Q: Can a faulty gas cap cause a P171 code? on 2005 Honda Accord

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Car was taking to mechanic that put a scanner to the vehicle and claimed a P171 code and the problem was a bad O2 sensor.
A gas cap was brought because the other was defective and allowing air to enter the tank. The has been running with no check engine light or code present for eight days. Is the problem with code resolved.
(3) Answers
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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.
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.
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.