diff --git a/engine/MUNGA/RECEIVER.cpp b/engine/MUNGA/RECEIVER.cpp
index 009a3d6..d2d8aba 100644
--- a/engine/MUNGA/RECEIVER.cpp
+++ b/engine/MUNGA/RECEIVER.cpp
@@ -102,6 +102,54 @@ void
 	{
 		(this->*handler)(what);
 	}
+	else
+	{
+		//
+		// UNHANDLED-MESSAGE TRACE (input-path audit, systemic cause #1).
+		//
+		// A message with no handler was dropped here in complete silence -- no log,
+		// no counter -- and a DEFAULT-CONSTRUCTED `MessageHandlerSet` has
+		// entryCount 0 and no parent chain, so it swallows EVERY id.  That is how
+		// the Avionics power bank (108 streamed records: 6 cockpit buttons + keys
+		// F5-F9), the searchlight toggle, the thermal sight, the crouch button and
+		// all four generator ON/OFF buttons were dead for months while looking
+		// perfectly wired.  This trace is the standing guard for the whole class:
+		// one line per (class, message id) pair, first occurrence only, so a newly
+		// dead control announces itself on the first boot instead of waiting for a
+		// player report.
+		//
+		// Deliberately once-per-pair: some ids are broadcast every frame, and this
+		// must never become a per-frame log.
+		//
+		static const int kMaxSeen = 64;
+		static struct { const char *name; int id; } seen[kMaxSeen];
+		static int seenCount = 0;
+
+		Derivation *derivation = sharedData->derivedClasses;
+		const char *class_name = (derivation != 0 && derivation->className != 0)
+			? derivation->className : "<unnamed receiver>";
+		const int id = (int)what->messageID;
+
+		int known = 0;
+		for (int i = 0; i < seenCount; ++i)
+		{
+			if (seen[i].id == id && seen[i].name == class_name)
+			{
+				known = 1;
+				break;
+			}
+		}
+		if (!known && seenCount < kMaxSeen)
+		{
+			seen[seenCount].name = class_name;
+			seen[seenCount].id   = id;
+			++seenCount;
+			DEBUG_STREAM << "[msg] UNHANDLED: " << class_name
+				<< " has no handler for message id " << id
+				<< " -- silently dropped (is its MessageHandlerSet chained?)"
+				<< std::endl;
+		}
+	}
 }
 
 //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/game/reconstructed/sensor.cpp b/game/reconstructed/sensor.cpp
index 7910fc8..e720756 100644
--- a/game/reconstructed/sensor.cpp
+++ b/game/reconstructed/sensor.cpp
@@ -100,6 +100,33 @@ Derivation
 		"Sensor"
 	);
 
+//
+// MESSAGE HANDLERS (Gitea #53 family / input-path audit systemic cause #1).
+//
+// This was a DEFAULT-CONSTRUCTED `MessageHandlerSet`, which is a total blackhole:
+// it has `entryCount = 0` and **no parent chain**, so `Find()` returns
+// `NullHandler` for every id (`engine/MUNGA/RECEIVER.h`), and `Receiver::Receive`
+// drops the message with no log, no warning and no counter.  The Sensor is the
+// cockpit's **"Avionics"** subsystem, and the streamed control mappings aim
+// **108 records across the 18 mechs** at it -- the MFD2 ENG1 page's power bank
+// (6 clickable buttons, keys F5-F9): route Avionics power to generator A-D, gen
+// mode, coolant cut.  Every one of them was silently swallowed.
+//
+// Sensor needs NO handlers of its own -- those ids (4-8) already exist on
+// `PoweredSubsystem`.  It only ever needed to CHAIN, which is exactly why the
+// byte-identical bank on ENG2 works: Myomers chains (myomers.cpp:88-100) and
+// Sensor did not.  Same idiom, function-local static so construction order is
+// safe.
+//
+Receiver::MessageHandlerSet&
+	Sensor::GetMessageHandlers()
+{
+	static Receiver::MessageHandlerSet messageHandlers(
+		0, 0,											// no ids of its own
+		PoweredSubsystem::GetMessageHandlers());		// copy-inherit ids 4-8
+	return messageHandlers;
+}
+
 Receiver::MessageHandlerSet
 	Sensor::MessageHandlers;
 
@@ -130,7 +157,7 @@ Sensor::AttributeIndexSet
 Sensor::SharedData
 	Sensor::DefaultData(
 		&Sensor::ClassDerivations,
-		Sensor::MessageHandlers,
+		Sensor::GetMessageHandlers(),		// was the EMPTY MessageHandlers (blackhole)
 		Sensor::AttributeIndex,
 		Sensor::StateCount
 	);
diff --git a/game/reconstructed/sensor.hpp b/game/reconstructed/sensor.hpp
index 8d69969..36c3a99 100644
--- a/game/reconstructed/sensor.hpp
+++ b/game/reconstructed/sensor.hpp
@@ -90,6 +90,10 @@
 		static Derivation ClassDerivations;
 		static SharedData DefaultData;
 		static Receiver::MessageHandlerSet MessageHandlers;
+		// The set the DefaultData actually hands out.  Chains
+		// PoweredSubsystem's ids 4-8 -- the bare `MessageHandlers` above is
+		// default-constructed and swallows every message (see sensor.cpp).
+		static Receiver::MessageHandlerSet& GetMessageHandlers();
 
