check engine light low idle bad gas mileage

What is your question?

I have a 2005 vue redline. Has about 125000 miles on it. bought it new. Just started idling really low (at least 500 rpm below normal). check engine light comes on. Normally gets about 21 mpg now 17 mpg if I'm lucky (Same driving conditions). Not a constant issue. Only seems to happen when my gas gauge is lower than 1/2. might be able to give you the code, but I will have to find it (just moved). Note: This vehicle is equipped with an electronic throttle body. I'm not sure if that helps. Thank you very much.