diff --git a/tests/domain/epc/test_main_heating_system_overlay.py b/tests/domain/epc/test_main_heating_system_overlay.py
index 3e2fe9317..aab952793 100644
--- a/tests/domain/epc/test_main_heating_system_overlay.py
+++ b/tests/domain/epc/test_main_heating_system_overlay.py
@@ -522,3 +522,17 @@ def test_community_boilers_drag_heat_network_category_and_a_community_gas_fuel()
     heating = simulation.heating
     assert heating.main_heating_category == 6
     assert heating.main_fuel_type == 20
+
+
+def test_oil_room_heater_decodes_to_the_post_2000_code() -> None:
+    # A modern standalone oil room heater is SAP Table 4a code 623 ("Oil room
+    # heater, 2000 or later", no boiler). It must decode to 623, not the Gas CPSU
+    # the under-populated taxonomy forced (ADR-0041).
+
+    # Act
+    simulation = main_heating_overlay_for("Oil room heater, 2000 or later", 0)
+
+    # Assert — SAP Table 4a code 623.
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.sap_main_heating_code == 623