A catalytic converter is an important part of a car's emission control system, and it can cause P0171 if it is not working properly. The converter converts harmful pollutants in exhaust gases into less harmful compounds before they are released into the atmosphere. A faulty converter can cause an increase in emissions, which can trigger the P0171 code. There are other potential causes of P0171, so it is important to have the vehicle diagnosed by a qualified technician to determine the root cause of the problem.