Today, I installed a heated O2 sensor (sensor 1) inplace of the old faulty heated sensor. I also checked the spark plugs, changed the oil, and took the black box off (2200) the top to check the TB.

Tonight though, I through the code P0137. That has to do with low voltage on the rear O2 sensor. Since I didnt touch the rear O2 sensor, what might have caused this? Is the sensor just bad all of a sudden?

the sensor could be bad or your cat could be clogged. I would try resetting the computer and see if it comes back up again. If it does, one of the two are suspect.