Engine fault - Exhaust feedback sensor

Hi

Driving along  and I got an "Engine Faul!t Workshop" message, flashing light and the car went into limp mode.

A fault code P046C00 came up on a mate's  diagnostic machine: Exhaust feedback sensor 1; implausible signal, permanent, Priority 2, fault frequency counter 1, fault delay counter/driving cycle 255, distance indicator 98236

I have no idea what any of that means!. Grateful for advice.