problems with 2005 mazda 3 hb???

i was wondering if anybody else is experiencing this...my 2005 mazda 3 hb, 2.3 lt engine is only getting 20 mpg instead of the 23 mpg when i bought it...guy at the mazda shop says it's because i'm running my air, but i ran my air 3 years ago as well. also, i had a cilinoid(?) replaced last year(my check engine light came on and my car would not start), my dash lights have gone a little dim, stereo light is not on and steering wheel lights are dim as well, and a little bit of a burning smell. this car has only 25,600 miles on it. do i have a lemon???
yep, i bought the car new in june of '05.