Thanks ZippyNH. But the mechanic did not removed any sensors. They just make two dummy holes so that it can act normally. Btw it happened even when I have my custom made cat (or performance cat which leaked) pre-installed.

 

The 02 sensor measures partially burnt fuel...a removed/failed catylatic converter (precat on the header, and regular further down)...there are 2 sensors...the first one measure how much fuel needs to be added/subtracted from the motor to adjust the mixture to run optiomally.....the second one measures if the catylatic converter is burning the leftover fuel, concert co to co2, and doing various things to nitrious oxides....
If the cats are removed/failed, or the sensor old/aged, or anything in the system has been contamanated with leaded fuel...it will report low efficeny/failure. Simply having the sensor plugged in does not mean much....the first o2 sensor is needed for engine management, the second one monitors the emissions system...
Some highflow/ performance cats have had this issue from what I have read...the 02 simulator will surpress the light/code...

 

okay so what shall i do now? i have discussed with mynes and he told me to bring it to the bmw dealership and get my ride re-flash to its factory setting. I am also curious about the innovate MTX-L since it includes one O2 sensor (Bosch) and if I were to purchase it, I am afraid it is not compatible with my ride (the case of scangauge2). and If it is compatible, should I purchase another O2 Sensor? Thanks