[Davescort97]

P0 171 indicates the fuel mixture is too lean. Many people with K&N air filters end up with this code because they over oiled the flter and it contaminated the MAF (Mass Air Flow) sensor. When the MAF gets dirty it will throw the 171 code.

1. Replace your air filter
2. Get a can of electrical contact cleaner at parts store or WalMart Automotive.
3. Disconnect the air hose from the air box to access the MAF.
4. Spray the MAF wire with the cleaner and don't touch it with anything.
5. Wipe out the interior of the air hose.
6. Reset your computer by disconnecting the negative battery terminal for 5 minutes. If the engine light comes on again it could be for the following:

1. Low fuel pressure due to a weak pump, pressure regulator or plugged filter.
2. Dirty injectors.
3. Vacuum leak at the intake or hose connections.
4. Leaky EGR valve
5. Leaky PCV valve or hoses.

You can get this code on many Fords because of the DPFE (Differential pressure sensor) The DPFE misreads the EGR flow as too low and the computer increases the amount of EGR flow causing a leaning affect on the fuel mixture.

I wish I could say it's this or that but PO171 can have many different causes.