diff --git a/minimateplus/sensor_check.py b/minimateplus/sensor_check.py
new file mode 100644
index 0000000..7432ea8
--- /dev/null
+++ b/minimateplus/sensor_check.py
@@ -0,0 +1,146 @@
+r"""Decode the Blastware sensor self-check waveforms from a series-3 event binary.
+
+Reverse-engineered 2026-09-15 against 7 BE12844 (MiniMate Plus) oracle events.
+After the main waveform record-chain and the trailing metadata / per-channel
+calibration records, the binary carries four length-prefixed records tagged
+0x3c-0x3f: the sensor self-check traces the unit records when it pulses each
+sensor before monitoring.  Blastware draws these as the little waveforms in the
+"Sensor Check" strip on the right of the Event Report.
+
+  * 0x3c / 0x3d / 0x3e = Tran / Vert / Long geophone ring-downs (a damped
+    oscillation at the geophone's resonance, ~7-8 Hz at 1024 sps).
+  * 0x3f            = MicL, a pulse train at the mic self-test frequency
+    (~20 Hz), whose zero-crossing frequency is BW's mic "Channel Test" freq.
+
+Record framing (per record, all four chained by their length prefix)::
+
+    [len:2 BE][id:1][00 00][Nchan:1][12-byte header][delta stream][40 02][6B]
+    \_________________ payload (len bytes) _______________________________/
+
+The delta stream is ``payload[20 : len-8]`` (the ``40 02`` terminator sits at
+``len-8``, followed by 6 trailing bytes).  It uses the exact same 10/20/30/00
+delta-block tags as the main waveform codec
+(:mod:`minimateplus.waveform_codec`), decoded here from an implicit anchor of 0
+— so the traces come out in the same 16-count raw units as the main waveform
+(LSB = 0.005 in/s at Normal range for the geophones).
+"""
+from __future__ import annotations
+
+from typing import Dict, List
+
+from minimateplus.waveform_codec import walk_body
+
+# Record id → channel.  Order mirrors the trailing per-channel calibration
+# records (Tran / Vert / Long / MicL), confirmed against BW's sensor-check
+# frequencies on all 7 oracle events.
+_ID_TO_CHANNEL = {0x3C: "Tran", 0x3D: "Vert", 0x3E: "Long", 0x3F: "MicL"}
+_CHAIN_IDS = (0x3C, 0x3D, 0x3E, 0x3F)
+
+_HEADER_LEN = 20    # payload bytes before the delta stream
+_TRAILER_LEN = 8    # 40 02 terminator + 6 trailing bytes after the stream
+
+
+def _s4(nib: int) -> int:
+    """Sign-extend a 4-bit nibble delta."""
+    return nib - 16 if nib >= 8 else nib
+
+
+def _i8(byte: int) -> int:
+    """Sign-extend an 8-bit int delta."""
+    return byte - 256 if byte >= 128 else byte
+
+
+def _decode_delta_stream(buf: bytes) -> List[int]:
+    """Accumulate a 10/20/30/00 delta-block stream from an anchor of 0,
+    stopping at the 0x40 terminator.
+
+    Mirrors the block semantics in
+    :func:`minimateplus.waveform_codec.decode_waveform_v2` (fully decoded &
+    byte-exact as of 2026-05-11); see that module for the format details.
+    """
+    out: List[int] = []
+    cur = 0
+    for blk in walk_body(buf, 0):
+        fam = blk.tag_hi & 0xF0
+        if fam == 0x10:
+            # nibble deltas, high nibble first
+            for byte in blk.data:
+                for nib in ((byte >> 4) & 0xF, byte & 0xF):
+                    cur += _s4(nib)
+                    out.append(cur)
+        elif fam == 0x20:
+            # int8 deltas
+            for byte in blk.data:
+                cur += _i8(byte)
+                out.append(cur)
+        elif fam == 0x30:
+            # 12-bit signed deltas, packed as tag_lo/4 groups of 6 bytes
+            for g in range(blk.tag_lo // 4):
+                grp = blk.data[g * 6:(g + 1) * 6]
+                if len(grp) < 6:
+                    break
+                high_word = (grp[0] << 8) | grp[1]
+                for k in range(4):
+                    nib = (high_word >> (12 - 4 * k)) & 0xF
+                    v = (nib << 8) | grp[2 + k]
+                    if v >= 0x800:
+                        v -= 0x1000
+                    cur += v
+                    out.append(cur)
+        elif fam == 0x00:
+            # RLE zero-delta run (wide form carries the high nibble in the tag)
+            run = ((blk.tag_hi & 0x0F) << 8) | blk.tag_lo
+            out.extend([cur] * run)
+        elif fam == 0x40:
+            # segment / record terminator
+            break
+    return out
+
+
+def _find_chain(body: bytes):
+    """Locate the four length-prefixed sensor-check records.
+
+    Returns a list of ``(offset, id, length)`` or ``None``.  The chain is
+    validated by walking the ids 0x3c → 0x3d → 0x3e → 0x3f via their own length
+    prefixes, so a stray 0x3c byte in the waveform data cannot match.
