Hi there hope i can be of assistance,
Is this a petrol or diesel?
What engine size is this?
Does the car display any symptoms, ie missfire, smoking, odours?
Thanks for the reply,
Ok i think the best thing to have done now is an actual emmisions test, preferably while the car is down on power.
This will show us if there are any lamba sensor failures, and shows if the cataylitic converter.
The down stream oxygen sensor is only a monitor for the catalytic converter and dosent effect performace/fuelling. If this is showing 'weak' as in an excess of air, then have the exhaust system inspected in detail for any remote leak, and also check the air intake system, especially the trunking after the MAF sensor.
Ok ill opt out and open this upto other experts, some of which may have the experience your looking for.
Thanks for your time.