O2 sensor location

[houghtonregis]

Hi, prior to my bypass op I fitted a pre-cat o2 sensor to our 2007 E87 120i.
The tech guy said two sensor were faulty, and if I`m not mistaken he said both were on the right hand side of the engine.
Unfortunately he has moved interstate so I cant ask him.
Now that I feel up to fitting the second one I`m ready to give it a go.
Can anyone advise where the other one is located, I think nearer the front of the engine.
Thanks.

[MarcoZandrini]

Before you dive into replacing the sensors, what codes are showing up? I ask as typically a bad sensor will cause issues upstream. The exception is when the sensor’s heating circuit is at fault. So get the engine computer (DME in BMWspeak) scanned for codes and post them here. Brisbane is a large city so I’m sure that there are several reputable independent shops that specialize in bimmers/beemers.