It's my understanding that the upstream O2 sensor is close enough to the exhaust valves that if it detects an interruption in the flow of hot gases, it tells the computer and the computer in turn combines that info along with the info it receives from the crank position sensor and can thereby define which cylinder is missing. So, a missfire for ANY reason, not just ignition failure, would throw a code. At least that's what I've been led to believe. I'd like it if someone knowledgeable could verify that for me.