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P-0171 Code and Fluctuating Oxy Sensor 1

Old 03-13-2019, 11:24 AM
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Default P-0171 Code and Fluctuating Oxy Sensor 1

2014 Chevy Malibu LT 2.5L4 No mods
This car began experiencing a P-0171 (Lean bank 1) code two years ago (34 months old and ~30,000 miles). Since that time, it has been in two different GM dealers shops eight times to resolve the issue. Actions to date include: clear code, ensure Top Tier fuel at every fill-up, MAF sensor cleaning, two times fuel injector cleaning, multiple checks for intake and vacuum leaks, replacement of split vacuum hose, replacement of PCV valve, update of fuel system firmware, etc. Each service has resulted in code returning within one to two days. Frustrated with not being able to remotely start car, I decided to get an OBDII to Bluetooth adapter to read and reset the P-0171 code. When looking at the sensor outputs, I noticed one of the two oxygen sensors fluctuating wildly, while the other oxygen sensor is very steady. I also noticed that the fuel trim is also fluctuating. I don't know if one is driving the other, or if something else is the problem. I have never noticed a performance issue, only the check engine light. I have included two charts from my OBDII Android software. One shows the two oxygen sensors (Oxy 2 steady, but going from 0.75 to 0.8 as I pull into parking place and put vehicle into park, vs. Oxy 1 fluctuating from 0.05 to 0.80 volts), and the other shows the two oxygen sensors and the fuel trim (F/T and Oxy 1 fluctuating while Oxy 2 steady).

Any help with this would be appreciated. I'm thinking that the Oxy 1 sensor (or its connection) has a problem, but I am too ignorant and inexperienced to know.








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Old 03-13-2019, 10:09 PM
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What is the fuel pressure?
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Old 03-14-2019, 01:40 AM
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Your oxygen sensors are doing exactly what they should.
You need to learn why the sensors are there and what they do. I'm not going to spend hours trying to teach you. Look it up.
Fuel pressure as asked by Cathedral is a place to start for your lean code.
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Old 03-14-2019, 09:16 AM
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40-44 psi driving steady at 40 mph, gradual movement with no swinging. 52-58 psi when parked, gradual movement with no swinging. O2-1 sensor and short-term fuel trim swinging as in charts in both situations. O2-2 sensor and LTFT both steady.
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Old 03-14-2019, 09:21 AM
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Pardon my ignorance, I have replied to Cathedral. Yielding to your great understanding, and in a brief response, can you please explain why, while the car is idling and driving down the road, the O2-1 sensor swings wildly while the O2-2 sensor is very steady?
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Old 03-14-2019, 10:17 PM
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O2 sensor 1 is reading the oxygen in the exhaust....this sensor data is used to control fuel air mixture.....the rapid changing is a normal waveform because the PCM is trying to maintain a 14.7/1 fuel mixture(if lean, it adds fuel, which probably over corrects and it becomes rich, so then it leans fuel, which over corrects and then it adds fuel again....thus the constant up/down waveform).....so when the PCM sees the exhaust lean, it adds fuel, when it sees it rich, it decreases fuel....it does this by constantly changing the pulse width of the injectors.......thus the rapid changing wave from of the O2 sensor 1......

The waveform of O2 sensor 2 is normal....it is reading the output of Oxygen from the catalytic converter as it uses and stores oxygen, to reduce emissions.....this second sensor is not used for fuel metering like O2 sensor 1 is.......it is used to check catalytic efficiency.....if the waveform from the second sensor "mirrored"(was the same as) the waveform of O2 sensor 1, that would mean your cat converter was no good.....

As for short term and long term readings, the short term will change more rapidly than long term...normal readings usually -10% to +10% for long term.......

If LT is greater than +10%, that means the PCM thinks the A/F mix is too lean and it is adding fuel to correct this......

Hope this helps you to understand....

Shame on the dealership.....if they could not correct this you should have gone Lemon Law on them, or at least contacted GM Customer Assistance, to fix this......GM should be able to fix their own vehicles....you let them off the hook if you did not go back....if not satisfied with the work of one dealership, they would direct you to another one and give you a loaner until your vehicle is fixed....

Old saying, the squeaky wheel gets the grease....
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Old 03-14-2019, 10:57 PM
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Thanks Tech II
i appreciate the education and the additional insight. Very informative. I am in contact with GM "Customer Service".
While driving home tonight, I noted that the LTFT was running in excess of 20. Wish I wasn't having to track this stuff down myself, but I don't know where to turn after so many dealer visits
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