 	//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 	// BT segment / base-state compatibility shims.
diff --git a/scratchpad/avionics.py b/scratchpad/avionics.py
new file mode 100644
index 0000000..5262329
--- /dev/null
+++ b/scratchpad/avionics.py
@@ -0,0 +1,92 @@
+"""Verify the Sensor/Avionics handler-set fix (input-path audit systemic cause #1).
+
+The MFD2 ENG1 page's power bank -- 6 clickable buttons, keys F5-F9 -- sends
+message ids 3-8 to subsystem 14 ("Avionics", class Sensor).  Sensor's
+MessageHandlerSet was DEFAULT-CONSTRUCTED (entryCount 0, no parent chain), so
+Find() returned NullHandler for every id and Receiver::Receive dropped them in
+silence: 108 streamed records across the 18 mechs, all dead.  Sensor now chains
+PoweredSubsystem::GetMessageHandlers() (ids 4-8) which chains HeatSink (id 3).
+
+PASS  = a [gensel] line naming Avionics after an F-key press.
+Also prints any [msg] UNHANDLED pairs the new trace catches.
+
+Kills only the PID it spawns.
+"""
+import ctypes
+import os
+import re
+import subprocess
+import sys
+import time
+
+KEYUP = 0x0002
+VK = {"K": 0x4B, "F5": 0x74, "F6": 0x75, "F7": 0x76, "F8": 0x77, "F9": 0x78,
+      "C": 0x43, "H": 0x48, "N": 0x4E}
+user32 = ctypes.windll.user32
+
+REPO = r"C:\git\bt411"
+LOG = os.path.join(REPO, "scratchpad_avionics.log")
+if os.path.exists(LOG):
+    os.remove(LOG)
+
+env = dict(os.environ)
+env.update({
+    "BT_START_INSIDE": "1",
+    "BT_KEY_NOFOCUS": "1",
+    "BT_DEV_GAUGES": "1",
+    "BT_FIRE_LOG": "1",     # [gensel] lines
+    "BT_HEAT_LOG": "1",
+    "BT_MODE_LOG": "1",     # [mode] preset page changes
+    "BT_LOG": LOG,
+})
+proc = subprocess.Popen(
+    [os.path.join(REPO, "build", "Release", "btl4.exe"), "-egg", "LAST.EGG"],
+    cwd=os.path.join(REPO, "content"), env=env,
+    stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
+print("pid", proc.pid)
+
+def tap(name, hold=0.10):
+    vk = VK[name]
+    user32.keybd_event(vk, 0, 0, 0)
+    time.sleep(hold)
+    user32.keybd_event(vk, 0, KEYUP, 0)
+    time.sleep(0.25)
+
+try:
+    deadline = time.time() + 200
+    while time.time() < deadline:
+        if os.path.exists(LOG) and "first frame" in open(LOG, errors="replace").read():
+            break
+        time.sleep(2)
+    else:
+        print("FAIL: never reached the mission"); sys.exit(2)
+    time.sleep(6)
+
+    # page MFD2 through its preset pages, pressing the bank on each
+    for page in range(5):
+        tap("K")
+        time.sleep(1.0)
+        for k in ("F5", "F6", "F7", "F8", "F9"):
+            tap(k)
+        time.sleep(1.5)
+        print("  page %d: bank pressed" % page)
+    # and the always-active coolant controls for comparison
+    tap("C"); tap("H")
+    time.sleep(3)
+finally:
+    proc.terminate()
+    time.sleep(1)
+
+t = open(LOG, errors="replace").read() if os.path.exists(LOG) else ""
+gensel = re.findall(r"^\[gensel\].*$", t, re.M)
+unh    = re.findall(r"^\[msg\] UNHANDLED.*$", t, re.M)
+print("\n=============== RESULT ===============")
+print("[gensel] lines (Sensor fix working):", len(gensel))
+for l in gensel[:10]:
+    print("   ", l[:130])
+print("\n[msg] UNHANDLED pairs caught by the new trace:", len(unh))
+for l in unh[:14]:
+    print("   ", l[:130])
+print("\nVERDICT:",
+      "PASS -- Avionics messages now reach a handler" if any("Avionics" in l or "Sensor" in l for l in gensel)
+      else ("handlers ran but not on Avionics" if gensel else "no [gensel] -- still not reaching a handler"))