There's two things to consider. First, there's an oxygen sensor in front of the catalytic converter in the exhaust system. The Engine Computer uses its readings to fine tune the fuel / air mixture going into the engine. If there is consistently too much fuel going in, that will be detected, a diagnostic fault code will be set related to that, and the computer will turn on the Check Engine light to tell you. Too much fuel increases emissions, and any fault code related to something that could adversely affect emissions must turn the Check Engine light on. If the light is not on while you're driving, the computer is happy with the exhaust gas content.
Even with a properly-running engine, a sticking brake will greatly increase fuel consumption. To check for that, stop on a slight incline, push in the clutch, and release the brake. The car should creep downhill on its own. On a flat surface you should be able to push the car by hand. If neither of these occurs, suspect a sticking brake caliper.
Sunday, January 26th, 2014 AT 10:12 AM