most people think that the lower the engine coolant temperature gauge needle is,the safer their engine is.but it is not always true and might even damage the engine.in fact ,i know a lot of ppl who got their engine damaged because of this misconception.if your temeperature gauge suddenly drops down,then be alarmed because it indicates a leak and loss of coolant.since the temperature sensor senses the engine coolant temperature, so if there is leak and loss of coolant then your sensor will not be in contact with coolant and it will just read the air temperature.so the needle will drop suddenly and u will become happy.but your happiness will go away after 5 mints once you see your head gasket smoke coming from under bonnet.next thing will be gasket replacement or even engine overhaul.so always be careful of sudden gauge needle fall while driving.immediately stop your car if your temp needle falls suddenly while driving.