The Coolant Expansion Tank is part of the car engine cooling system system. We have 3 associated problem symptoms connected to it.
There is coolant around the expansion tank, the area around the tank is wet and possibly dripping onto other parts of the engine. The drip often appears to come from the cap or the pipes entering the tank.
The engine temperature warning light shown in the picture (or one very similar depending on the car manufacturer) has appeared on the dashboard and stays on.
The engine temperature reading on the dashboard gauge is noticeably higher than usual. It doesn't seem to be caused by sitting in traffic on a hot day, it is generally just higher all the time.