I have a 2001 xsara Picasso 1.8 petrol and it has fluctuating co2 emmissions what I want to know is which o2 sensor to change as there is a up and down stream sensor the lambda and hydrocarbons are fine and the o2 is better at idle but not right so if anyone can suggest any solutions I would be very grateful

Dave, June 2012

Either will affect the emissions you need to get it on to a code reader to see if it is faulting either sensor

nigel, June 2012