2006 Honda Civic ECU problem?

I have a honda civic 2006 Lx and I modified it by install greddy ti-c intake few weeks later I have an aem cold air intake. Before I install the aem cold air intake, I got good gas mileage so I decided to remove the cold air intake. The problem is after I installed my oem air box filter, the gas mileage drop as when I still have the cold air intake in my car.

I just want to know what is the problem?
does it because the ECU reading the setting with CAI?

how to reset the ecu?
some people say just disconnect the fuse, but which fuse?

thanks