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

modules/app_obd.py

@@ -406,6 +406,49 @@ def _persist_dtc_state():
         print("Failed to persist DTC state to SQLite:", error)
 
 
+def clear_dtc_codes():
+    """Clear stored DTCs on the ECU (OBD mode 04) and reset local state.
+
+    Returns a dict with a "status" key ("cleared", "not_connected", or
+    "error") so the HTTP route can map it to a status code. Never raises —
+    the serial path can fail in normal use (e.g. adapter yanked mid-request).
+    """
+    with _connection_lock:
+        candidate = connection
+    if candidate is None or not _is_car_connected(candidate):
+        return {
+            "status": "not_connected",
+            "error": "OBD is not connected to the car ECU.",
+        }
+
+    global dtc_codes, mil_on, dtc_updated_at, mil_updated_at
+    try:
+        with _query_lock:
+            candidate.query(obd.commands.CLEAR_DTC)
+    except Exception as error:
+        _mark_connection_lost(candidate, "CLEAR_DTC", error)
+        return {"status": "error", "error": str(error)}
+
+    # Reset the live snapshot so the UI reflects the cleared state immediately.
+    # If a fault is still physically present, the next sensor poll (within a
+    # few seconds) re-populates these — which is the correct behavior.
+    dtc_codes = []
+    dtc_updated_at = time.monotonic()
+    mil_on = False
+    mil_updated_at = time.monotonic()
+    app_events.socketio.emit("obd_dtc", [])
+    app_events.socketio.emit("obd_mil", {"on": False, "dtc_count": 0})
+
+    # Also clear the persisted snapshot so it doesn't show stale active codes.
+    try:
+        from modules import app_sql
+        app_sql.clear_dtc_state()
+    except Exception as error:
+        print("Failed to clear persisted DTC state:", error)
+
+    return {"status": "cleared"}
+
+
 def obd_sensors():
     with obd_sensor_lock:
         while not stop_sensors.is_set() and _is_car_connected():