Well last night while running around town i noticed my air fuel gauge on my turbo timer went real lean, and when i ran it up around 5 grand it started breaking up as it went into vtec, it was super lean then the gauge read 10.0 and was blinking red, so then it threw a code, for the o2 sensor, I just rebuild the motor from one that was using 4 quarts of oil a week, and i never replaced the o2 sensor, could this be my cause for running lean??? I droped my friend off, cut the car off then drove it home, no code, not lean anymore, and ran like a top? anything else that can cause this???