A P0171 or P0174 trouble code tells you the fuel mixture is running lean (not enough fuel and/or too much air). This type of problem can be confirmed by looking at the Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) values with a scan tool. Normally, STFT and LTFT should be plus or minus 5 to 10 from zero. If STFT and/or LTFT are more than about +12, it indicates the engine is running lean. Negative numbers (-12 or more) indicates the engine is running rich.
A lean fuel condition can be caused by:
* Low fuel pressure due to a weak pump or leaky fuel pressure regulator. Use a fuel pressure gauge to check fuel pressure at idle. If fuel pressure is less than specifications, your fuel filter may be plugged, your fuel pump may be failing or have a bad wiring connection, or the fuel pressure regulator may be leaking.
* Dirty fuel injectors. Cleaning the injectors with a fuel system additive, or having the injectors professionally cleaned may solve your problem.
* Vacuum leaks at the intake manifold, vacuum hose connections or throttle body.
* Leaky EGR valve. Check the operation of EGR valve and system, and for a buildup of carbon under the valve.
* Leaky PCV Valve or hose. (Check valve and hose connections)
* Dirty or defective Mass Airflow Sensor (MAF). Try cleaning the MAF sensor wires or filament with aerosol electronics cleaner. Do NOT use anything else to clean the sensor, and do not touch the sensor wires.