stewil

I was driving along the other day when i noticed what looked like white smoke coming from the back of my 2.0 HDI Xantia - my dad seemed to think it was probably just water in the exhaust burning off. The 'smoke' disapeared, but yesterday as i was driving, the 'Auto diagnosis' light came on and my car wouldn't rev over 3000 RPM. The car itself is running fine, but obviously performance is pedestrian at best. I can accelarate to about 2500rpm then up to 3000 the acceleration drops off. I spoke to a Citroen dealer yesterday to try and get in for a diagnosis but they can't see me till the 1st of Nov, but mentioned that if I'm in the AA (I am) i should be able to get them out to check the car over. Before I do this, has anybody else run into this problem/know what it might be and (worst of all) what it might cost? Thanks in advance