected_by source — including the
+    ///     NMX sources, which name the layer that noticed the failure, not a transport failure.
     /// </summary>
+    /// <param name="rawDetectedBy">The raw MxStatusSource value.</param>
     [Theory]
-    [InlineData(MxStatusCategory.Unspecified, 0x80000000u)]
-    [InlineData(MxStatusCategory.CommunicationError, 0x80050000u)]
-    [InlineData(MxStatusCategory.ConfigurationError, 0x80890000u)]
-    public void FromMxStatus_SuccessZero_DetailZero_RoutesByCategory_NotDetectedBy(MxStatusCategory category, uint expected)
+    [InlineData(-1)] // Unknown
+    [InlineData(0)]  // RequestingLmx
+    [InlineData(1)]  // RespondingLmx
+    [InlineData(2)]  // RequestingNmx
+    [InlineData(3)]  // RespondingNmx
+    [InlineData(4)]  // RequestingAutomationObject
+    [InlineData(5)]  // RespondingAutomationObject
+    [InlineData(77)] // outside the documented range -> Unknown
+    public void FromMxStatus_DetailZero_EverySource_RoutesByCategory(int rawDetectedBy)
     {
-        var s = new MxStatusProxy { Success = 0, Detail = 0, RawDetectedBy = 3, Category = category };
-        StatusCodeMap.FromMxStatus(s).ShouldBe(expected);
+        Map(MxStatusCategory.OperationalError).ShouldBe(StatusCodeMap.Bad);
+        Map(MxStatusCategory.SoftwareError).ShouldBe(StatusCodeMap.Bad);
+        Map(MxStatusCategory.OtherError).ShouldBe(StatusCodeMap.Bad);
+        Map(MxStatusCategory.Unspecified).ShouldBe(StatusCodeMap.Bad);
+        Map(MxStatusCategory.ConfigurationError).ShouldBe(StatusCodeMap.BadConfigurationError);
+        Map(MxStatusCategory.SecurityError).ShouldBe(StatusCodeMap.BadUserAccessDenied);
+        Map(MxStatusCategory.CommunicationError).ShouldBe(StatusCodeMap.BadCommunicationError);
+
+        uint Map(MxStatusCategory category) => StatusCodeMap.FromMxStatus(
+            new MxStatusProxy { Success = 0, Detail = 0, RawDetectedBy = rawDetectedBy, Category = category });
+    }
+
+    /// <summary>
+    ///     #872: the shape the old rule got wrong — an operational failure the responding automation object
+    ///     detected (raw 5, the live write-refusal source) is not a communication error.
+    /// </summary>
+    [Fact]
+    public void FromMxStatus_DetailZero_RespondingAutomationObject_OperationalError_IsNotCommunicationError()
+    {
+        var s = new MxStatusProxy
+        {
+            Success = 0, Detail = 0, Category = MxStatusCategory.OperationalError,
+            DetectedBy = MxStatusSource.RespondingAutomationObject, RawDetectedBy = 5,
+        };
+        StatusCodeMap.FromMxStatus(s).ShouldBe(StatusCodeMap.Bad);
+    }
+
+    /// <summary>
+    ///     #872 positive control: a communication-category row stays BadCommunicationError when detected by
+    ///     the requesting LMX (raw 0) — the source the old rule treated as "never a comms error".
+    /// </summary>
+    [Fact]
+    public void FromMxStatus_DetailZero_RequestingLmx_CommunicationError_IsCommunicationError()
+    {
+        var s = new MxStatusProxy
+        {
+            Success = 0, Detail = 0, Category = MxStatusCategory.CommunicationError,
+            DetectedBy = MxStatusSource.RequestingLmx, RawDetectedBy = 0,
+        };
+        StatusCodeMap.FromMxStatus(s).ShouldBe(StatusCodeMap.BadCommunicationError);
     }
 
     /// <summary>Verifies that all-zero status maps to Bad status code.</summary>