The MIL (or CEL) came on about 1000 miles ago. The codes are 25 & 26 (engine rich and engine lean). It is a new Jasper engine with about 6K on it. The light comes and goes, but mostly stays on. I had the exhaust checked for leaks. Monday someone is going to check the O2 sensor for me. The only other symptom is that after setting for awhile it starts rough and there is a smell of fuel (http://www.tundrasolutions.com/forums/#). Once it is warmed up, that goes away and it starts and runs fine. Oh, and my mpg has gone down an average of 1 mpg. Please tell me what else to check and how to check it, keeping in mind I am not a mechanic and have to hire someone and explain it to them. Thanks!!

How good is yer airflow sensor? Any obstructions since it's the flap type? I'd be leaning towards a vacuum leak. See if there's a shop around you that has a smoke machine that they are qualified to use. A smoke machine like the Smoke Wizard (http://www.smokewizard.com/) can assist in the detection of vacuum leaks that could cause the intermittent lean/rich codes yer ECU is picking up. ;)

Fix the vacuum leaks and then move on to the next diagnosis. :D

I would even lean towards a bad fuel regulator or the hose going to it. ;)