2002 model ford focus 1.4litre reving very high.How can i fix it?

i have a ford focus 1.4 2002 model.When it is sitting idle.The rev,s won,t drop below 4000 rev,s.I,have had the air intake valve changed and the idle control valve changed and it is still doing the same thing.Could it be the ecu unit that is faulty or could it be something else.