Deterioration of exhaust emissions????

[JonbmwX5]

Hi everyone,

I startup the 3.0p x5 e53 this morning and the car shaked quite a lot for a couple of minutes. it was like a judder then a yellow engine light, which in the manual says Deterioration of exhaust emissions, have it checked.
The shake feels like it is coming from the engine.

I have the car booked in at hughes brothers in derby on the 27th for a belt change over before the bad weather, but my wife needs to use the car for the school run over the next few days, do you think it will be ok to drive? whats likley to be the cause? a lamba senser? (had the car read last year and this code pop up once but the chap cleared)

(im off to do a search to find out how to read the codes and how to find them)

any ideas anyone??
thanks guys