Problem with 53 plate Focus 1.6 PETROL?

Hi,

The cat went on my car, I think due to a misfire caused by a faulty spark plug. The cat broke up and would rattly.

This caused an engine mgt light to come on in the car. I got a replacement exhust which included the cat fitted 4 months ago.

Now periodically the engine light is coming on, the error being Cat bank 1 below effeciency threshold.

What could the problem be? Obviously it could be the cat but the AA man seems to think its probably not and has mentioned the EGR valve.

Does anyone have any opinion on if the EGR valve would causer this issue?

Thanks

Paul.