Q: Got a trouble code P0171.

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Got code P0171. Plugged in my OBD II scanner so I could monitor the O2 sensor function. Immediately I noted that while at normal operating temperature, the Bank 1, # 2 O2 sensor was not cycling as it should. It was keeping a constant voltage output of about .7-.8 volts. To me it appears the O2 sensor has failed and needs to be replaced. Just wanted to bounce this of someone to see if I'm on target.

My car has 39000 miles.
My car has an automatic transmission.

The purpose of the No. 2 sensor is to monitor the efficiency of the catalytic converter. Consequently, if everything is functioning correctly, the No. 2 sensor is supposed to report a constant voltage. If the No. 2 sensor waveform mimicked the No. 1 sensor (which IS supposed to fluctuate around an average) that would mean that the catalytic converter has failed. So, if you see the front (No. 1) O2 sensor wave form switching from high to low (rich to lean) while the post converter sensor displays close to a straight line, that is an indication that the catalytic converter is functioning as intended. On the other hand, if the post converter sensor has the same waveform as the front sensor, that is a tip off that the converter is not functioning.


With regard to Diagnostic Trouble Code P0171 that is a serious code which requires prompt attention because continued operation could, in fact, damage the converter. Typically, P0171 will set if any of the following faults occur: faulty fuel pressure regulator; weak fuel pump; clogged fuel filter; faulty powertrain control module (rare to very rare); vacuum leak; faulty injector(s); faulty oxygen sensor(s); or faulty mass air flow sensor. If you request a check engine light diagnostic a certified mechanic from YourMechanic will pinpoint the cause of the fault that caused the code to set. Once the faulty part or component is identified, it is explained to your satisfaction and the mechanic will let you know of the cost to repair. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.

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