V4 tacoma with poor milage

I have a V4 2004 with less than 50k miles. For the first year I got 20.5 mpg then I slacked off checking. Last September I was about to replace the front brakes, ox sensor, and sparkplugs, so a week before, I checked and got 20.1. When I got it back, mpg was 19.1. Now it goes between 16.5 and 19. My mechanic swapped out the ox sensor thinking it was that. It wasnt. Some suggested that it was just the winter months, but when it was new I still got 20.5 in the winter. Tires - new same as original brand. Airfilter - fine. Heater - fine......Any ideas ?

Check the fuel filter..repalce if it has not been done.

Thanks, I didn't think of that one

re-install the other 2 cylinders! lol

What kind of plugs did you use?