Oxygen Sensor replacement on C7 A6 3.0T

[Tom Cruise]

Bought a new toy recently, an OBD Bluetooth dongle and played around to explore what use has it got.











I created a few pages on the app to analyse the O2 sensors as I have been getting poor fuel consumption. Here's one of the page with 2 graphs that maps the post catalytic oxygen sensors (Sensor 2). The data are snap shot of the sensor reading during real live driving in traffic condition that goes through lots of variety of throttle input. Basically, the falling lines represent the moments where I was decelerating and foot off throttle.







The TOP graph maps the Bank 1 (driver side) Sensor 2 reading while the bottom graph maps the Bank 2 (pax side) Sensor 2.







To illustrate, bank 1 exhaust is the exhaust for cylinder 1,2&3 located at the driver side. While bank 2 refers to the exhaust for cylinder 4,5&6 at the passenger side. Each set of exhaust has its own catalytic converter. Each converter has 2 oxygen sensors. Sensor 1 being the pre-cat sensor just after the manifold. Sensor 2 located on top and middle of the catalytic converter.



Being a V6 engine both side should produce similar if not close reading especially during cruising deceleration (very lean) or during acceleration (rich). However, I noticed Bank 2 Sensor 2 is retarded and slow to react. See the graph of the lower map at the last raise of voltage (engine went into idling, zero load and vehicle stopped at traffic), it happened later than the top graph. Next is the lack of details (absence of peak) in the bottom graph compared to the top especially at the centre X axis area.



This have previously thrown out CEL in Feb but after I cleared the fault it did not happen again. Which means the problem is not bad enough and the sensor has not failed completely. Despite so, it does have an effect and contribute to higher FC (though only a small % effect as fuel trim are mainly command by sensor 1) and in the long run killing the catalytic converter with the rich mixture. If left unattended, eventually it will damage the cat, which the replacement cost will be in the range of $2k - $3k. Thus, I rather keep my sensor in good condition and let the engine does its work properly.



The graph plot helps to pin point the problem as the fault code just refers to Bank 2 below threshold efficiency. This could means a plugged catalytic converter (worst case), faulty sensor 1 (pre-cat) or faulty sensor 2 (post cat).





Mode 07: Pending emission related fault codes



Address 7E8: 1 Fault Found:

P0430 - Catalyst System; Bank 2: Efficiency Below Threshold





With clear evidence based on the graph that the bank 2 post cat sensor has aged and is retarded, I proceed to indent the post cat sensor part number 06E906265AA. At this juncture I have driven with the retarded sensor for 2 months and about 2,000km since I last erase the fault code.









Will write in the next post on the replacement steps and experience....





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[Tom Cruise]

Before I move on to the actual replacement procedures, I wish to elaborate further on the cause and effects.

As mentioned in the previous post that a damage or aged oxygen sensor is equivalent to a slow acting poison, killing the expensive catalytic converter slowly while consuming more fuel and costing more for every km travelled.



That, is the consequence and effect. It is thus also worthwhile and important to talk about the cause. Specifically, what causes the oxygen sensor to fail.



Oxygen sensor are designed to last at least 120,000km and could work properly maybe up to 160,000km. That's about it considering the Super extreme hot and toxic environment it resides in, but typically you would experience problems sooner than that. My previous A4 had a failed sensor 1 at 80k and current A6 failed sensor 2 also at 80k.

Over time, an oxygen sensor may become coated with byproducts of combustion, such as sulfur, fuel additives, oil ash, etc. This contamination causes the sensor to lose its ability to produce voltage and send the right signal. i.e. This reduces the sensor's ability to respond quickly to changes in the air/fuel mixture. The sensor slows down and becomes "sluggish".

At the same time, the sensor's output voltage may not be as high as it once was, giving the false impression that the air/fuel mixture is leaner than it actually is. The result can be a richer-than-normal air/fuel mixture under various operating conditions that causes fuel consumption and emissions to rise.

A rich operating condition causes the converter to run hotter than normal. If the converter gets hot enough (above 900 degree Celsius), the catalyst substrate inside may actually melt forming a partial or complete blockage. The result can be a drastic drop in high rpm performance or stalling in severe case because of a buildup of backpressure in the exhaust system.

Using low-quality petrol may also speed up the oxygen sensor failure. And if you are skipping maintenance, especially things like spark plug (60k servicing) and air filter replacement (every 30k), you are increasing the likelihood of incomplete fuel combustion, which in turn leads to more dirt and grime in your emissions system. A bad ignition coil that causes frequent misfire will add to the likelihood of incomplete fuel combustion. Note that Audi ignition coil starts to degrade as early as 50,000km and most failure occurs around 70k - 90k in the past (pre 2010 batch).

