My 2008 Efi Cultus was giving me O2 Circuit Malfunction whenever I scanned it. There was no engine check light ever, so to see whether the O2 sensor is even working or not I disconnected the sensor wire while the engine was running and I observed zero response as if the O2 sensor is not even connected in the first place. This is my first experience with O2 sensor shouldn't it have showed any sign that it was disconnected? Also what should I do should I just clean it or what?