It is always difficult to judge what is going on without further information.
The extra information would be the CO2 and O2 levels measured and the lambda number.
Based on the above figures I would say a defective catalyst.
Any fault codes present in the engine ecu?

What's the history? Any problems with sensors/spark plugs/coils?
How to test a catalyst: http://www.picoauto.com/tutorials/cat-efficiency.html