The Nissan Path

a while back I replaced the front driver side catalytic converter with a aftermarket converter. ever since I did this I have been getting ECL codes P0420 and P0057. My book says P0420 is catalyst system efficiency below threshold (bank 1). P0057 is HO2S heater control circuit low (bank 2 sensor 2).
Can anyone tell me what these codes mean? How to fix? I need an emissions test soon and think this might be an issue.

Thanks

The O2 heater code is due to a problem with the internal heater of the O2 sensor or the circuit to the sensor. I would replace the sensor (I prefer NTK sensors which I get from Rockauto.com), erase the codes, and then see if the converter code comes back. If it does, you probably have a faulty converter. Some aftermarket converters are a bit sketchy, so it's best to stick with a known name brand, like Walker or Bosal. I've heard of some problems with some Magnaflow converters on Nissans.