Well first off, what was the code # and why didn't "your" mechanic investigate it further? If the code is cleared and returns after a few days, or few miles, that means the computer stored a hard fault and you should be able to read the code and start your diagnose from there......(the reason why the computer is telling you this so you don't have to play hide and seek in guessing what to replace)

ANYWAY...if it's reading a lean condition it's most likely one of the 2 oxygen sensors. Left or right...I would replace both...and start from there...one bad oxygen sensor will cause the poor fuel milage you describe...when the sensor is working, you get the better fuel milage......


If your mechanic wasn't able to figure that out or didn't explain it to you correctly, i would find another mechanic....fast

 

Right On!!

 

A lean condition is when your engine does not receive enough fuel for a PROPER combustion. Hence the oxygen sensor determines whether your engine is running lean or rich and makes the proper adjustments...
Lean condition will "burn" holes in your piston and or burn out your valves, also, a engine running lean will run hotter, creating more problems with in the cooling system and burning out your cats....easy fix...check oxygen sensors, fuel filter and use only fuel octane rated for your engine. A lean engine can actually seize.

The opposite of that is an engine running rich....meaning too much fuel and not enough air. Running rich will also decrease your engines performance level giving you poor fuel economy, black smoke out the tail pipe and possibly clogging your cats. How will too much fuel give you poor performance? Easy, your combustion chamber gets too much fuel and doesn't have time to burn it all in the power stroke...actually decreasing performance and wasting fuel. (An occasional backfire will let you know you have fuel sitting in the mufflers waiting too explode) Easy fix, check oxygen sensors, fuel injectors may not be misting fuel properly, air filter, and change oil once you have fixed the problem,

The sensors, hence the name "oxygen sensors" sense the amount of oxygen coming out of the exhaust via the heat generated and read in volts.
A properly running engine will run a bit lean to give to your the proper fuel economy, performance and smooth running engine. If one of those is offset then it will greatly effect the others...anyway, this is only a tip of the ice berg, there is more to this system and how the computers control your fuel system....and how everything works.....so if you're not sure and your mechanic can't fix it...like I said, find another one fast...