My van a couple weeks ago had the check engine light come on and the code was p1155 which I interpreted to be the air/fuel sensor by the rear catalytic converter. I put in the sensor, but by the time I put it in the engine started missing at idle and flashing all kinds of misfire codes. I hear a hissing of an exhaust leak from somewhere which makes me think it could be the catalytic converter being clogged. Seems like it is having a hard time breathing. Could I have fouled out the cat with not replacing the sensor rite away. Any info would be great.

If you run it for a long time with it missing, it can hurt the cat, but a few weeks of normal driving should be OK. And I don't think the cat should plug up, just stop working. It's each to check anyway. Just take it off and shine a light through it.