The function of the catalyst is to convert NOx to other stuff, so that is probably not the first place to look for a problem. You can check the O2 sensor signal with a multimeter, but once again, I am not able to give you the technical information that you need. If you have no fault codes, it may be possible that there is some mechanical/electrical problem at the throttle body that is causing air starvation at low revs. Another thought...if the engine was not hot when the low speed test was performed the test results might be normal. If so, try a retest with a hot engine. Run a bottle of Techron fuel treatment then change the oil. This is not the most informed advice that you can get, but these are the things that I would look at if it was my car.