My '03 2.3 keeps throwing P0171 code, running too lean. Between the shop and I at this point we've tried quite a few things:

Round 1:
- Old Upstream O2 sensor was bad, replaced with Motorcraft.
Assumed that was the problem's source, but it came back.
- Replaced PCV and hoses too, because they were old.

Round 2:
- Cleaned the MAF and replaced the air filter.
- Found vac leak at the intake manifold #4, replaced gaskets.
- Replaced valve cover gaskets, too. Test drive, code came back.

Round 3:
- Replaced MAF. Used a Delco, I think. OEM was not available.
Code disappeared for a day, then came back this morning.

My fuel pressure looked fine. What else should I be looking at? Not sure where to go from here. Overall it seems like things are running smoothly. Way better now than before this all started, but still getting codes.