diff --git a/domain/epc/property_overlays/main_heating_system_overlay.py b/domain/epc/property_overlays/main_heating_system_overlay.py
index b8c945393..6d25d979c 100644
--- a/domain/epc/property_overlays/main_heating_system_overlay.py
+++ b/domain/epc/property_overlays/main_heating_system_overlay.py
@@ -28,7 +28,6 @@ are left UNKNOWN until modelled. Unresolvable values produce no overlay.
 
 from __future__ import annotations
 
-import logging
 from typing import Optional
 
 from domain.modelling.simulation import EpcSimulation, HeatingOverlay
@@ -36,8 +35,6 @@ from domain.sap10_calculator.tables.table_12a import (
     OFF_PEAK_IMPLYING_HEATING_CODES,
 )
 
-logger = logging.getLogger(__name__)
-
 # Off-peak (Economy 7) meter. Electric storage / CPSU systems charge overnight at
 # the low rate and cannot run economically on a single-rate meter; "Dual" lets
 # the §12 dispatch resolve the specific tariff (storage 7-hour, CPSU 10-hour).
@@ -418,21 +415,19 @@ def main_heating_overlay_for(
     category = _category_for(code)
     control = _control_for(code)
     if category is None or control is None:
-        # A system-replacing override should stamp a complete, coherent companion
+        # A system-replacing override must stamp a complete, coherent companion
         # set; an unmapped category or control leaves the field unset, so the
-        # effective cert inherits the REPLACED system's value (a silent mis-rating
-        # — e.g. a room heater keeping a storage heater's category/control). We do
-        # not yet have confident defaults for every archetype, so log-and-continue
-        # (matching `flag_fuel_mismatch`) to make the gap visible for review
-        # rather than raise mid-run.
-        logger.error(
-            "Landlord main_heating_system %r (SAP code %d) overlaid with an "
-            "incomplete companion set (category=%s, control=%s); the effective "
-            "cert may inherit the replaced system's value — known gap, review",
-            main_heating_value,
-            code,
-            category,
-            control,
+        # effective cert inherits the REPLACED system's value — the silent
+        # mis-rating class ADR-0048 exists to stop. Every mapped archetype now
+        # carries confident defaults (ADR-0050/0052/0053), so an incomplete set
+        # can only mean a new archetype landed without choosing its companions:
+        # fail loudly at overlay time rather than ship an incoherent cert
+        # (ADR-0048's intended end-state, #1444 Part C).
+        raise ValueError(
+            f"Landlord main_heating_system {main_heating_value!r} (SAP code "
+            f"{code}) resolves an incomplete companion set (category="
+            f"{category}, control={control}); add the archetype's SAP Table 4a "
+            f"category and conservative Table 4e control before mapping it"
         )
     return EpcSimulation(
         heating=HeatingOverlay(