diff --git a/scripts/monitor_dbus_signals.py b/scripts/monitor_dbus_signals.py
index c2d3a75..289fad8 100755
--- a/scripts/monitor_dbus_signals.py
+++ b/scripts/monitor_dbus_signals.py
@@ -685,10 +685,6 @@ def _check():
         if _PROPERTIES is None:
             return []
 
-        assert isinstance(_CALLBACK_ERRORS, list)
-        if _CALLBACK_ERRORS:
-            return _CALLBACK_ERRORS
-
         if _MO is None:
             return []
 
@@ -730,7 +726,16 @@ def _check():
 
         return diffs
 
-    result = _check()
+    assert isinstance(_CALLBACK_ERRORS, list)
+    if _CALLBACK_ERRORS:
+        print(os.linesep.join(_CALLBACK_ERRORS))
+        sys.exit(3)
+
+    try:
+        result = _check()
+    except Exception as exco:  # pylint: disable=broad-except
+        print(f"{exco}")
+        sys.exit(4)
 
     assert isinstance(result, list)
     if not result:
diff --git a/testlib/infra.py b/testlib/infra.py
index 139e304..812653e 100644
--- a/testlib/infra.py
+++ b/testlib/infra.py
@@ -464,6 +464,21 @@ def run_check(self, stop_time):
             time.sleep(sleep_time(stop_time, 16))
             self.trace.send_signal(signal.SIGINT)
             (stdoutdata, stderrdata) = self.trace.communicate()
+
+            if self.trace.returncode == 3:
+                raise RuntimeError(
+                    "Failure while processing D-Bus signals: "
+                    f'stderr: {stderrdata.decode("utf-8")}, '
+                    f'stdout: {stdoutdata.decode("utf-8")}'
+                )
+
+            if self.trace.returncode == 4:
+                raise RuntimeError(
+                    "Failure while comparing D-Bus states: "
+                    f'stderr: {stderrdata.decode("utf-8")}, '
+                    f'stdout: {stdoutdata.decode("utf-8")}'
+                )
+
             msg = stdoutdata.decode("utf-8")
             self.assertEqual(
                 self.trace.returncode,