Does anyone know what is wrong here?

In September '04 my engine light starting coming on and the engine was
losing power to the point of stalling. I immediately took it to the local
Honda dealer. To fix it, the oxygen sensor had been replaced. Sure enough,
the engine light does not come on anymore and the engine seems to be running
normally EXCEPT fuel economy has been nearly halved!

There has been no change in driving habits. I took it back to the dealer
yesterday and all diagnostics passed and the Civic has been given a clean
bill of health. Something else must have happened here that perhaps
vehicles diagnostics cannot test. Any clues?