it means that your engine is not getting enough fuel, or your computer thinks that it's not getting enough fuel.
looking at your sig, one thing i can see that could possibly attribute to that is the K&N.
if you are in fact running lean it could be the cause of your poor milage. but the only way that will happen is if you have a misfire AND the PCM is trying to compensate.
when you run lean you're acting getting less fuel and more air. what that means is that the fuel that IS making it into the cylinder will be burnt off better, hotter, and cleaner than a normal mixture. BUT!!! this means the heat and pressure inside the cylinder wall is greater and will severely decrease engine life. Also, the lubrication is degraded and can cause scoring, and pitting on the cylinder walls.
as far as a fix goes, well my knowledge doesn't run that deep on these engines, so maybe someone will chime in...and keep in mind, what i listed above is a worst case kinda thing...there are alot of sensors that can crap out that can cause this code..
just my .02
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