I've been researching using pre and post cat O2 waveforms to diagnose misfires and ran across this quote from the article, Intermittent Engine Misfire Analysis:
Keep in mind that the downstream, post-catalyst oxygen sensor can often be used to differentiate between an ignition- or fuel-related misfire. To illustrate, a misfire caused by a lean A/F mixture will drive the downstream voltage lower because an excess of oxygen is passing through the catalytic converter. In contrast, an ignition misfire will drive the downstream O2 voltage higher because raw fuel and oxygen is being oxidized in the catalytic converter.
I'm having a little trouble understanding exactly what's going on in the case of an ignition misfire. In that case you also have excess oxygen passing both sensors I would think. I don't quite understand how the operation of the cat in each case affects how the post cat waveform looks.