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Right, I had a code throw up for 02 sensor before cat bank 1, changed the sensor then was fine until a month later, changed again and the eml light kept coming up, found out the 30amp fuse in the ecu compartment was blown so i replaced that and still the same code every single time i start the car, used a electrical tester on the sensor and reads the correct figures yet still the same fault code every start up without fail, no vac leaks, car runs fine plenty of power just the same constant code which has now spread to the after cat sensor which i also tested and they're perfectly fine, I'm at a loss anyone have any suggestions?
2001 330ci (no secondary air pump its mapped out)
 

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That fuse is likely for the O2 sensor heater circuits. Each sensor has a heater circuit which gets the sensor up to temperature quickly. All heater circuit are in a single fuse. This is done so that the engine computer (DME in BMWspeak) can go into the closed loop mode quickly after a cold start. You’re going to have to find out which O2 sensor has a shorted heater circuit. Be advised that you need to label both sides of each connector where the sensor cable plugs into the engine wiring harness. Swapping bank 1 and bank 2 sensors, especially the pre-cat sensors, will have you chasing your tail as the DME will through a weird mix of codes.
 
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