1999 Ford Explorer P0133 Code: Causes, Solutions & Fix for Lean Condition
My 1999 Ford Explorer is throwing up trouble codes P0133, P0171, and P0174 after replacing the left-side oxygen sensor (O2 sensor) in the exhaust system. The right O2 sensor appears to be new and recently installed. I replaced both sensors with plug-and-play types that match OEM specifications—these are not universal soldered sensors. The vehicle ran smoothly for about four days, but now it struggles to start and runs poorly at idle. It seems to improve slightly when RPM increases. Could this be due to a vacuum leak causing a lean mixture in both engine banks? I've already verified the O2 sensor types match the vehicle's specifications. Any advice on diagnosing or fixing this issue would be greatly appreciated.
Check the trouble codes section of this forum for common causes and solutions. A vacuum leak is a likely culprit leading to lean conditions in both engine banks. If the vehicle runs poorly at idle but improves when RPM increases, focus on identifying air leaks. Additionally, ensure the Mass Air Flow (MAF) sensor and Throttle Body (TB) are clean and functioning properly. Verify that the O2 sensors installed are compatible with the 1999 Ford Explorer's engine system.