diff --git a/py_driver/driver.py b/py_driver/driver.py
index e57c41a..36371f9 100644
--- a/py_driver/driver.py
+++ b/py_driver/driver.py
@@ -235,6 +235,10 @@ def update_state(self, tube=None, quick=False, initial=False):
         raise SensorError(f"Failed to retrieve new state, got {state}.")
 
     def _underflow_check(self):
+        """
+        Checks whether underflow occurs, i.e., the outflow pump is pumping while water is lower than
+        lowest allowed level. This check prevents the outflow pump from pumping air.
+        """
         TAG = "UNDERFLOW_CHECK"
         while True:
             if not self.ready():
@@ -274,6 +278,11 @@ def _underflow_check(self):
             else:
                 if DEBUG:
                     print(f'{TAG}: No underflow detected in watchdog.')
+
+            # Stop checking underflow.
+            if self.stopped:
+                break
+
             sleep(UNDERFLOW_CHECK_INTERVAL)
 
     def _read_lines(self):