Q: Cooling fan won't stop running.

Hi there. The cooling system can leak from so many different spots that it can be hard to diagnose. If the problem occurred right after the thermostat and gasket were replaced then the work may not have be performed properly. If this was occurring prior to the parts being replaced then the leak may be from someplace else. If you cannot see the leak visually then a more in depth inspection must be done. A mechanic would opt for either a pressure test or a dye test to find the source of the leak. As for the fan running all of the time, this can occur if the coolant level is low or if the engine is overheating. The coolant leak should be located first and repaired to see if the fan begins working properly. Getting some help from a certified mechanic, like one from YourMechanic, who had a cooling system pressure tester can make this a lot easier.