O2 Sensors 4.2 V8 Quattro
 

Hi,

I have a CEL being brought on by Bank 2 Sensor 2 heating element. I have searched and searched and cannot find a guide on how to replace this on the 4.2 V8. Can anyone shed any light on its location and a P/N ?

I don't see what year your 4.2 is so I'm just letting you know where I found mine (2002, c5, probably doesn't matter as I don't think Audi redesigned the 4.2):
Bank 1 is passenger side, Bank 2 is driver side. Sensor 1 is pre-cat, Sensor 2 is post-cat

Therefore Bank 2, sensor 2 is the O2 sensor that sits after the catalytic converter on the driver's side. You should be able to spot this one easily when you lift the car up and look at the exhaust pipe.

Hi, Im in the U.K so Bank 2 will be passenger side - LH. And I should be able to see this O2 sensor from under the car yes ?

There are a total of 4x O2 sensors. 2 are located after the catalytic converted which you can see from the bottom of the car. The other 2x O2 sensors are at the top of the exhaust where it comes out of the engine - those are only visible and accessible from the engine bay. It should be obvious when you lift the car up.
So to answer your question = yes :)

Had a butchers under the motor this morning. Still don't see any O2 probes ! On my BMW 3 series it was plain as day ! I can see the 2 cats but no probes or wiring. Guess I need it up on a ramp yeah ?

Yes, I would lift it up or put it on a ramp or jack stands. The o2 sensors will be slightly angled upwards on the exhaust pipe so you need to look around the pipe after the catalytic converters. They are not hanging down or on the side, so peaking at it from the side without getting under it will not be very fruitful.

So all sorted !! But after a fair stramash!! Ended up getting a mobile mechanic to do it and what a job. First he removed the correct one but undid the wrong plug and said it was different. I then pointed out it was the brown plug not the black on the l/h side. So he refits the sensor and try's to unplug the other one (Bank 1) and says it's seized and the exhaust will have to come off. I then have a discussion and tell him the plug is the brown one on the firewall. He goes under and I pull the cable to discover it's connected to the one he had off first time. So sensor replaced and connected to correct plug and happy days ! Just as well I was there.

Nice! I do remember the different plug colors under the hood and that tracing the wire before unplugging is definitely the smarter move :D
Those O2's can be hard to take out after many years of heat and weather but glad you got it resolved.