Code P0171 system lean

[cralx2k]

I'm trying to fix the code P0171 on the Ford Contour 99 2.0 LX myself. The car is running perfectly. It has new O2 sensors, fuel filter, sparks, spark plugs, full clean injectors. I don't see any vacuum leaks or loose/cracked hoses.

I need some tips on to how to do a better tests on vacuum leak, PCV valve and ERG valve.

The car has been running for 2 years with P0171 on it and doesn't have performance problems.

Thanks all.