The fault does not jeopardise safety, but could lead to flickering dashboard lights or, at worst, failure of electrical systems. The recall affects 61,130 cars built between September and March 5. The free alterations take half an hour.
A Ford spokesman said: 'The Focus is fitted with a new type of alternator designed to prolong battery life by charging at different voltages depending on temperature. If water gets into the system there could be electrical problems.'