Oxygen Sensor/Cat Problems
 

I am pulling CEL's P1131 & P0421: O2 Sensor before driver's side Cat - Internal Resistance too High, Warm Up Catalyst Bank 1 Efficiency Below Threshold - Intermittent, respectively.

I've recently replaced deteriorated rubber vacuum hosing and checked the corrugated PVC hosing for leaks without spotting any trouble areas. Replacing the hosing increased performance noticeably initially but car is again running a bit sluggish. I've attached photos of my work and the related VAG COM info.

Can I pass the "Readiness" tests and still need to replace the O2 Sensor and Cat?

The "threshold" issue can be programmed out. That will probably take care of your other issue. See your dealer or a very good indy.

Bob

Pulled Sensor 1 from Bank 2 (pre Catalytic Converter, driver's side) and it looked pretty bad off. The electrical male/female connections were pretty dirty too which could have contributing to the intermittent signal. Ended up denting the Sensor canister and had to replace the guy regardless of the wear and tear. Went on Ebay and grabbed two pre-cat converters with connection hubs for $160.

I will post some pics for anybody looking to bust the 02 sensor repair on their own. Next steps: breaking out the channel locks for MAF cleaning, Bank 1 sensor 1 02 sensor, clear the CEL's and pray that the Catalysts aren't boned.

I've read many posts about the C5 Engine Control Unit (ECU) software update. Sounds like the update can reduce the Check Engine Light (CEL's) codes thrown but my car is/was not running well.

I think I can alter the threshold using the VAG COM software but I'll have to do some more reading.

(Sorry for spelling out the acronyms but it can really help an individual using the forum search function.)

The threshold limit changes only gets rid of the code, it does not change how your car runs. If it was running like crap, then you are on the right path (maybe) to recovery.

Good luck,

Bob

I now have smoke streaming out of the engine compartment which seems to be originating somewhere beneath the intake hosing just past the Mass Airflow Sensor.

I just changed the O2 Sensor driver side pre Cat and attempted to do the same on the passenger side but after a great deal of unsuccessful torquing decided to give up for the time being.

The smoke smells like exhaust but I am not certain. Could this be a fouled Cat? Could I have detached or punctured a hose/exhaust in my futile attempts to remove the passenger side 02 Sensor? Any help would be appreciated.

Did you get your Pre-cat replacements from Ebay welded in? Im skeptical about that to begin with, but warm-up catalyst specifically refers to the pre-cats. I would just replace the entire system or gut them out if you're having smoking problems. Those little guys can easily get clogged and deflect the flow of your exhaust and we all know how close that it is to your turbos exhaust turbine. Just looking at the simple possibilities.

I attached a few photos of the Front 02 Sensor removal process and will provide a brief outline below.

Bank 1 - Sensor 1 - (Passenger's Side - Pre Cat)

1. Remove Intake hose after Mass Airflow Sensor.
2. Remove Vacuum Line attached to One Way Check Valve. This is a J shaped rubber attachment which runs a plastic hose to the Driver's side One Way Check Valve (looks like a bell one would attach to a bicycle) and Secondary Air Injection Solenoid. The O2 Sensor in question is located beneath and adjacent to the One Way Check Valve.
3. Remove the two bolts that hold in place the plastic housing to which the various sensor connectors are attached against the firewall (I think it's the firewall). Slide the plastic harness toward the center of the engine compartment carefully.
4. If necessary move the three hoses that run through a plastic router beneath the sensor harness' location prior to step 3.
5. Break out your sensor wrench attachment and slide the O2 Sensor cable through the attachment's slot.
6. Attach your wrench extensions to the attachment. I used a 24'' extension without much luck. I eventually added a 6 inch extension that gave me the angle and leverage I needed to move the sensor.
7. I ended up getting the sensor loose while the car was running. I tried heating the car and stopping the engine but could not get the bolt to budge. Torquing while running the engine was the only way I could pull it off.
8. Once the bolt comes loose let the car cool and remove the sensor by hand.
9. Remove the old sensor connection from the female ECU connection. I cut the old cable sensor cable to aid the removal of the sensor from the engine compartment.
10. Screw in your new sensor and connect the male attachment to the female ECU connection after finding a suitable routing scheme for your new sensor wires. Zip tie the excess cable appropriately.
11. Reattach and replace items removed for accessibility purposes.

Bank 2 Sensor 1 - Driver's Side - Pre Cat
1. Remove coolant expansion tank and carefully place out of harm's way.
2. Remove Vacuum Line attached to One Way Check Valve. This is a J shaped rubber attachment which runs a plastic hose to the Passenger's side One Way Check Valve and Secondary Air Injection Solenoid. The 02 Sensor in question is located beneath and adjacent to the One Way Check Valve.
3. Follow steps 5 through 11 listed in Bank 1 Sensor 1 removal outline.


I was extremely winded after removing the sensors. I ended up cutting the female ECU connection for Bank 1 Sensor 1 after correctly removing Bank 2 Sensor 1 and replacing. Be cautious before cutting wires if you are a putz like this hombre.

