When was it last serviced? If not recent do a full service, new oil and filter, replace air filter, new sparkplugs, check the crankcase breather pipework, if you only normally do short stop-start trips take it out for a run on a 70mph road but keep it the lower gear to get the rev's as near to the revs red line for at least 20 mins, this will get the catalytic converter red hot and burn off any carbon deposits, You could also try "cataclean" additive, some people say it's a waste of money but I've seen positive results.
If this all fails then your looking at new cat's and new lambda sensors
nigel, December 2011
