diff --git a/bin/debug/scan_artefact_check.py b/bin/debug/scan_artefact_check.py
index 0552e26d..83390f67 100755
--- a/bin/debug/scan_artefact_check.py
+++ b/bin/debug/scan_artefact_check.py
@@ -429,6 +429,51 @@ def check_line_motion_from_motor(signals, motors, meta) -> None:
         return
     ratio = spans / median
     print(f"    samples per line : ~{per_line}   median line span : {median:.6g}")
+
+    # Enough geometry to check this file against the scan that was intended.
+    # A step size that disagrees with the scan command means either the wrong
+    # axis has been identified or the motion was not what was asked for -- and
+    # both make everything below meaningless.
+    grid_pts = int(meta.get("frames_per_trigger") or 0)
+    print(f"    fast axis range  : {arr.min():.6g} .. {arr.max():.6g}")
+    if slow_name is not None:
+        sarr = signals[slow_name]
+        print(f"    slow axis range  : {sarr.min():.6g} .. {sarr.max():.6g}"
+              f"  ({found} lines -> {(sarr.max() - sarr.min()) / max(1, found - 1):.4g} per line)")
+    if grid_pts:
+        print(f"    implied step     : {median / grid_pts:.4g} per point"
+              f"  ({median:.4g} over {grid_pts} points)")
+        print("    ^ compare with the step size in the scan command. A mismatch means")
+        print("      the wrong axis, the wrong units, or motion that was not commanded.")
+
+    # Whole-line spans only catch a line that never moved. An axis that stalls
+    # part-way and then continues still spans the full range, so look for runs of
+    # samples that did not advance.
+    stalled = []
+    for i, sg in enumerate(segs):
+        if sg.size < 4:
+            continue
+        step = np.abs(np.diff(sg))
+        typical = float(np.median(step))
+        if typical <= 0:
+            continue
+        frozen = step < 0.1 * typical
+        run, longest = 0, 0
+        for f in frozen:
+            run = run + 1 if f else 0
+            longest = max(longest, run)
+        if longest >= 3:
+            stalled.append((i, longest, sg.size))
+    if stalled:
+        print(f"\n    within-line stalls: {len(stalled)} lines contain a run of >=3"
+              " consecutive samples that did not advance")
+        report_distribution(np.array([i for i, _, _ in stalled]), found, "within-line stalls")
+        for i, longest, size in stalled[:10]:
+            print(f"      line {i:>5}  {longest} frozen of {size} samples")
+        print("    A line can span its full range and still have stopped part-way;")
+        print("    this is the partial-stall case the span check cannot see.")
+    else:
+        print("    no within-line stalls either (no run of >=3 frozen samples).")
     if per_line < 5:
         print(f"    WARNING: only ~{per_line} position samples per line. The motor trace")
         print("    is far sparser than the data, so a partial stall can hide between")