I attached the wrong photo in the post above. This is a shot of the engine compartment from the front with the wrench attached to Bank 1 Sensor 1 (Passenger Pre-Cat).

The smoke running coming from the passenger side of the engine compartment arrived prior to my replacement of the Stock 02 Sensor. The new sensor part numbers are OEM: 078906265M & Bosch: 16 287.

The new sensors fit snugly and I tightened them up a bit with sensor wrench. I didn't go nuts after hand tightening as they are so difficult to remove. After getting the new O2 sensor in the passenger side slot I am not seeing smoke. There is a considerable amount of heat running off the passenger side of the engine compartment but I couldn't see smoke this morning. I am thinking the smoke was due to oil residue burning off as my paw prints were all over the exhaust line. Is this possible?

Photos of the sensors removed are attached. Bank 1 Sensor 1 is showing a great deal of white oxidation while Bank 2 Sensor 1 has black residue all over the tip. Not sure what the white (smoke side)/black residue mean in terms of malfunction indication.

Something like Anti-seize copper-based paste is the answer to threads subject to high temperature (exhaust-related most likely).
http://www.germanautoparts.com/image...23130323d4c4/f

The sensors came with a good bit on them. I figured adding any more to the mix really wouldn't help all that much. It just seems that you have so much heat running through the area that going too tight would cause problems with or without the anti-seize.

So I believe the smoke is leaking oil. Pulled the PVC breather hose to find and obscene amount gunk in every hose. Pressure builds and you get leaks apparently. How can I clean my valve covers? Gasoline soak? The plastic housing connected to the intake manifold was gunky too. Is it easy to pop the intake cover off and lean the manifold? I assume it needs it. Any other things I should be thinking about because of the gunky PVC?

Hello I have a 2003 Audi A6 I get error codes P0421 and P0431. I guess my O2 sensors have gone bad. I went to the mechanic and he has asked for $1000 which is a little too much for me. Would anybody here be able to help replacing the O2 sensor either with a video or a step by step documentation I will really appreciate it.
Thanks

Oh man - I just replaced mine 2 weeks ago but I didn't take any pictures, sorry. I have the 4.2 and the passenger side was a b*tch.

For the passenger side you need to remove the MAF/intake hose to at least have some sort of access to it. At this point you might want to disconnect the sensor already from the ECU. Get a very long extension (at least 12" or longer!) and a regular O2 socket with a swivel head. First take the socket and guide it onto the O2 by hand. This might take a while and be patient (took me 60 minutes because I dropped the socket like 10 times). Once the socket is on, get the extension and swivel head into it and start using the ratchet (use pb blaster if it is stuck). Once it is out, screw the new one in by hand, attach the socket, use the ratchet again (or preferably torque wrench). This literally took me almost 2h hours due to the roomy restrictions in that area. I'm sure now that I have done it it will be under 1h.

The drivers side takes like 10 minutes:
I used a crowfoot O2 socket for this side for easier access. First remove the coolant reservoir, disconnect the O2 connector going to the ECU and just like above guide the socket onto the sensor with your hand first, then attach the ratchet and remove it.

With both sides I used the extension and roughly the same angle from the center back end of the engine. This was the best access for both in my case.

FYI earlier in this thread there was a much more detailed list of how to attack this: https://www.audiforums.com/forum/aud...2/#post1319256

Check it out if you need additional help or simply ask more questions :)
Good luck

Thank you so much for the instructions. Would you know which oxygen sensors would fit in the 2003 Audi A6 3.0 on the drivers side and passenger side. I did research and looks like driver side has Bosch 16132 Oxygen Sensor, but I am not quite sure of the passenger side.

The easiest way to make sure you buy the right ones is to check the part numbers @ Bosch Vehicle Part Finder

There are 3 listed for your car:
17353 Oxygen Sensor - Upstream 2 per car
16978 Oxygen Sensor - Downstream Right 1 per car
16132 Oxygen Sensor - Downstream Left 1 per car

If you want to replace the pre-cat ones (before the cat, which was the instructions I posted) then you need 17353. Those are identical, so all you need to do is buy 2 of them.

16978 and 16132 are for post-cat (after the cat).

The Bosch Oxygen sensors with the numbers I posted are direct fit and no modifications are needed. If you decide to go with the universal ones (different part numbers and a little bit cheaper) then you will have to make your own connector and figure out the wires. I wouldn't go cheap here and just spent the extra dollars to get the "plug and play" version unless you love wiring ;)

yep, thanks I think I ll go with Bosch parts since I am not into wiring and stuff. Also by any chance would you know for the error code P0421 and P0431 which oxygen sensors would I need to replace?

I mean pre cat or post cat? I am sorry for asking so many questions but this is fairly new to me.. I really appreciate all your help. Thank you!!!:):)

Now with these codes it doesn't really specify which ones but since they are related to the cats only I'd suggest the post cats. Another thing you probably don't want to hear is that if your still have the same error after replacing the O2s then it is possible the actual cats are bad (and they are expensive $$$$). Also the O2s should be replaced once you reach the 100k mark (or when you get the CEL).
So if you have more then 100k and those things and have never been replaced, do all 4 at the same time since it is always better to have them within the same age range.

