Hi there hope i can be of assistance.
Ok using 'real time' display this should be quickly diagnosed.
The fault is rarely the lambda sensor.
By viewing the upstream (before Cat) and downstream (after cat) we can monitor the cats performace.
When the engine is warm, and the car is on idle, the upstream lambda sensor should oscillate bnetween 0.2v and 0.9v. The down stream lambda sensor should stick at around 0.3v.....flat lined.
If the down stream sensor oscillates the same as the upstream sensor, then the cat isnt working and needs replacing.
Cats can fail due to poor spark plugs/ignition system so also think about replacing the spark plugs also.
As the vehicle is 2006 this wont be covered by warranty.
If the cat is diagnosed to be the fault, this can be done by your local workshop and will be considerably cheaper. The cat is a part fo the manifold and costs around £250 from Ford, im unsure of other makes costs, but this will be cheaper.