2000 Ford Taurus CODE P0171 BANK 1 & P0174 BANK 2

On many Fords, a P0171 and/or P0174 Lean Code may sometimes appear because of a bad Differential Pressure Sensor (DPFE). This sensor monitors EGR flow, and is located on the engine near the EGR valve. There are two hoses that connect the sensor to the tube that runs from the exhaust manifold to the EGR valve. The sensor misreads EGR flow and the computer increases EGR which has a leaning effect on the fuel mixture. The fix is to replace the DPFE sensor.

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)

* Dirty fuel injectors. (try cleaning the injectors)

* Vacuum leaks at the intake manifold, vacuum hose connections or throttle body. (Use a vacuum gauge to check for low intake vacuum)

* Leaky EGR valve. (Check operation of EGR 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)

Last year I had code 0401 insufficient flow come up and the dealer changed the DPFE sensor.
Does this sensor have somethingto do with p0401 & p0171??

If the EGR is cracked open it will create a leaning effect-

DPFE will cause lean condition

I did change the DPFE sensor last week ( new ) and also cleaned the throttle body...was alittle dirty and then I cleared the codes, Car ran ok for that 1 day then the next day the car ran rough The RPMs would go high then low then the car would stall out 2 or 3 times until it got warm. Then the check engine came back on, showing code P171 bank 1 & P174 bank 2 = running lean condition.
Can't find the PCV valve ?? What would be the next step ??
note: 2000 ford taurus sel model.

I also had my car stalling out multiple times at red lights and just at idle.., well they told me to change O2 sensors, the fuel injectors, fuel filter, the fuel pump and hope the problem would be fixed. I found out what it was...., at O'reilly auto parts it is a $12.00 dollar part but they have to order it. It was the PCV valve elbow connected to the intake manifold.