1999 Mitsubishi Colt 99 Mitsubishi Colt 1.3 fuel economy

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.

Don't throw random parts at a problem. That is the most expensive and least effective way to diagnose a problem. My reason for mentioning the oxygen sensor was to explain what it does, and that if it was not working properly, the Check Engine light would be on. To say that a different way. If the Check Engine light is not on while you're driving, the oxygen sensor is working as it is supposed to and it's reporting normal values.

I'm inclined to make a dragging brake my first suspect. If no problem is found there, your mechanic can connect a scanner to view live data to see if all the other sensors are reporting normal operating conditions. In particular, if the engine is running too cold, the point may not even be reached at which the Engine Computer begins adding the oxygen sensor's readings to the fuel metering calculations.