OBD II Trouble Code P0171 indicates that the oxygen sensor in bank 1 detected a lean condition (too much oxygen in the exhaust, or not enough fuel in the exhaust). On the Ford Expedition engine (V6/V8/V10) Bank 1 is the side of the engine that has cylinder #1.
Both P0171 and P0174 are related. They both indicate a lean fuel condition, but on opposing sides of the engine.
P0171 could be triggered by the following:
1. The MAF (Mass Air Flow) Sensor is dirty or faulty. Cleaning the MAF sensor with electronics cleaner (comes in a spray can) may resolve this issue.
2. Possible vacuum leak downstream of the MAF sensor. You should inspect all vacuum and PCV hoses.
3. Possible fuel pressure related issues (check for dirty fuel filter and correct fuel pressure).