+    """
+    for p in range(len(body) - 6):
+        if body[p + 2] == 0x3C and body[p + 3] == 0 and body[p + 4] == 0:
+            q = p
+            recs = []
+            ok = True
+            for expect in _CHAIN_IDS:
+                if q + 3 > len(body) or body[q + 2] != expect:
+                    ok = False
+                    break
+                length = int.from_bytes(body[q:q + 2], "big")
+                recs.append((q, expect, length))
+                q = q + 2 + length
+            if ok and len(recs) == 4:
+                return recs
+    return None
+
+
+def decode_sensor_check(raw: bytes) -> Dict[str, List[int]]:
+    """Decode the four sensor self-check traces from a series-3 event binary.
+
+    Returns ``{"Tran": [...], "Vert": [...], "Long": [...], "MicL": [...]}`` in
+    raw decode units (same 16-count LSB as the main waveform), or ``{}`` if the
+    binary carries no sensor-check block (a histogram event, a non-series-3
+    file, or a unit/firmware that doesn't store it).
+    """
+    strt = raw.find(b"STRT")
+    if strt < 0 or len(raw) < strt + 21 + 26:
+        return {}
+    body = raw[strt + 21: len(raw) - 26]
+    chain = _find_chain(body)
+    if not chain:
+        return {}
+    out: Dict[str, List[int]] = {}
+    for off, rid, length in chain:
+        payload = body[off + 2: off + 2 + length]
+        if len(payload) < _HEADER_LEN + _TRAILER_LEN:
+            continue
+        stream = payload[_HEADER_LEN: length - _TRAILER_LEN]
+        out[_ID_TO_CHANNEL[rid]] = _decode_delta_stream(stream)
+    return out
diff --git a/tests/test_sensor_check.py b/tests/test_sensor_check.py
new file mode 100644
index 0000000..b39924a
--- /dev/null
+++ b/tests/test_sensor_check.py
@@ -0,0 +1,67 @@
+"""Blastware sensor self-check waveform decode (minimateplus.sensor_check).
+
+Reverse-engineered 2026-09-15 against 7 BE12844 (MiniMate Plus) oracle events.
+After the main waveform record-chain and the trailing metadata / per-channel
+calibration records, a series-3 binary carries four length-prefixed records
+tagged 0x3c-0x3f: the sensor self-check traces the unit records when it pulses
+each sensor before monitoring (Blastware draws these as the little waveforms in
+the "Sensor Check" strip on the right of the Event Report).
+
+  * 0x3c / 0x3d / 0x3e = Tran / Vert / Long geophone ring-downs.
+  * 0x3f            = MicL, a pulse train at the mic self-test frequency.
+
+The self-check injects a fixed pulse, so the response is near-identical across
+events — asserted here as an invariant shape (damped one-sided ring-down for
+the geophones, a multi-pulse train for the mic).
+"""
+from pathlib import Path
+
+import numpy as np
+
+from minimateplus.sensor_check import decode_sensor_check
+
+FIXDIR = Path(__file__).parent / "fixtures" / "fft-oracle-2026-09-14"
+EVENTS = sorted(p.name for p in FIXDIR.iterdir())   # 7 BE12844 event binaries
+
+
+def _decode(name):
+    return decode_sensor_check((FIXDIR / name).read_bytes())
+
+
+def test_all_four_channels_present():
+    for name in EVENTS:
+        sc = _decode(name)
+        assert set(sc) == {"Tran", "Vert", "Long", "MicL"}, name
+
+
+def test_geo_channels_are_damped_ringdowns():
+    # Each geophone self-check is a large one-sided deflection (~-990 raw) that
+    # rings back and damps toward a settled value well above the trough.
+    for name in EVENTS:
+        sc = _decode(name)
+        for ch in ("Tran", "Vert", "Long"):
+            tr = np.asarray(sc[ch], dtype=float)
+            assert 240 <= len(tr) <= 260, f"{name}:{ch} n={len(tr)}"
+            assert abs(tr[:3].mean()) < 50, f"{name}:{ch} starts off-baseline"
+            assert tr.min() < -800, f"{name}:{ch} min {tr.min()}"
+            assert tr.max() < 60, f"{name}:{ch} unexpected positive swing {tr.max()}"
+            # damped: settles between the trough and zero, well above the trough
+            assert tr.min() < tr[-1] < 0, f"{name}:{ch} end {tr[-1]} not between trough and 0"
+            assert abs(tr[-1]) < 0.6 * abs(tr.min()), f"{name}:{ch} not damped, end {tr[-1]}"
+
+
+def test_mic_channel_is_a_pulse_train():
+    for name in EVENTS:
+        tr = np.asarray(_decode(name)["MicL"], dtype=float)
+        assert 235 <= len(tr) <= 255, f"{name} mic n={len(tr)}"
+        # larger dynamic range than the geo ring-down, and swings both ways
+        assert tr.min() < -1500, f"{name} mic min {tr.min()}"
+        assert tr.max() > 100, f"{name} mic max {tr.max()}"
+        # multiple pulses: several deep local minima
+        deep = (tr[1:-1] < tr[:-2]) & (tr[1:-1] < tr[2:]) & (tr[1:-1] < -800)
+        assert int(deep.sum()) >= 4, f"{name} mic pulses {int(deep.sum())}"
+
+
+def test_returns_empty_when_no_sensor_check_block():
+    assert decode_sensor_check(b"not a blastware file") == {}
+    assert decode_sensor_check(b"") == {}