Late last year the fuel economy of our 2005 Citroen C4 2.0-litre petrol started increased from its usual mid-9s to high 10s. A warning light then came on indicating a problem, which was diagnosed as the front oxygen sensor. For a time after this, while I waited for the replacement part to arrive from overseas, the warning light disappeared and the car seemed quite happy. But since I installed the replacement sensor the fuel economy has worsened to around 11 L/100 km. What do you make of this, and should I have replaced both sensors?
Problems don't fix themselves, you had a problem and I'm guessing you still have it after replacing the faulty sensor. I would have the car checked again in an effort to trace the cause of the problem.