Running Lean?

Ok, i replaced my O2 sensor to a heated one about 8 months ago because of my header. then about a month or 2 ago, my check engine light came on for high voltage. i got a new soldering gun for x-mas and redid the connections for the 3rd time, so im guessing the high voltage problem is fixed (so far, so good). but in the past 3 days or so, a code has been thrown for system too lean. what do you guys think has caused this? ive narrowed down my guesses to possible dirty MAF sensor, bad fuel filter, or the O2 sensor is just no good anymore. thanks guys