2001 Citroen C3 Fuel Economy

2001 peugot 206.milege 57787. I have a gas consumption problem with this car as it consump 1 gallon per 18 mile. i changed the mass air flow sensor, fuel injector cleaning,diagnostic computers tells no problem with O2 sensor but the check engin check lump sometimes turn on and turn off when i restart my car plz tell me what can i change to manage it.