diff --git a/packages/domain/src/domain/sap/rdsap/tests/test_golden_fixtures.py b/packages/domain/src/domain/sap/rdsap/tests/test_golden_fixtures.py
index 616e7d70..a61187cf 100644
--- a/packages/domain/src/domain/sap/rdsap/tests/test_golden_fixtures.py
+++ b/packages/domain/src/domain/sap/rdsap/tests/test_golden_fixtures.py
@@ -162,3 +162,52 @@ def test_golden_cert_stays_within_tolerance(expectation: _GoldenExpectation) ->
         f"±{_PE_TOLERANCE_KWH_PER_M2} (expected ≈{expectation.expected_pe_resid_kwh_per_m2:+.2f}). "
         f"Notes: {expectation.notes}"
     )
+
+
+# Cert 0390 lodges Firebird Boilers S 150-200 oil boiler at PCDB index_number
+# 9005 (Table 105 winter eff 86.4%). End-to-end mapper → cert_to_inputs chain
+# must surface that PCDB winter efficiency on `inputs.main_heating_efficiency`
+# rather than falling back to the Table 4a oil-boiler category default.
+_PCDB_CHAIN_EXPECTATIONS: tuple[tuple[str, int, float], ...] = (
+    ("0390-2954-3640-2196-4175", 9005, 0.864),   # Firebird oil PCDB-listed
+    ("7536-3827-0600-0600-0276", 17679, None),   # Vaillant gas PCDB-listed
+    ("0300-2747-7640-2526-2135", 17992, None),   # gas PCDB-listed
+    ("8135-1728-8500-0511-3296", 17702, None),   # gas PCDB-listed
+)
+
+
+@pytest.mark.parametrize(
+    "cert_number, expected_pcdb_id, expected_winter_eff",
+    _PCDB_CHAIN_EXPECTATIONS,
+    ids=lambda v: v if isinstance(v, str) else "",
+)
+def test_api_to_domain_mapper_preserves_main_heating_index_number(
+    cert_number: str, expected_pcdb_id: int, expected_winter_eff: float | None
+) -> None:
+    """The full API JSON → EpcPropertyData → CalculatorInputs chain must
+    preserve `main_heating_index_number` end-to-end so the PCDB precedence
+    cascade (Appendix D2.1) fires correctly. Pins:
+
+    1. EpcPropertyDataMapper.from_api_response surfaces the PCDB pointer
+       on `sap_heating.main_heating_details[0].main_heating_index_number`.
+    2. cert_to_inputs resolves Table 105 record by that ID and applies the
+       winter efficiency to `inputs.main_heating_efficiency`.
+
+    Schema versions ≥17_1 carry the field on their dataclass; schema 17_0
+    hardcodes None in the mapper (the field didn't exist in that schema's
+    EPC API contract). The 4 corpus golden certs are all post-17_1.
+    """
+    # Arrange
+    doc = _load_cert(cert_number)
+
+    # Act
+    epc = EpcPropertyDataMapper.from_api_response(doc)
+    inputs = cert_to_inputs(epc, prices=SAP_10_2_SPEC_PRICES)
+
+    # Assert
+    main = epc.sap_heating.main_heating_details[0]
+    assert main.main_heating_index_number == expected_pcdb_id
+    if expected_winter_eff is not None:
+        assert inputs.main_heating_efficiency == pytest.approx(
+            expected_winter_eff, abs=1e-3
+        )