What is code PO174 and PO171? on 1999 Ford Explorer

CHECK ENGINE light comes on. When this happens I get code(s)PO171 andPO174

3 answers
OBD II Code P0171 and P0174 = Fuel System Too Lean (Bank 1 or 2).

There is very nice information about these codes in the RepairPal Encyclopedia; http://repairpal.com/OBD-II-Code-P0171-and-P0174
System too lean bank one and two, likley vacuum leak.
Think intake manifold o-rings...if it's a 4.0L SOHC engine.