No specifics Peter but have you just refuelled the car? Clutching at straws but just wondering if it's like the GEMS system on the later +8's, they are very sensitive to overfilling, mine did it all the time. I am sure you know but just in case remember pre cat sensors are used for fuel trim and post cat sensors are used to confirm cat operation (simplified but that's a good starting point). From that and considering what they actually do then it would suggest that the code reading shows an over fuel on one bank as likely......that may not however actually mean over fuelling! What it most often means is an excess of HC on one bank (unburned fuel). The adaptive system will have tried to reduce the injector pulse width but it's adaptive window is limited and once you drop below a specific Ms signal then the MIL light comes on and a code is stored. Over fueling affects the HC as when the purge valve is opened for the carbon canister it draws fuel rather than fumes into the plenum chamber.
If, and fingers crossed for you, it's like a GEMS then a few drive cycles can reset the light (but not erase the code). Other possibilities if you have not just topped the fuel tank to look at are cat and or sensor heat up time (if you have heated Lambda sensors the heating elements can often fail, the system ignores the cold start signal but if the sensor is not at operating temperature it would give the correct voltage signal to the ECU.
Not a lot you can do yourself other than bang in a few drive cycles and if the light does not reset then your right in your guess it's a back to dealer jobbie......good luck!
