Okay I have a 2000 prerunner 2x4 Trd 3.4L. roughley 80,000 miles. The only engine mods it has are an k&n intake and magnaflow exhaust. My check engine is on the OBD code is P0171 (system too lean (bank 1)). Which would refer to the first O2 sensor. So I bucked up and bought both 02 sensors (genuine toyota parts) and installed them. Then i reset the computer, and everything was good. After about 20 miles its back on, and I'm frustraited as hell after spending $400 on 02 sensors, please someone help me, I'd greatley appreciate it.