I'd like to make a thread in the future about the critical current AFR sensor and how it works. Also, most people don't know that, in these types of systems (i.e. those which utilize an air/ fuel ratio sensor instead of a heated oxygen sensor as the primary), the ECM/ PCM also uses the secondary O2 sensor to modify the air/ fuel ratio. So, for those running without the secondary plugged into the bung, or for those using defoulers, the ECM/ PCM cannot modify the air/ fuel ratio optimally. It's not like back in the day with a basic primary heated oxygen sensor which was solely used for air/ fuel ratio modification. In that system, the secondary was used solely for determining catalytic converter efficiency. That's not the case anymore. Secondary has become an agent in air/ fuel ratio modification, so it's important for it to be placed in the exhaust stream, as it comes from the factory.