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diff --git a/tests/domain/sap10_calculator/worksheet/_elmhurst_worksheet_001431_case38.py b/tests/domain/sap10_calculator/worksheet/_elmhurst_worksheet_001431_case38.py
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+++ b/tests/domain/sap10_calculator/worksheet/_elmhurst_worksheet_001431_case38.py
@@ -0,0 +1,116 @@
+"""Mapper-driven cascade pin against the Elmhurst P960-0001-001431
+"simulated case 38" worksheet — a mains-gas dwelling with a code-117
+regular boiler (1979-1997, winter 66%), control 2102 (programmer, no room
+thermostat → −5pp interlock → (206)=61%), and a **mains-gas condensing gas
+fire secondary** (SAP code 611).
+
+This is the realistic re-generation of "simulated case 37": case 37 lodged
+the same dwelling's code-605 gas fire on BIOGAS (7.60 p/kWh), which the
+Elmhurst Summary export cannot carry (it lodges only the secondary SAP
+code, not its sub-fuel — see `_elmhurst_secondary_fuel_from_sap_code`), so
+the mains-gas modal default left a +7 SAP gap that was purely the biogas
+sub-fuel. With a mains-gas secondary the whole cascade reproduces the
+worksheet EXACTLY, confirming the boiler-efficiency / control-2102 /
+secondary handling is all correct.
+
+Like 000565 / the _rr cases / case 20 / 21, this fixture does NOT hand-
+build the EpcPropertyData: it routes the Summary PDF through
+ElmhurstSiteNotesExtractor + from_elmhurst_site_notes so the pin exercises
+the WHOLE extractor + mapper + calculator pipeline.
+
+Source: user-simulated PDFs at `sap worksheets/golden fixture debugging/
+simulated case 38/`. The Summary is mirrored into the tracked
+`backend/documents_parser/tests/fixtures/Summary_001431_case38.pdf` so the
+test runs without depending on the unstaged workspace.
+
+Worksheet pin targets (P960-0001-001431, "11a. SAP rating" block):
+- SAP value (un-rounded, before (258) integer rounding) = 60.9152
+- (272) Total CO2, kg/year                              = 5801.0770
+
+Per [[feedback-zero-error-strict]] + [[feedback-continuous-sap-tolerance]]:
+pins are abs <= 1e-3 against the worksheet PDF (the worksheet prints the
+SAP value to 4 dp).
+"""
+
+from __future__ import annotations
+
+import re
+import subprocess
+from pathlib import Path
+from typing import Final
+
+from backend.documents_parser.elmhurst_extractor import ElmhurstSiteNotesExtractor
+from datatypes.epc.domain.epc_property_data import EpcPropertyData
+from datatypes.epc.domain.mapper import EpcPropertyDataMapper
+from domain.sap10_calculator.calculator import calculate_sap_from_inputs
+from domain.sap10_calculator.rdsap.cert_to_inputs import cert_to_inputs
+
+# parents[0]=worksheet/, [1]=sap10_calculator/, [2]=domain/, [3]=tests/,
+# [4]=repo root.
+_SUMMARY_PDF: Final[Path] = (
+    Path(__file__).resolve().parents[4]
+    / "backend" / "documents_parser" / "tests" / "fixtures"
+    / "Summary_001431_case38.pdf"
+)
+
+LINE_258_SAP_VALUE_CONTINUOUS: Final[float] = 60.9152
+LINE_272_TOTAL_CO2_KG_PER_YR: Final[float] = 5801.0770
+_PIN_ABS: Final[float] = 1e-3
+
+
+def _summary_pdf_to_textract_style_pages(pdf_path: Path) -> list[str]:
+    """Convert a Summary PDF into the per-page text format the
+    ElmhurstSiteNotesExtractor expects (label/value token sequences).
+    Mirror of the helper in the other `_elmhurst_worksheet_*` fixtures.
+    """
+    info = subprocess.run(
+        ["pdfinfo", str(pdf_path)], capture_output=True, text=True, check=True,
+    ).stdout
+    m = re.search(r"Pages:\s+(\d+)", info)
+    if m is None:
+        raise RuntimeError(f"Could not parse page count from {pdf_path}")
+    page_count = int(m.group(1))
+    pages: list[str] = []
+    for i in range(1, page_count + 1):
+        layout = subprocess.run(
+            [
+                "pdftotext", "-layout", "-f", str(i), "-l", str(i),
+                str(pdf_path), "-",
+            ],
+            capture_output=True, text=True, check=True,
+        ).stdout
+        tokens: list[str] = []
+        for line in layout.splitlines():
+            if not line.strip():
+                tokens.append("")
+                continue
+            parts = [p for p in re.split(r"\s{2,}", line.strip()) if p]
+            tokens.extend(parts)
+        pages.append("\n".join(tokens))
+    return pages
+
+
+def build_epc() -> EpcPropertyData:
+    """Route the simulated case-38 Summary through extractor + mapper.
+    No hand-built EpcPropertyData — the extractor and mapper are part of
+    the test target."""
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_PDF)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+    return EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+
+
+def test_case38_mains_gas_secondary_reproduces_the_worksheet_sap_and_co2() -> None:
+    # Arrange — the full extractor -> mapper -> calculator pipeline on the
+    # simulated case-38 Summary (mains-gas boiler 117 + mains-gas
+    # condensing gas-fire secondary 611).
+    epc = build_epc()
+
+    # Act
+    result = calculate_sap_from_inputs(cert_to_inputs(epc))
+
+    # Assert — the SAP-rating block reproduces the worksheet exactly.
+    assert (
+        abs(result.sap_score_continuous - LINE_258_SAP_VALUE_CONTINUOUS)
+        <= _PIN_ABS
+    )
+    assert abs(result.co2_kg_per_yr - LINE_272_TOTAL_CO2_KG_PER_YR) <= _PIN_ABS