Hi Dave

In a rush again but here goes...

In my experience a failed CAT ( unless it is restricting the exhaust gases ) will not cause a higher CO than 0.7% so you appear to have a fueling issue here. Maybe the CAT just can't cope, hence the rear sensor switching ?

I would like to see the lambda readings as well as the CO% when the sensors are switching and flat lining. The lambda reading tells you if the system can achieve correct AFR, so its important to look at this and the O2 sensor patterns at the same time. The sensor behaviour and/or the fuel trim figures will show you how hard the system is working to achieve lambda 1.000.

I would also be doing a manifold vacuum test and an exhaust backpressure test.

Dave F