since i can not find the right up, here i go.
I had the same problem as you, but it happened after i installed my n1.
To keep it short and simple, i took it to a diagnostic guy, who told me exactly what was wrong. Its not your secondary o2 sensor. While taking the n1 out you may have pulled on the power wires of the secondary o2, thus causing it not function.
This is what you do:
Get under your car, and push the secondary o2 sensor clip wire which is on the chasi of the car, push it in. Like squeeze it in. That way if any power wires were loose it will give off power.
Your ATTS light will remain on until you clear the CEL
once you do as i mentioned above, make sure you touch all the other clips the secondary o2 is connected to.
Then reset the ecu, by removing the battery terminals etc.... and try it..
if you have hard time understanding this email me with your cell, and i will help you over the phone....