P0171 System Too Lean Bank 1 Fix: DIY Solution for 2001 Nissan Pathfinder LE

I'm grateful to many forum users who shared solutions online, so I've documented the steps that successfully resolved my issue to help others facing similar problems. CAR MODEL: 2001 Nissan Pathfinder LE — Original owner, well-maintained with regular servicing at Nissan dealerships. ISSUE SYMPTOMS: • Check Engine Light illuminated • OBD-II Diagnostic Code: P0171 – System Too Lean Bank 1 • Fuel efficiency dropped to as low as 14 MPG • Car failed smog test due to emissions failure DIY REPAIR STEPS: 1) A local Nissan dealership in San Jose, CA initially diagnosed the issue as an emissions system problem, estimating $650 for diagnosis and repair — likely involving either the Mass Air Flow (MAF) or Oxygen (O2) sensor. Given that my vehicle is 16 years old, I decided to attempt a DIY fix using online resources. 2) After researching forums, I identified common causes: clogged air filter, dirty MAF sensor, or faulty O2 sensors. Starting with the most accessible and inexpensive component, I replaced the engine air filter — which was severely dirty. A replacement filter costs around $20 on Amazon and takes only 10 minutes to install. The filter is easily accessible and has numerous YouTube tutorials available. 3) After a few days, I had a mechanic clean the MAF sensor for $35. However, the Check Engine Light returned within a couple of days. 4) I purchased a genuine replacement MAF sensor (Model MF4W00, 3.5L V6 OEM fit) from AIP Electronics at $44. The housing was too difficult to remove and unnecessary — so I simply replaced only the sensor itself. Installation took about 10 minutes. 5) I returned to the mechanic for OBD2 code clearing and reset of the Check Engine Light. 6) For several days, the car ran roughly. After switching from 85 or 91 octane gasoline to regular 87 octane fuel, fuel efficiency improved significantly — now exceeding 18 MPG. The vehicle runs smoothly and has passed a recent smog test. The Check Engine Light remains off after over a month. I’ve saved more than $700 and gained confidence in handling future car repairs independently. PROPOSED CAUSE: I believe the root cause was MAF sensor failure. After my last oil change, a mechanic failed to properly seal the oil gasket, resulting in oil leakage onto the garage floor. I initially ignored this for several days, assuming it would resolve on its own. However, the leak persisted. During that time, engine oil fumes were likely drawn into both the air filter and MAF sensor — potentially damaging the sensor beyond cleaning or repair. This may have triggered the P0171 code after a few days. This sequence of events appears to have initiated the entire chain of issues. Therefore, I strongly recommend that any vehicle showing oil leaks be repaired promptly, with immediate replacement of the engine air filter if necessary.