If there was a performance issue the CEL would likely be on. If it’s not, the the type of fuel being used, driving habits, or ambient conditions are the most likely cause. The use of AC does effect fuel economy BTW.
Know that altering the engine, like blocking off the idle passage can effect fuel economy. The less you mess with it the better. Also, I highly recommend OE parts whenever possible.
You might also find this info helpful. http://ericthecarguy.com/faq/finding-and-fixing-the-causes-of-poor-mpg