diff --git a/scripts/audit/anomalies.py b/scripts/audit/anomalies.py
index 3317b9712..da334d564 100644
--- a/scripts/audit/anomalies.py
+++ b/scripts/audit/anomalies.py
@@ -175,15 +175,47 @@ def _zero_works_post_differs(a: PropertyAudit) -> Optional[str]:
 def _effective_lodged_divergence(a: PropertyAudit) -> Optional[str]:
     """The Effective baseline is far from the lodged accredited figure (≥15 SAP).
     Often legitimate (overrides / pre-SAP10 rebaseline), but worth a look — a big
-    gap can also mean a bad override or a calculator divergence."""
+    gap can also mean a bad override or a calculator divergence.
+
+    Lodged scores below 13 are handed off to ``implausible-lodged-score`` — they
+    indicate corrupt upstream EPC register data, not a modelling divergence."""
     if a.effective_sap is None or a.lodged_sap is None:
         return None
+    if a.lodged_sap < 13:
+        return None
     gap = a.effective_sap - a.lodged_sap
     if abs(gap) < 15:
         return None
     return f"effective {a.effective_sap:.0f} vs lodged {a.lodged_sap:.0f} (Δ{gap:+.0f}, reason={a.rebaseline_reason})"
 
 
+@check("implausible-lodged-score", Severity.LOW)
+def _implausible_lodged_score(a: PropertyAudit) -> Optional[str]:
+    """Lodged SAP score is below 13 with a large effective-vs-lodged gap — the
+    lodged figure is implausible upstream EPC register data, not a modelling bug.
+
+    Floor of 13 is justified against the portfolio-796 distribution: every lodged
+    score below 13 either matches a known Class C exemplar (SAP 1: pids 727752,
+    727008, 731205) or shares the same bad-data fingerprint — implausibly low
+    lodged paired with a reasonable effective in band D/E (pids 710449 SAP 5,
+    722562 SAP 7, 725377 SAP 9, 727457 SAP 10, 723364 SAP 12). SAP 13 and above
+    are left in scope as ambiguous.
+
+    The gap guard (≥15) mirrors ``effective-lodged-divergence`` so the two checks
+    partition the same population without overlap."""
+    if a.lodged_sap is None or a.effective_sap is None:
+        return None
+    if a.lodged_sap >= 13:
+        return None
+    gap = a.effective_sap - a.lodged_sap
+    if abs(gap) < 15:
+        return None
+    return (
+        f"lodged SAP {a.lodged_sap:.0f} is implausible (< 13) — upstream EPC data,"
+        f" not a modelling bug (effective {a.effective_sap:.0f}, Δ{gap:+.0f})"
+    )
+
+
 @check("impossible-sap-over-100", Severity.HIGH)
 def _impossible_sap_over_100(a: PropertyAudit) -> Optional[str]:
     """A SAP score above 100 is impossible (SAP caps at 100) — a calculator /
diff --git a/tests/scripts/test_audit_anomalies.py b/tests/scripts/test_audit_anomalies.py
new file mode 100644
index 000000000..280b7c3a9
--- /dev/null
+++ b/tests/scripts/test_audit_anomalies.py
@@ -0,0 +1,94 @@
+from typing import Optional
+
+import pytest
+
+from scripts.audit.anomalies import (
+    PropertyAudit,
+    _effective_lodged_divergence,
+    _implausible_lodged_score,
+)
+
+
+def _make_audit(
+    *,
+    lodged_sap: Optional[float],
+    effective_sap: Optional[float],
+    rebaseline_reason: str = "both",
+) -> PropertyAudit:
+    return PropertyAudit(
+        property_id=1,
+        uprn=None,
+        portfolio_id=796,
+        scenario_id=None,
+        scenario_goal_band=None,
+        lodged_sap=lodged_sap,
+        lodged_band=None,
+        effective_sap=effective_sap,
+        effective_band=None,
+        rebaseline_reason=rebaseline_reason,
+        post_sap=None,
+        post_band=None,
+        cost_of_works=None,
+        energy_bill_savings=None,
+        energy_consumption_savings=None,
+        solar_sap_points=None,
+        solar_bill_savings=None,
+        n_measures=0,
+    )
+
+
+class TestEffectiveLodgedDivergence:
+    def test_silent_when_lodged_sap_below_floor(self) -> None:
+        # Arrange — Class C exemplar: lodged=1, effective=76 (gap +75, but lodged is implausible upstream data)
+        audit = _make_audit(lodged_sap=1.0, effective_sap=76.0)
+
+        # Act
+        result = _effective_lodged_divergence(audit)
+
+        # Assert
+        assert result is None
+
+    def test_fires_when_lodged_sap_at_or_above_floor_with_large_gap(self) -> None:
+        # Arrange — lodged=20 (band G, plausible), effective=65 (band D), gap +45
+        audit = _make_audit(lodged_sap=20.0, effective_sap=65.0)
+
+        # Act
+        result = _effective_lodged_divergence(audit)
+
+        # Assert
+        assert result is not None
+        assert "65" in result
+        assert "20" in result
+
+
+class TestImplausibleLodgedScore:
+    def test_fires_when_lodged_sap_below_floor_and_gap_large(self) -> None:
+        # Arrange — Class C exemplar: lodged=1, effective=76 (gap +75)
+        audit = _make_audit(lodged_sap=1.0, effective_sap=76.0)
+
+        # Act
+        result = _implausible_lodged_score(audit)
+
+        # Assert
+        assert result is not None
+        assert "1" in result
+
+    def test_silent_when_gap_below_threshold(self) -> None:
+        # Arrange — lodged=9, effective=5 (gap -4, below ±15): both low, not a divergence anomaly
+        audit = _make_audit(lodged_sap=9.0, effective_sap=5.0)
+
+        # Act
+        result = _implausible_lodged_score(audit)
+
+        # Assert
+        assert result is None
+
+    def test_silent_when_lodged_sap_at_or_above_floor(self) -> None:
+        # Arrange — lodged=13 is on the floor boundary; effective=70 gives large gap
+        audit = _make_audit(lodged_sap=13.0, effective_sap=70.0)
+
+        # Act
+        result = _implausible_lodged_score(audit)
+
+        # Assert
+        assert result is None