diff --git a/pm3py/core/lf.py b/pm3py/core/lf.py
index 2baacb5..da0e008 100644
--- a/pm3py/core/lf.py
+++ b/pm3py/core/lf.py
@@ -25,8 +25,15 @@ class T55xxCommands:
         }
 
     async def writebl(self, block: int, data: int, page: int = 0,
-                      password: int | None = None, downlink_mode: int = 0) -> dict:
-        """Write a T55xx block."""
+                      password: int | None = None, downlink_mode: int = 0,
+                      test: bool = False) -> dict:
+        """Write a T55xx block.
+
+        ``test`` uses the test-mode write (downlink opcode 01) instead of the standard opcode 10.
+        Test mode is the datasheet's reconfiguration path (§5.10.3) — it can rewrite a tag that a
+        normal write can't (a corrupted/locked config), provided the master key still permits it
+        (allowed for key 9, denied once key 6 is set). Use it for recovery, not routine writes.
+        """
         pwd = password if password is not None else 0
         # Firmware t55xx_write_block_t: data(4)+pwd(4)+blockno(1)+flags(1).
         # flags: 0x01 PwdMode, 0x02 Page, 0x04 test, 0x18 downlink_mode<<3.
@@ -35,6 +42,8 @@ class T55xxCommands:
             flags |= 0x01
         if page:
             flags |= 0x02
+        if test:
+            flags |= 0x04
         flags |= (downlink_mode & 0x03) << 3
         payload = struct.pack("<IIBB", data, pwd, block, flags)
         resp = await self._t.send_ng(Cmd.LF_T55XX_WRITEBL, payload, timeout=5.0)
diff --git a/tests/test_lf.py b/tests/test_lf.py
index d541ef3..aa15497 100644
--- a/tests/test_lf.py
+++ b/tests/test_lf.py
@@ -57,6 +57,17 @@ def test_lf_t55_writebl():
     # PwdMode(0x01) | Page(0x02) | downlink 2<<3 (0x10) = 0x13
     assert flags == 0x13
 
+def test_lf_t55_writebl_testmode():
+    t = AsyncMock()
+    lf = LFCommands(t)
+    t.send_ng.return_value = make_response(Cmd.LF_T55XX_WRITEBL, 0, b"")
+    asyncio.get_event_loop().run_until_complete(
+        lf.t55.writebl(block=0, data=0x00148040, test=True))
+    _, _, blockno, flags = struct.unpack("<IIBB", t.send_ng.call_args.args[1])
+    assert blockno == 0
+    assert flags & 0x04                     # test-mode bit set (downlink opcode 01)
+    assert not (flags & 0x01)               # no password mode by default
+
 def test_lf_t55_wakeup():
     t = AsyncMock()
     lf = LFCommands(t)