Also worth to note that given the fundamental design of the oxygen sensor, the Zirconium dioxide ceramic bulb can also be quite fragile and susceptible to damage from severe shock or constant severe vibrations (bad engine mount or frequent knocking or misfire).



One thing for sure, if you are reading this post, you probably are one of the Audi owner that also experience engine oil consumption. If the excessive oil consumption you experienced is quite bad, and your car is near to end of warranty period or already beyond 3 yrs then you better be worry as the contaminants of phosphorus from excessive oil consumption will cause premature oxygen sensor failure.

However, it’s difficult to spot a failing oxygen sensor. While it will inevitably lead to decreased FC, but it’s usually not drastic enough for an average driver to notice. The decrease is gradual and happens over time, so unless you are keeping records of every top up, you will likely miss these signs. A bad or failing O2 sensor can also cause you to fail the LTA emissions test during vehicle inspection.

To summaries the general cause of oxygen sensor failure:
1. Contaminants from engine oil burnt
2. Contaminated Air or Low quality fuel
3. Contaminants from Incomplete combustion (misfire, degraded ignition coil, rich mixture vicious cycle)
4. Excessive vibration from worn engine mount

With reference to the cause, here's the possibility of reference for my sensor failure:
1. Consumed 1L of engine oil per 7000km
2. Link to my other post, I reported on recent replacement of broken air pipe may be a good contributing factor of contaminants
3. At 80,000km some of the 6 ignition coils may have degraded. Pending further investigation.
4. Recently replaced engine mount due to excessive vibration.

While, I have replaced the sensors, it's also important that I looked through the possible cause and addressed all 1-4 to prevent further failure occurrence.

[Tom Cruise]

Audi C7 A6 3.0T V6 oxygen sensor removal
Bank 2 Sensor 2

(Some may wonder why do I DIY? Part interest, part to save half the cost and the deciding factor is because I requested my regular workshop to replace the sensor but they chose to ignore my request despite I sent 3 massages. I don't usually like to chase ppl so I chose to DIY instead of being left in limbo)

Sharing on the process and procedures can be quite abstract as the sensors connector is located beyond sight and takes a lot of touch feel to locate it then disconnect.

The steps however is pretty straightforward and took me about 20 minutes of trial and error to complete. I saw from other Ang moh forums the process they took sibeh Chim and some claim it take 4 hours, some remove lots of engine parts to access. *faint* And most mentioned using what liquid wrench etc... Which we don't really heard of in Singapore. Maybe they mean WD40 but still it's unnecessary and avoid it for fear of contamination in cat or causing a fire when the exhaust becomes hot when driving.

Basic steps outline as follows:
1. Remove oxygen sensor connector from the bracket
2. Disconnect the sensor from the connector
3. Remove the old sensor using a socket and wrench
4. Replace the new sensor into exhaust
5. Connect the connector and clip it back to the bracket holder

What u can expect:
1. Very hard to break the torque of the sensor as its factory tightened with 50lbs/ft force (will elaborate later) and prolong heating that almost fuse the sensor thread to exhaust
2. Hard to reach the connector
3. Brittle plastic connector clip due to prolong exposure in high temperature that most likely will chip off when trying to disconnect. Just the clip that break off and is not critical. So don't worry.

Tools required :
1. Oxygen Sensor socket (special tools see pic below)
2. Socket extension bar (I used 25mm + 15mm)
3. Socket wrench or T-bar preferred due to high torque required to remove the old sensor
4. Cheater bar (hollow strong metal tube to extend the length of the wrench handle to increase leverage force)

This is the oxygen sensor socket. It's a normal 22mm deep socket except with a cut slot at the side to allow the wire to pass thru. Otherwise with the big fat connector of the sensor, it's not going to thread through the 22mm diameter socket.


This is how the tools look like after assembled. Ignore the flexible joint attached before the socket. Shouldn't use the flexible joint to break the torque of the old sensor as I'm pretty sure the joint will gives way.


For illustration on cheater bar applications, here are 2 photos found online to give an idea. Basically, it's a hollow pipe that slot over the handle of the wrench so we increase the length of the handle in order to achieve a force of 50lbs/ft or about 70 N/m force (70kg of force at 1 meter length of lever).


I will elaborate on the steps in my next post. It will be quite descriptive as I did not take much photos while doing it. Nonetheless, will try to google some photos online to give a better illustration.


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