P0137 with no cats
 

Hi, before everyone gets mad cause I took my cats out, I am installing new cats in a month from now. I just figured I'd run my truck catless for a bit to try it out.
Installed straight pipe yesterday and added spark plug non foulers to space the o2sensor. Picked up a P0137 yesterday.

From what ive read that should indicated a bad O2 sensor driver side behind cat. My question is: is that code only for a bad O2 sensor that needs replacing or could it have anything to do with bad reading due to no cats.

Thanks for any help and I'll be updating after a month letting everyone know what it was like without cats. I can tell you for starters...its loud.