this happened to my A3 (53 plate, 2.0 petrol), actually it was the final straw in a very long list of things that made me sell it. when the nox went i was getting intermittant loss of power and 20mpg on a good day.
my garage were really good, guy lifted the car up, showed me the sensor on the exhaust, in his opinion he reckoned i was going to be back year in year out fixing same fault as corrosion had set in.
best price i found was here:- http://www.lambdapower.co.uk/
but i sold before i fixed it, and the leaking coolant issue, and replaced the pads and discs…………….etc etc.