I have a 2002 Ford Mondeo (I live in the UK). I have noticed recently that the cooling fan keeps switching on far more frequently than it used to. In fact, until now, I had never noticed the fan coming on at all. The temperature gauge shows normal and the car runs fine. I suppose I may have a faulty thermostat but could it be anything else?

. Flush the cooling system and refill with a 50/50 mixture of manufacturer-recommended antifreeze and distilled water. Change coolant every 50,000 km or 3 years, whichever comes first. A well-maintained cooling system should be super clean inside, without any signs of phosphate scale.

. Read this link for insight:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-3.html#post235973

It's possible that the cooling fan coming on more frequently indicates changes are taking place in the cooling system. Ensure it's the cooling fan (radiator), not the condenser fan (A/C), that's coming on. Review your maintenance program. Allowing an aluminum head engine to overheat can warp the head, damage the valves, and blow the head gasket. If the problem persists, consider replacing the radiator--the heart of the heat-transfer process.

Thanks for that but I don't think the engine is overheating as the fan comes on even when I have only travelled, say, half a mile from cold. Also, the gauge shows normal.

Check/replace the Engine Coolant Temperature (ECT) Switch, not the ECT Sensor.