P0171 is System Too Lean (Bank 1) and P0174 is System Too Lean (Bank 2). Per the service manual, these are some items to check:
- Fuel system
- Excessive fuel pressure.
- Leaking/contaminated fuel injectors.
- Leaking fuel pressure regulator.
- Low fuel pressure or running out of fuel.
- Vapor recovery system.
- Induction system:
- Air leaks after the MAF.
- Vacuum Leaks.
- PCV system.
- Improperly seated engine oil dipstick.
- EGR system:
- Leaking gasket.
- Stuck EGR valve.
- Leaking diaphragm or EVR.
- Base Engine:
- Oil overfill.
- Cam timing.
- Cylinder compression.
- Exhaust leaks before or near the HO2Ss.
I think this subject has come up before on this forum. You may want to use the search feature for this forum and see what other's have found to resolve the issue.
When you determine your problem, please post back with your resolution as well.
Good luck!
-Rod