picasso ecu

Having 'engine fault' light on dash board and lack of power, I recovered my picasso to my main dealer who diagnosed a motorised throttle failure, This was supposedly solved by replacing a new unit. When I started the engine to drive home 'engine fault' still appeared on dashboard, the mechanic explained he had road tested the car and everything seemed fine and the problem must be the ecu ( some type of computer) which is going to be really expensive. Do any readers know if this is a common fault,or had similar experiences. The car has done 25000 miles on 03 plate so I can't see it being down to wear and tear