the second O2 sensor is sensing that the converter is not doing its job... so you'll get a CEL... search the forum for a way to "bypass" the second O2 sensor.

Only a really high-priced ($300 or so) High-flow converter will allow the second sensor to work properly. My friend put a DC (I think, it was $329 plus tax) converter on his 99 LX and never threw any lights.