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Hi everyone! I’m new here and trying to diagnose a problem with my 2016 Altima 2.5. I had a dirty MAF sensor and fixed that problem, but now the good old PO171 code is showing. I have found no vacuum leaks (as best as I can tell, anyway). Here are the readings for my O2 sensors:
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Do these look normal under idle? Car is idling rough and sputters under acceleration.
Thank you for any help you can give!!
 

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P0171 points indirectly to the front sensor, which is actually an A/F type and not HO2S. The lean/rich cutoff is 2.20V, above that is lean. So if that reading was at idle it's normal, they usually run a tad lean. However, P0171 is a "performance" code and can be caused by a lot of things. There is a bulletin for A/F codes, NTB16-117:


It doesn't specifically mention P0171, but we've seen the PCV Valve and/or old ECM firmware cause A/F-related codes that weren't listed in the bulletin.
 

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Here are some other possible causes to look at:
  • Intake system vacuum leaks. Attach a vacuum gauge to a full vacuum source. With the engine fully warmed up, the reading at idle should be 18 - 20 InHg. At 3,000 RPM, it should be up around 21 InHg. If readings are under 18 InHg, check the intake manifold nuts to make sure they are tight. The gasket may have failed; spray a water mist at the gasket to see if the gauge reading changes. Also check the intake plenum bellows at the throttle valve and at the MAF for cracks or loose clamps.
  • Dirty fuel injectors. Run a good injection cleaner, like BG products 44K, through the system; give the cleaner about a week or two to do it's job.
  • Incorrect fuel pressure. Tee-in a temporary fuel pressure gauge between the fuel feed hose and the fuel rail. The reading at idle should be 51 psi.
 
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