P0171 and P0420 Diagnosis on 2007 Nissan Quest: Fuel Pump or Sensor Issue?
Hello, My check engine light came on about a month ago. I took the vehicle to Firestone for service, where they diagnosed a P0171 code. The mechanic recommended replacing both oxygen sensors and cleaning the mass airflow (MAF) valve. After the repair, the check engine light returned the next day. I returned for another diagnosis, which confirmed the P0171 code again. They then replaced the MAF valve, claiming it was faulty. However, during a test drive of 30 miles, they did not monitor the fuel gauge and ran out of gas—requiring a tow. They later suggested that the issue might be the fuel pump. I had my fuel pump replaced two years ago with no issues since. After returning home, the check engine light came back on three hours later. I contacted Firestone again to explain the recurring issue. The mechanic responded that it must be the fuel pump. I requested a diagnostic scan from a local auto parts store, which confirmed both P0171 and now P0420 (catalytic converter system bank 1). I also contacted Nissan, where I learned about a recall related to inaccurate gas gauge readings—this was addressed by a technician. After the recall fix, the dealership stated there were no fuel pump issues. However, when I returned to Firestone, they had no clear diagnosis or solution. They asked if Nissan could help and offered additional charges for repairs. I believe their diagnostic process failed to detect the same codes within hours of driving—this is inconsistent with professional standards. When I spoke to a manager, they agreed that the fuel pump is not the root cause but admitted they have no clear path forward. I’ve been refunded and am now back at square one with both P0171 and P0420 codes. I’m concerned about driving out of town during holiday season and don’t want to pay a large sum at the dealership. However, I will proceed if necessary. The vehicle has two new oxygen sensors and a replaced MAF valve. What might they have missed in their diagnosis? Thanks for any advice or insights on this issue.
Firestone is known for quality tires, but their diagnostics on engine issues may not be reliable. P0171 indicates a lean condition in the fuel mixture. A proper diagnosis should include checking fuel trim values from O2 sensors, fuel pressure, vacuum leaks, and MAP sensor readings—especially since you’ve already had these checked by Nissan. If no scanner or live data is used, troubleshooting becomes guesswork. P0420 (catalytic converter efficiency) often follows a lean condition like P0171; resolving the root cause of the lean issue is essential before addressing catalytic converter performance. A professional technician should re-evaluate all engine parameters—including fuel pressure and O2 sensor response—before making repairs. Without live data, any fix based on assumptions (like replacing parts without validation) is likely ineffective. This situation appears to involve a missed diagnosis or inadequate testing. A proper diagnostic process would include reviewing real-time sensor readings and verifying the underlying cause of the lean condition. For reference: https://www.obd-codes.com/p0171