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.
thanks for your answer, when you said this
"Ok using 'real time' display this should be quickly diagnosed"
do you mean a ford dealer can read the flow above and below the cat. i dont mind going to a ford dealer to get these readings confirmed because my local garage sold me the car and it is still under warranty, and i feel he does not want to honour this as the labour involved in replacing the 06 focus cat is quite extensive. i really want to get this matter resolved, i bought the car in february the light has been coming on since march my warranty is expiring and i cant sell the car on until this problem is sorted. thanks for listening to my rant.
OK i was unaware this had a private warranty. This shoudl be covered by them as its not a service item and is usually caused by an previous engine missfire.
The real time display can be done at Ford, or by any workshop who has the correct equipment to read the real time display. Obviously Ford are guarenteed to have the equipment.
Basically they view the voltages in a real time table. This is what the ECU (main computer) reads, and adjusts the fuelling accorindlgy.