The ecu will run in openloop. Basically lots of fuel, because your secondary O2 sensor is telling your ecm that your primary O2 sensor is giving an unreliable reading. All the O2 sensor simulator is, is a resistor put in line with your secondary O2 sensor, basically simulating the effect the catalyst would have on your O2 levels, and tricking the ecm's comparator circuits into thinking that everythings hunkydorry. Of course you're going to be illegal for emissions purposes, etc etc... and the fine is $25,000 dollars and jail time for modifying or removing a stock catalytic convertor, etc etc. Thats just the standard disclaimer which I'm sure you already knew though. :thumbup: