O2 sensor issue after head gasket change

I did head gaskets on my 99 2.5d  I have a check engine light for O2 sensor. Looking at torque, it is bouncing all over the place,  it is stable coasting at 0. other than that it is from .1 to .9 completely random. O2 sensor 2 is at 0.3v constant Is there anything I could have missed that would cause this? Or is it just a bad 02 sensor. I did have a leak at the exhaust manifold before the head gasket change. and have fixed that. 

Thanks