diff --git a/components/radiator_auto.rules b/components/radiator_auto.rules
index a327d9f..298ece3 100644
--- a/components/radiator_auto.rules
+++ b/components/radiator_auto.rules
@@ -42,25 +42,95 @@ AutoRad_ForceVentSignal
 	AllowUncategorized = false
 }
 
+// "value is not zero", written the way vanilla writes it (the thermal pump's
+// UtilizationToggle). The signals are only ever 0 or 1, so this is equivalent
+// to a 0.5 threshold while sticking to a form the game demonstrably honours.
 AutoRad_IsHolding
 {
 	Type = ThresholdToggle
 	ValueFrom = AutoRad_HoldSignal
-	MinAmount = 0.5
+	MinAmount = 0
+	MaxAmount = 0
+	Invert = true
 }
 
 AutoRad_ThermalAmmoInUse
 {
 	Type = ThresholdToggle
 	ValueFrom = AutoRad_ThermalAmmoSignal
-	MinAmount = 0.5
+	MinAmount = 0
+	MaxAmount = 0
+	Invert = true
 }
 
 AutoRad_ForceVent
 {
 	Type = ThresholdToggle
 	ValueFrom = AutoRad_ForceVentSignal
-	MinAmount = 0.5
+	MinAmount = 0
+	MaxAmount = 0
+	Invert = true
+}
+
+// --- Self-test probes -------------------------------------------------------
+//
+// Three versions of this mod have now loaded cleanly and done nothing, each for
+// a different reason, so these bisect the remaining possibilities directly.
+// Each drives an indicator, and an indicator is drawn while its toggle is OFF,
+// so in every case a visible icon means that probe FAILED.
+
+// A: does ThresholdToggle behave as expected at all? No network involved.
+AutoRad_ProbeOne { Type = StaticValue; Value = 1 }
+AutoRad_ProbeThresholdOK
+{
+	Type = ThresholdToggle
+	ValueFrom = AutoRad_ProbeOne
+	MinAmount = 0
+	MaxAmount = 0
+	Invert = true
+}
+
+// B: can a mod-defined category carry a value at all? The radiator broadcasts
+// and receives this one itself, so no second part is involved.
+AutoRad_ProbeBroadcast
+{
+	Type = NetworkValue
+	ValueSource = AutoRad_ProbeOne
+	Categories = [ auto_radiator_selftest ]
+	IncludeInBlueprints = false
+}
+AutoRad_ProbeReceive
+{
+	Type = NetworkValueReceiver
+	Categories = [ auto_radiator_selftest ]
+	Mode = Max
+	AllowUncategorized = false
+}
+AutoRad_ProbeSelfLoopOK
+{
+	Type = ThresholdToggle
+	ValueFrom = AutoRad_ProbeReceive
+	MinAmount = 0
+	MaxAmount = 0
+	Invert = true
+}
+
+// C: does receiving work for a category vanilla already broadcasts? The
+// radiator's own NetworkedThermalDissipationLoadValue publishes this one.
+AutoRad_ProbeVanillaCat
+{
+	Type = NetworkValueReceiver
+	Categories = [ thermal_load_dissipation ]
+	Mode = Max
+	AllowUncategorized = false
+}
+AutoRad_ProbeVanillaCatOK
+{
+	Type = ThresholdToggle
+	ValueFrom = AutoRad_ProbeVanillaCat
+	MinAmount = 0
+	MaxAmount = 0
+	Invert = true
 }
 
 AutoRad_VentAllowed
diff --git a/mod.rules b/mod.rules
index 1ee0916..2744a47 100644
--- a/mod.rules
+++ b/mod.rules
@@ -16,7 +16,7 @@
 
 ID = ganter.auto_radiator
 Name = "Auto Radiator"
-Version = 1.3.0
+Version = 1.4.0
 CompatibleGameVersions = ["0.30.4c", "0.30.4"]
 ModifiesGameplay = true
 Author = "Tobias Ganter"
@@ -85,6 +85,9 @@ Actions
 			[NetworkedHeatBuffer, AutoRad_HoldSignal, 0]
 			[NetworkedHeatBuffer, AutoRad_ThermalAmmoSignal, 0]
 			[NetworkedHeatBuffer, AutoRad_ForceVentSignal, 0]
+			[NetworkedHeatBuffer, AutoRad_ProbeBroadcast, 0]
+			[NetworkedHeatBuffer, AutoRad_ProbeReceive, 0]
+			[NetworkedHeatBuffer, AutoRad_ProbeVanillaCat, 0]
 		]
 	}
 
@@ -180,6 +183,35 @@ Actions
 					Size = [1, 1]
 				}
 			}
+			// Probe A: mine icon visible -> ThresholdToggle itself misbehaves.
+			{
+				Toggle = AutoRad_ProbeThresholdOK
+				AtlasSprite
+				{
+					File = &/INDICATORS/NoMines
+					Size = [1, 1]
+				}
+			}
+			// Probe B: crew icon visible -> a mod-defined category carries
+			// nothing, even broadcast and received on the same part.
+			{
+				Toggle = AutoRad_ProbeSelfLoopOK
+				AtlasSprite
+				{
+					File = &/INDICATORS/NoCrew
+					Size = [1, 1]
+				}
+			}
+			// Probe C: EMP icon visible -> receiving fails even for a category
+			// vanilla already broadcasts on this very part.
+			{
+				Toggle = AutoRad_ProbeVanillaCatOK
+				AtlasSprite
+				{
+					File = &/INDICATORS/NoEmp
+					Size = [1, 1]
+				}
+			}
 		]
 	}
 ]