The car will usually drive, with hesitation, rough idle and worse economy. Have it looked at rather than ignored — running lean raises combustion temperatures, and a sustained lean condition can damage valves and the catalytic converter. If a workshop proposes replacing the oxygen sensor as a first step, ask what the fuel trim values were and whether a smoke test for vacuum leaks was done.
The module has been adding fuel to compensate for a persistently lean mixture on bank 1, and has run out of authority to add more. This is one of the most frequently encountered codes in the whole standard, and one of the most frequently misdiagnosed — the instinct is to replace the oxygen sensor that reported the problem, when the sensor is usually the only part working correctly. Something is either letting unmetered air in, or not delivering the fuel that was asked for.
The diagnostic shortcut worth knowing: compare fuel trim at idle against trim at higher engine speed. A vacuum leak has a large effect at idle, when manifold vacuum is high and total airflow is low, and its influence fades as the throttle opens — so trim that is badly positive at idle but near normal at speed points at a leak. A fuel delivery problem behaves in the opposite direction, getting worse as demand rises. That single comparison separates the two largest families of cause before any part is touched.
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
P0171 says bank 1 is running lean and the ECU has run out of authority to correct it. The code names the symptom. The fuel trims name the cause — and reading them takes ten minutes...
Sign in or create an account to add what you found about P0171.