Unfortunately I cannot guarantee that replacing just the O2 will make you CEL go away since as stated above there could be more then 1 reason you have the CEL. I read that this can also happen if your engine is running rich. Anyhow for now if you want to go the easy route, check for vacuum leaks and replace all 4 o2s.

I have about 77000 miles on my car and pretty positive the O2s have never been replaced. I will go ahead and replace the post 02's first and then if the CEL disappears great else will have to replace the other 2 and check for vacuum leaks. I really appreciate all your advice and will let you know if I have more questions. Thanks

Wasting $ on O2's = no good
 

I never pay the $100 + that the actual O2 w/ OEM plug for the O2's. A $30.00 a sensor solution would be to find any Bosch "universal" O2 sensor. You simply remove the old sensor, cut the wire from the old one all the way at the bottom basically touching the sensor, and splice the wires of the new sensor with the old wire that has the plugged attached.

The Pre cat O2's are the ones that take most of the beating and will need to be changed at some point. If your post cat O2's are having problems something else is the culprit, probably your cats.

Yup, I'm afraid of that too :(
Good luck Audi9090.

Thank you so much the instructions. I have a 2002 Audi A6 that seems need to replace the o2 bank2 sensor 1 due to CEL. However, since I am new with o2 sensor, I couldn't figure which one it is. I saw two O2-like sensors right behind the engine. One is straightly attached to the exhaust pipe by looking from the hood and the other is attached slightly from the left side which is closer to engine. Can you tell me which is the bank2 sensor 1?

Also, I am not sure which connector is the ECU for o2 bank2 sensor 1 since I couldn't track wire from the sensor as the room is so restricted for my hand. Pictures would be great help.

I'll check if I can take 2 or 3 pictures for you tomorrow and post them but to give you some information right now:
Bank 2 is the drivers side (so the easier one) which requires you only to remove the coolant reservoir. The wire connector is also easy to locate since it is the only one zip tied to the wall of the back side of your engine compartment. Once you have the reservoir off you should see it right away.

I would advise you to replace both bank 1 and 2 at the same time (depending on mileage and time of last replacement of both). It's best to have 2 O2 front sensor at about the same age so the engine doesn't get confused when it spots different readings for the engine (in 2 different banks).

Thanks for the reply. I have finally manage to replace the bank 2 sensor 1. What is job! The CEL is gone now and hasn't back on after driving over 50 miles today.

Just for an update, I replaced the bank 2 sensor and fixed the code p0050, but today CEL was back on again with code p0431. Any idea what I should do now?

Going to resurrect this in case you havent been helped.

Ive gone through replacing 02's first also as a precaution... but they are almost NEVER the problem. I havent personally experienced this fix yet, because ive been saving money, but ive heard from tons of other Audi A6 guys, and performance dealers near by (Like NGP in Maryland).

The sensors throwing the code are Pre-cat 02's. But they are likely fine. In most cases they can go well over 100k miles. The Problem is the pre-cat itself. As the pre-cat ages the honeycomb begins to fail... but ONLY to a MINOR extent. The most minimum degradation to the pre-cats will throw the light. Ive had mine on for over a year it goes on and off every week or so sometimes it's off for almost a whole month. There is no performance or gas-mileage change. It's really just an annoying Audi thing.

The 02s are set up in the Audi almost retarded anyways. There is a pre-cat 02 then the pre-cat, then another 02 and then the MAIN CAT. The system should really only measure the 02 from before and after the MAIN CAT but Audi did not manufacture the car this way.

FIX = have your pre-cat's gutted out and welded back in so its clear (illegal, but ... you know), or you can purchase something like Piggie Pipes which is the same process already completed by Vast Performance. It relocates the main-cat 02 sensor BEHIND it (to the back end of the car exhaust) so that it prevents the CEL and the 02 sensor gets a proper reading. This is honestly how the Dealer should have built the car but they didnt.

Hello ,

I have a 2003 Audi A6 3.0L FWD could anyone tell me whats the bosch part number for Oxygen sensor Bank1 sensor 2?? or could some give me a link where I can order it I will really appreciate it. also can I get my audi repaired for cheap some where in or around Phoenix, Arizona.

Please let me know.

THanks

Best shot is to go online and use the many databases available to determine the part number specific to the engine size/yr etc.

Oxygen Sensor - Auto Parts Warehouse

Thank you for your reply. I visited the website but nowhere on the website it tells whether it is Bank 1 sensor2 or not. The error code keeps telling bank 1 sensor 2. Does any know what is the Bosch part number for it, will really appreciate it?

thanks

I had the same problem with finding B2S1 oxygen sensor for my 2003 A6 3.0L FWD.
I did as follows:
- I took a very close look at the electric connector (or number of wires in case you want a general purpose one).
- I search on amazon.com by "oxygen sensor before catalytic converter" and then they have a feature to check compatibility by year/make/model and I purchase the one having the same electric connector.
It worked for me.

In your case, B1S2, I think you have to search by "oxygen sensor after catalytic converter".