I had a check engine light come on due to a o2 sensors which were giving a slow response but this was actually caused by a leaking flexi pipe which had been there for some time.
This case can lead to incorrect air fuel ratio which increases fuel consumption.
The leak has been there for a while but the light only came on much later. Would there have been poor fuel consumption gradually from earlier or does the computer only change the air fuel ratio when the light comes on. Car never felt sluggish to me until I saw the light.
Also what if the o2 sensor had been the problem(not the leak), would fuel consumption get worse gradually or does the computer only know when the light comes on?