ok, so ive been getting terrible terrible gas mileage, like.. 140 to a tank. Bad power, and some erratic idle. Im wondering if it could be my 02 sensor and if so, should i replace upstream or the downstream? My CEL was on for about a week and shut off. Tried to scan it today, and no codes showed up. Help

 

upstream controls fuel mixture. downstream validates that the cats are functioning properly.

 

I would do plugs ,wires ,air filter first .....if your car was throwing a code for an o2 sensor it should still show up as a stored memory code . Your code was probably not related to an o2 sensor and it cleared itself after so many starts ......

 

Yeah, you need to have some kind of reference that the small stuff is all within maintenance parameters. New- plugs, wires, fuel filter, air filter, ect.. Suck a can of Seafoam up the pcv valve to clean the upper intake manifold.
Next time it throws a code it will be specific, and then you can perform a diagnostic test.

 

tried to test it and no codes came up. I have changed the rear fuel filter, the air filter, oil change, and changed the spark plugs all within the past month, and still no luck....

 

I just read the Chiltons manual for your discription:
It refers to the fuel filter, when you replaced it, did you see "rusty " colored fuel come out when it was tilted? That would indicate another filter is need to be installed. The other thing mentioned is ignition- Improperly gaped plugs (.052), incorrect plugs installed (BEST:NGK, Autolite, Motorcraft Copper Core or Platinum.), brakes binding & not fully releasing, dirty fuel system*, vacuum hose leak to upper intake manifold.
-*Seafoam the upper intake manifold while engine is running. Put a straw inside the can, pull out the pcv valve, and let the pcv valve slowly suck in the Seafoam.
-*Inside the gas tank- Pour a can of "BG 44K" (Any service shop or dealer sells it) Or pour a full can of Seafoam into the gas tank.
-****Try it! It wont hurt and will help better diagnose any other issue there might be.****
Vacuum leaks around the EGR are very, very common. They are plastic and brittle. Break easily. Good luck, and update.