MP4.2+6.0

Inspired by a current thread, I am posting some tips that are in common with the 4.0T's, either A8 or S8. Starting points assume you have a check engine light (CEL), have used a quality scan tool (VCDS, etc.) to pull the codes, and the relevant code is tied to an oxygen (O2) sensor.

Last summer I had a code for this. I first cleared it a few times, but it recurred a trip or two later. 2015 S8, about 55K miles at time. I concluded it logically was an O2 sensor. I looked into the job and realized two of the sensors are easy to do, and two very hard. (Very hard if you use the complete sensor with connector; read on). The difficulty is the sensor set for each bank (motor side) of the engine consists of one "easy" and one "very hard." I had a post-cat (S2) error, but knew from experience it was likely a hidden pre cat S1 issue, so that gave me this mix of one easy, one very hard if I wanted to do the whole side/bank and be done with it. I ended up doing all 4 to avoid likely repetition in next 50K+ miles or 7 or 8 years. Basically once I opted to deal with one of the "hard ones" underneath labor wise, I just dealt with both of those at once, leading to all four.

Learning:

1. Do NOT assume a code for an S2 (post cat) sensor is necessarily that S2. Experience says it is likely actually S1 on that Bank, and ECU can't directly understand S1 is faulty without comparing it to S2. S2 codes can mean cat is no good, but subtly can also mean it is actually S1 that is bad. S2 rich is more likely a pointer at a bad cat (or that S2 sensor). S1 also has a heating element (why it has more wires in the connector), so if you have a direct S1 code, it may relate to that heating component or otherwise be bad directly.

2. Bosch is the OES for the 4.0T sensors. They are factory correct for connector color code and wiring lengths. No need to pay much higher dealer prices for the identical sensor in a Bosch package. Once I had the part number for the Bosch, little problem finding availability and a good price. IIRC, mine were from rockauto.com @ USD $55 to $65 each (as of 1/24), depending on which exact one it is. DISCOUNTED US Audi dealer is $230 to $430 EACH, depending on position That sourcing to me was a no brainer given the huge delta in price relative to Audi dealer, and that Rock Auto likely sources from Bosch directly--to avoid counterfeit or other product risk. Also big part of my just doing all 4 at once--time to do work and long term trouble free vehicle are more way valuable to me at those relatively low parts prices.

3. Know that the plugs for the sensors are "easy" for the two connectors on the USA/Continental Europe drivers side, and very hard for the other side. NOT sensors, but rather their CONNECTORS. What is counterintuitive is the S2 sensor CONNECTOR for bank 1 (USA passenger side--B1 S2) ends up on the drivers side of the motor in very easy reach, along with the pre cat S1 for Bank 2 (B2 S1). But then the painful connectors are B1 S1 and B2 S2 that are buried down above the USA passenger side inner CV joint in a very hard to access location. See shop manual--even direct view of them is blocked by combination of engine subframe, CV joint and axle and other things. Work is tedious at the connectors, heavily by feel and not for the uninitiated. As simple tests of skill, if you EITHER are not comfortable working by feel/quasi blind, OR use screwdrivers to undo Audi connector locks or fasteners (and then may well break them...), this isn't for you.

4. See the picture below of my S8. It is focused on the "easy" connectors on driver's side of motor. Those are to the right of the oil fill and in plain sight. If you look at your actual 4.0T motor, you will see the exhaust leaving the turbo on either side and the S1 sensors at turbo area. But then that side's head pipe basically does almost a U turn back by the firewall. Then that turbo's converter is actually closest to the OPPOSITE side of the motor. Why in turn exactly how the sensors lay out relative to the connectors is confusing.

Net, B2 S1 and B1 S2, easy and proceed per diagnostics AND knowledge to nonetheless suspect B1 when you actually have a B2 code.

B1 S1 and B2 S2, either be skilled with mechanics AND Audi connectors, or use a shop who is. OR ALTERNATELY, and contrary to my normal preference, consider splicing in the new sensor into the existing wiring and leave existing connect underneath in place. Reaching the sensor and the early part of the wiring on the "hard" side is still easy. It is the connector ends of these that is challenging. Given what these do, heat loads and that any install or wiring issues lead to CEL's, be sure nonetheless you have good wire splice connectors, the right tools and are experienced in high reliability wire connections. This would NOT be your simple shade tree twist it together, put black plastic electrical tape on it (or even old basic stereo type crimp connectors) and call it good. Instead, solder, shrink wrap, careful routing and specific splice area choice to avoid turbo heat, etc.


S8 4.0T pictured. Under the top engine trim cover, A8 4.0T should look almost the
same except for the extra S8 only second airbox and intake tube.

skiracer

Awesome write up! I am just getting ready to do this on my '14 S8 and your instructions will be invaluable. I have found the following Bosch part numbers for the four O2 sensors, are these the parts you used? I just want to be sure as the photo of the downstream sensors on the Bosch site does not have the correct connector pictured (but it does look correct on Rock Auto). Thanks again for taking the time to document/write this up!

Upstream
Qualifier: Cylinder(s) 1 to 4
Part Number : 17407

Position: Upstream
Qualifier: Cylinder(s) 5 to 8
Part Number : 17078

Position: Downstream
Qualifier: Cylinder(s) 1 to 4
Part Number : 16528

Position: Downstream
Qualifier: Cylinder(s) 5 to 8
Part Number : 16531

MP4.2+6.0

Yes, those are the same Bosch part numbers I just confirmed via my rockauto.com order still online. All matched, fit and connected correctly just like the originals I pulled. They are the OES, so even the connector ends locked back into their attachment points on the relevant parts of the engine hardware (USA driver's driver's side uppers in picture) and body (lower difficult ones on USA passenger side).

If you are doing all four, either review the shop manual on the passenger side ones, or at least know the inner CV joint heat shield on passenger side needs to come out. That is, unless you opt to splice those in to the existing passenger side connector leads as I flagged as a possible alternative. I was hopeful when I looked at instructions in advance I could skip that removal step, but they were correct. Straight forward to do with a decent set of tools, socket extensions, etc.; just takes a little more time.

A8L_New_England

.
What would it take for someone in a position of power to activate the "sticky thread" function in this forum? For God's sake, they even have them in the "off-topic" forum... why the heck can't we have them here???
.

BrandoMeats

Good write up! CEL was on a couple months back with P2098 B2 S2 fault code. I only went half way and decided to replace both post cat sensors, when in hindsight should’ve just done all 4 once discovering the location of the connectors for the right side of the car. Ordered 2 Bosch post cat sensors from Rock Auto. B1 S2 took less than 20 minutes, B2 S2 took a couple hours due to having to locate the connectors first, then having to remove the wheel, fender liner and CV axle shield to access connectors. In hindsight, should’ve done all 4 at once, but couldn’t be without the car and couldn’t wait for another order from Rock Auto. If you have one bad sensor code, do all 4 at the same time. You’ll thank the OP later, wish I had known this back then.