I cant figure out what It is. So far I have replaced plugs and wires, reset the ecu (stock) and I cant figure out what it is. For engine all i have is intake and exhaust. What things should I start checking. Could the o2 sensor go bad (I am not throwing any codes) Fuel filter? Cat? What could it be? I am not leaking anything. Last year I was getting at LEAST 30 mpg around town, 35-40 on the highway, now i am lucky to get 25 on the highway.