diff --git a/tests/domain/epc/test_main_fuel_overlay.py b/tests/domain/epc/test_main_fuel_overlay.py
index 2ffd9e29..6001ddc3 100644
--- a/tests/domain/epc/test_main_fuel_overlay.py
+++ b/tests/domain/epc/test_main_fuel_overlay.py
@@ -8,6 +8,7 @@ from __future__ import annotations
 
 import pytest
 
+from domain.epc.main_fuel_type import MainFuelType
 from domain.epc.property_overlays.main_fuel_overlay import fuel_overlay_for
 from domain.modelling.scoring.overlay_applicator import apply_simulations
 from tests.domain.sap10_calculator.worksheet._elmhurst_worksheet_000490 import (
@@ -76,3 +77,18 @@ def test_fuel_override_remaps_the_primary_systems_fuel_on_the_epc() -> None:
 
     # Assert — the calculator reads the primary fuel from main_heating_details[0].
     assert result.sap_heating.main_heating_details[0].main_fuel_type == 29
+
+
+@pytest.mark.parametrize(
+    "member", [m for m in MainFuelType if m is not MainFuelType.UNKNOWN]
+)
+def test_every_resolvable_fuel_value_decodes_to_a_code(member: MainFuelType) -> None:
+    # A classifier emits a MainFuelType value; if the overlay can't decode it the
+    # override silently no-ops. Every non-UNKNOWN member must resolve.
+
+    # Act
+    simulation = fuel_overlay_for(member.value, 0)
+
+    # Assert
+    assert simulation is not None
+