Removal of precat sensor

[Darryl5]

Hello,

I have intermittent EML/VSC Off warning lights on the dashboard and Torque Pro shows the O2 bank 1 sensor 1 to exceed maximum current for periods of time. I've tried removing the precat sensor to clean/inspect it but can't seem to shift the surrounding disc to get to the sensor's retaining nut (see image). The manifold heat shield is easy to unbolt but the disc appears to be a separate attachment. Other car models with this engine don't appear to have this disc.

Any advice gratefully received.

Darryl