Not your average SES P0171, P0174 error code...

[adnoid]

Funny - I just started to get these codes as well. 2002 3.0 with 202,000+ miles.

Last week I had a brief stumble in the morning, followed by the SES light on the drive to work. Other than the stumble the car drove just fine, with one significant difference - much BETTER gas mileage! I usually get around 19, I suddenly was getting 23+. When I was able to read the codes I got:

P0171 (System too lean, Bank 1)
P0174 (System too lean, Bank 2)
P0300 (Random/multiple misfire detected)
P1347 (Misfire cylinder #3 with fuel cutoff)
P1348 (Misfire cylinder #4 during start)
P1350 (Misfire cylinder #5 during start)

I noted them, cleared them & left the reader connected. The next day the SES returned with all the above codes and these:

P0302 (Cylinder #2 misfire)
P1083 (Fuel control mixture lean, Bank 1)
P1085 (Fuel control mixture lean, Bank 2)
P1342 (Misfire cylinder #1 during start)

I took it to my indie repair shop (good guys) and they replaced the pre-cat O2 sensors and the intake boot which had a crack in it. I could discern no difference in behavior of the car before or after the repair, but right after I picked it up my fuel consumption was back where I thought it should be. Then the next day I got the SES again with just P0171 and P0174, along with the reduced fuel consumption, albeit only up to about 21 MPG. I reset the codes again, and although the light has not come on again I can see P0171 and P0174 are pending.

What's interesting is that this began to happen immediately after I filled up at a station I don't usually go to, but I was low and running an unplanned errand. It was a Miristar and the pump said the fuel could contain up to 10% Ethanol - has anyone else experienced any such connection?

Still, the car runs great - fires right up without hesitation, not lacking power, smooth, and if not for the lights/codes and the increased mileage I'd have no idea anything was going on!