1999 1.6 16v Scenic running prob

[redbullracer]

Can anyone help?
I fitted a low milage engine to my mates scenic about 2 months ago after the T/belt broke and recked the engine.The car has been running brilliant up untill a couple of days ago.When driving and you put your foot down it wont rev above 4000 rpm and it also does the same while at a stand still.The other thing i noticed is that the engine management light aint comming on when this happens.The light does come on when you first turn on the ignition and goes out after it does the self test.He had this prob when he first got the car and it was repaired at renault and they said that it was a faulty sensor and that it was a common fault.He cant find the invoice that he got from renault when the job was done so we cant check it to see what sensor was replaced.
It would be great if some of you guys could help us out.