diff --git a/packages/domain/src/domain/sap/tables/pcdb/__init__.py b/packages/domain/src/domain/sap/tables/pcdb/__init__.py
index c4ac3528..a2fc3364 100644
--- a/packages/domain/src/domain/sap/tables/pcdb/__init__.py
+++ b/packages/domain/src/domain/sap/tables/pcdb/__init__.py
@@ -8,19 +8,70 @@ that the PCDB winter seasonal efficiency overrides the Table 4b
 category default — closing most of the cert-vs-rating efficiency gap
 documented in [ADR-0010 §4](../../../../../../../docs/adr/0010-sap10-calculator-spec-target-and-validation.md#4-pcdb-integration-is-promoted-from-session-c-to-a-prerequisite).
 
-This subpackage owns:
+Public surface:
 
-- `parser.py`: per-table row parsers (Tables 105, 122, 143, 362, 391,
-  313, 353, 506) that decode a CSV-shaped row into a typed record
-  dataclass with high-confidence fields named + the full raw row
-  preserved for forensics.
-- `etl.py`: walks the multi-table `pcdb10.dat` source, dispatches each
-  table's records to its parser, and writes one JSON file per table
-  under `docs/sap-spec/`.
-- `<table>.py` (planned): runtime lookup modules that import the JSON
-  and expose `gas_oil_boiler_record(pcdb_id) -> Optional[...]` style
-  functions for `cert_to_inputs` precedence cascades.
+- `gas_oil_boiler_record(pcdb_id)`: Table 105 lookup.
+- `GasOilBoilerRecord`: typed record dataclass.
+- `parser.py`: per-table row parsers (Table 105 typed; raw walker for the
+  other 7 tables).
+- `etl.py`: walks the multi-table `pcdb10.dat` source and writes one
+  newline-delimited JSON file per table under `docs/sap-spec/`.
 
 Reference: BRE PCDB pcdb10.dat (April 2026 revision); SAP 10.2
 specification (14-03-2025) Appendix D2.1.
 """
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+from typing import Final, Optional
+
+from domain.sap.tables.pcdb.parser import GasOilBoilerRecord
+
+__all__ = ["GasOilBoilerRecord", "gas_oil_boiler_record"]
+
+
+_REPO_SAP_SPEC_DIR: Final[Path] = (
+    Path(__file__).resolve().parents[7] / "docs" / "sap-spec"
+)
+_TABLE_105_JSONL: Final[Path] = (
+    _REPO_SAP_SPEC_DIR / "pcdb_table_105_gas_oil_boilers.jsonl"
+)
+
+
+def _load_table_105() -> dict[int, GasOilBoilerRecord]:
+    """Read the Table 105 NDJSON at import time and build a by-pcdb-id
+    dict. ~5MB / ~4000 rows; one-off ~50ms cost. The Python runtime
+    caches the dict so repeated lookups are O(1)."""
+    records_by_id: dict[int, GasOilBoilerRecord] = {}
+    with _TABLE_105_JSONL.open(encoding="utf-8") as f:
+        for line in f:
+            line = line.strip()
+            if not line:
+                continue
+            data = json.loads(line)
+            record = GasOilBoilerRecord(
+                pcdb_id=data["pcdb_id"],
+                brand_name=data["brand_name"],
+                model_name=data["model_name"],
+                model_qualifier=data["model_qualifier"],
+                winter_efficiency_pct=data["winter_efficiency_pct"],
+                summer_efficiency_pct=data["summer_efficiency_pct"],
+                comparative_hot_water_efficiency_pct=data["comparative_hot_water_efficiency_pct"],
+                output_kw_max=data["output_kw_max"],
+                final_year_of_manufacture=data["final_year_of_manufacture"],
+                raw=tuple(data["raw"]),
+            )
+            records_by_id[record.pcdb_id] = record
+    return records_by_id
+
+
+_TABLE_105_BY_ID: Final[dict[int, GasOilBoilerRecord]] = _load_table_105()
+
+
+def gas_oil_boiler_record(pcdb_id: int) -> Optional[GasOilBoilerRecord]:
+    """Table 105 lookup by `main_heating_index_number`. Returns None when
+    the cert's index number is not in Table 105 — caller falls back to
+    Table 4a/4b category defaults via `seasonal_efficiency(...)`."""
+    return _TABLE_105_BY_ID.get(pcdb_id)
diff --git a/packages/domain/src/domain/sap/tests/test_pcdb_lookup.py b/packages/domain/src/domain/sap/tests/test_pcdb_lookup.py
new file mode 100644
index 00000000..742debfe
--- /dev/null
+++ b/packages/domain/src/domain/sap/tests/test_pcdb_lookup.py
@@ -0,0 +1,41 @@
+"""Tests for the runtime PCDB lookup module.
+
+The lookup loads pcdb_table_105_gas_oil_boilers.jsonl at import time and
+caches it as a by-pcdb-id dict. Callers (cert_to_inputs) invoke
+`gas_oil_boiler_record(pcdb_id)` to obtain the typed record or None when
+the ID is not in the PCDB.
+
+Reference: BRE PCDB pcdb10.dat (April 2026); user-verified records.
+"""
+
+from __future__ import annotations
+
+from domain.sap.tables.pcdb import gas_oil_boiler_record
+
+
+def test_gas_oil_boiler_record_returns_verified_baxi_98() -> None:
+    """Baxi Heating Wm 20/3rs (user-verified against ncm-pcdb.org.uk):
+    winter SAP seasonal efficiency 66.0%, summer 56.0%, comparative HW
+    40.8%. Lookup by `main_heating_index_number = 98` returns the typed
+    record."""
+    # Arrange
+    # Act
+    record = gas_oil_boiler_record(98)
+
+    # Assert
+    assert record is not None
+    assert record.brand_name == "Baxi Heating"
+    assert record.model_name == "Wm"
+    assert record.winter_efficiency_pct == 66.0
+    assert record.summer_efficiency_pct == 56.0
+
+
+def test_gas_oil_boiler_record_returns_none_for_unknown_pcdb_id() -> None:
+    """`main_heating_index_number` values not in Table 105 return None so
+    `cert_to_inputs` can fall back to the Table 4a/4b category default."""
+    # Arrange
+    # Act
+    record = gas_oil_boiler_record(99999999)
+
+    # Assert
+    assert record is None