Poor Fuel Consumption

Question

Hi. I have Chevrolet Matiz 2007 UK.

I am experiencing very high fuel consumption (like 7KM per liter). It is supposed to be very high.

I recently went to local garage (that don't do this car works, but they have computer to check), he attached car with computer for auto diagnostic and it showed message like this:

Bank1: Fuel Tank blah blah blah...

I recently changed fuel filter and oil but of no gain.

I will go to authorized mechanic but first I want to know what can be the possible reasons for this?

More info:
Car temperature light is no turned on. (means temperature is OK).