Had a regular service done. Car overheated while driving round an indoor carpark, engine made a knocking sound, turned off immediately. Open bonnet to find service guys had not connected electric fan, plugged it in, it kicked it immediately. Had a mechanic check it, seems to think ok, waited for hours for engine to cool. Checked coolant before turning on, engine ran good on way home, temp was a little higher than normal but not excessive. Checked temp 1.5 hours after arrive home, temp had only dropped 20 degs, check temp after 6hrs had dropped another 20 deg. Why would it be taking so long to cool down? Maybe senor gone? What damage to engine do you think i have done?