header/o2 sensor problem

I installed a new header on my 98 honda civic ex. I couldnt get the old sensor out of the stock manifold so I bought a bosch OEM type one. The harness plugged right up to my factory harness. It threw a 01 code on my MIL. primary o2 sensor open or short in o2 sensor circuit, defective o2 sensor. I took the sensor back and got a new one. The exact same type. I reset my ECU and again it threw the same code. I just sent out for a factory honda sensor hoping that will work. There dosen't seem to be any leaks on my manifold gasket or anywhere. Anyone else have this problem? if so how did you fix it?