2006 Citroen C3 maintenance light comes on suggesting engine is overheating

Can anyone help my Grandad? His C3 keeps showing a maintenance symbol on the dash suggesting that the engine is overheating. He had it checked by a mechanic who changed a thermostat but that hasn't made any difference. The same garage can't establish anything as being mechanically wrong so could it be something as minor as a loose wire/ connection or could it be problems with the computer system operating in the vehicle? Please help us decide whether to stick with this car or consider buying another! Festive greetings & thanks in advance, Andy & Grandad Dennis