Add clear DTC codes button to Pi DTC page (FEAT-15)

tests/test_app_obd.py

@@ -1,5 +1,7 @@
 import unittest
-from unittest.mock import patch
+from unittest.mock import MagicMock, patch
+
+import obd
 
 from modules import app_obd
 
@@ -26,5 +28,49 @@ class CarConnectedFlagTest(unittest.TestCase):
             mock_set.assert_called_once_with("car_ever_connected", "1")
 
 
+class ClearDtcCodesTest(unittest.TestCase):
+    def setUp(self):
+        self.original_connection = app_obd.connection
+        self.original_dtc_codes = app_obd.dtc_codes
+        self.original_mil_on = app_obd.mil_on
+
+    def tearDown(self):
+        app_obd.connection = self.original_connection
+        app_obd.dtc_codes = self.original_dtc_codes
+        app_obd.mil_on = self.original_mil_on
+
+    def test_not_connected_returns_not_connected(self):
+        app_obd.connection = None
+        result = app_obd.clear_dtc_codes()
+        self.assertEqual(result["status"], "not_connected")
+
+    def test_connected_clears_ecu_and_resets_state(self):
+        fake_conn = MagicMock()
+        app_obd.connection = fake_conn
+        app_obd.dtc_codes = [{"code": "P0300", "description": "Misfire"}]
+        app_obd.mil_on = True
+        with patch.object(app_obd, "_is_car_connected", return_value=True), \
+             patch("modules.app_sql.clear_dtc_state") as mock_clear, \
+             patch("modules.app_events.socketio.emit") as mock_emit:
+            result = app_obd.clear_dtc_codes()
+        self.assertEqual(result["status"], "cleared")
+        fake_conn.query.assert_called_once_with(obd.commands.CLEAR_DTC)
+        self.assertEqual(app_obd.dtc_codes, [])
+        self.assertFalse(app_obd.mil_on)
+        mock_clear.assert_called_once_with()
+        self.assertTrue(mock_emit.called)
+
+    def test_query_error_returns_error(self):
+        fake_conn = MagicMock()
+        fake_conn.query.side_effect = RuntimeError("boom")
+        app_obd.connection = fake_conn
+        with patch.object(app_obd, "_is_car_connected", return_value=True), \
+             patch.object(app_obd, "_mark_connection_lost") as mock_mark:
+            result = app_obd.clear_dtc_codes()
+        self.assertEqual(result["status"], "error")
+        self.assertIn("boom", result["error"])
+        mock_mark.assert_called_once()
+
+
 if __name__ == "__main__":
     unittest.main()

tests/test_dtc_persistence.py

@@ -62,6 +62,22 @@ class DtcPersistenceTest(unittest.TestCase):
         self.assertEqual(app_sql.count_active_dtcs(), 0)
         self.assertIs(app_sql.get_persisted_dtc_state()["mil"], False)
 
+    def test_clear_dtc_state_clears_active_and_mil(self):
+        app_sql.save_dtc_state(True, [
+            {"code": "P0300", "description": "Random misfire"},
+            {"code": "P0420", "description": "Catalyst efficiency"},
+        ])
+        app_sql.clear_dtc_state()
+        state = app_sql.get_persisted_dtc_state()
+        self.assertIs(state["mil"], False)
+        self.assertEqual(state["codes"], [])
+        self.assertEqual(app_sql.count_active_dtcs(), 0)
+        # History rows are kept but no longer set.
+        rows = self.conn.execute(
+            "SELECT code, IS_SET FROM dtc ORDER BY code"
+        ).fetchall()
+        self.assertEqual(rows, [("P0300", 0), ("P0420", 0)])
+
     def test_first_seen_preserved_while_last_seen_bumps(self):
         first = datetime(2026, 2, 1, 8, 0, 0)
         later = datetime(2026, 2, 1, 8, 5, 0)