Can a faulty gas cap cause a P171 code?

most likely no. 171 is a lean code from a vacuum leak, mass air flow failure, pcv valve bad or low fuel pressure.. the gas cap is part of the evap system. Roy

Customer Concern: The check engine light is on with a code P0171. Tests/Procedures: 1. Follow Honda TSB # 06-039 for a software update for this code. Honda will reflash the Engine Control Module (ECM) so it does not falsely set a code P0171. 2. Check if both banks are lean, if both banks are lean look at the mass air flow sensor gramps per second at warm idle, normal is 3.1-3.3 grams per second and the manifold absolute pressure sensor voltage should be at .85 to .91 volts with a good 19-20 inches of engine vacuum.

Valve clearance check/adjustment is often over looked as part of proper servicing for this engine. A slightly tight valve causes a low MAP sensor reading indicating low manifold vacuum. Start with the basics and then look for vacuum leaks and that the EGR is operating correctly.