A Ford P0171 fault code usually indicates that the engine is receiving too much air or too much fuel. It is also possible for both of these conditions to exist simultaneously. The P0171 fault code in a Ford vehicle usually means the problem is in the passenger-side cylinder bank.
Various problems may trigger a P0171 fault code, including a dirty mass air flow sensor, a weak fuel pump or dirty fuel injectors. If a car exhibits this code, look for other symptoms of the problem, such as a rough idle or lack of power. The solution to this problem depends on the exact cause. Possible fixes include cleaning the mass air flow sensor or replacing a hose.