hey guys...

i just finished up my exhaust work, reset my ecu, then my CEL came on!

i have a 99 cali spec and i got 4 codes for all 4 O2 sensors. I removed three of them when i installed my WS Y pipe but thats all i touched. (on upper flange of Y pipe, the front of the Y, and on the manifold in front of the Y)

I had to remove the cat but i dont remember seeing a 4th sensor near there.

Anyone have any ideas what caused this or what i could do to fix this short of replacing ALL 4 of them? (i dont feel like dropping 400 bucks on this if i dont have to)

Thanks