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Old 01-08-2008, 09:24 AM   #1
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Default Oxygen Sensor replacement (pre-cat) on 2001 A4 2.8

I need to replace some o2 sensors on my car. I have a 2001 2.8quattro (30v), and I cannot find anything related to the procedure. I know they most likely screw out of the exhaust, but wasn't sure what else I may need in regards to sealing them (teflon tape? anti-seize?) and/or o-rings.

Is there a FAQ on this? I didn't see one, although I did find a very helpful article on the tech articles about how to diagnose which one is bad (getting a heater circuit malfunction on bank 1, for sensor 1 and 2).

Another question is that I think I may just replace both pre-cat sensors, as my understanding (from a previous thread) is that they are the two which determine fuel mixture, and I have been seeing some horrible gas mileage (I am replacing coolant switch along with the o2 sensors).

Thanks in advance.
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Old 01-08-2008, 10:19 AM   #2
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Default The bosch oxygen sensors i got for my 2.8 came with anti-sieze already on the threads.

The link is a post about how i replaced my pre-cat o2 sensors.

The precat O2 sensors in conjuntion with the MAF are used by the ecu to determine fuel mixture. When o2 sensors go bad the car defaults to a programmed fuel trim which allows the car to run, but not as efficiently as it can with the o2 sensors.

What kind of gas mileage are you getting now and how are you calculating MPG?
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Old 01-08-2008, 02:30 PM   #3
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Default Getting 23-24 highway, 15-16 stop and go.

Before I would get 18 stop and go, and 28 or so on the highway (beltway, ~60mph).

If it's very cold outside, I can smell gas out of my exhaust when I first start the car. Maybe it's something else? I was also going to replace the coolant switch, but my temperature guage never fluctuates from straight-up (as normal).

I just got back inside from testing the resistance on the sensors, and all 3 (green, brown, and black) show 11.5 ohms, which should be fine. I guess I need to vag again, maybe the codes are clear now? Any idea what might be going on?

FYI, I used this article to determine how to read the resistances: http://www.audiworld.com/tech/eng.html
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Old 01-08-2008, 03:14 PM   #4
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Default In regards to your linked post...

I have a green, brown/red and black plug on the passenger's side of the firewall, all were 11.5 ohms of resistance. After reading your post I found a black plug (4 pin, exactly like the others) closer to the driver's side, under the coolant resevoir... it had 9.5 ohms resistance.

Was this the other pre-cat os2 sensor plug?

However, still doesnt' matter, none of the o2 sensors "appear" to be bad judging from these results (the heating element, anyhow).
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Old 01-08-2008, 08:28 PM   #5
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Default 2 parts

First is the heater which preheats the O2 sensor so it reads faster and you get into your emissions control loop faster. Either it works or not. It'll throw a code if open or shorted to ground. Even if it were bad once the car is warm it will work just fine.

Second is the sensor itself. The only static test is short to ground for the sensor itself. You really have to measure voltage and with a scope at that to see if they're working. A DVM will give you a good idea if it's any good or not.

You should:

See if you have any codes for any of the O2's.
See what the voltages are for each O2 to see if they're close ot range.
Look at long term trim to see if it's trying to compensate.

How old are they?
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Old 01-09-2008, 04:21 AM   #6
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Default 2 questions :-)

Thanks for your input.

2 questions:
1) What is a DVM?
2) How can I check the voltage on the o2 sensors?

The car has 90k miles, I bought it at 48k... I have never replaced them. So unless there was some issue with them (faulty), then I doubt they have ever been changed. I think the recommendation is 100k?
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Old 01-09-2008, 05:24 AM   #7
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Default DVM = Digital Volt Meter, Check all the sensors, they should read pretty close to one another

Checking it out with VAG Com is the best way to check the health of the sensors. search around in the VAG forum and the B5 forum, theres tons of info on what measuring blocks to look at and whatthey should read.

In my case, i was getting codes from both the O2 sensors and the MAF and ended up replacing both.

If you do end up replacing, I picked up both front O2 sensors (Bosch) for around 100 bucks a pop from autohausaz.com

I can give you a more details on the install if you need it.

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Old 01-10-2008, 04:02 AM   #8
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Default Poor mileage? Coolant temp sensor checked?

If you are just guessing, there are cheaper places to start.
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Old 01-10-2008, 05:20 AM   #9
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Default ^ What he said

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Old 01-10-2008, 06:07 AM   #10
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Default FYI, I average around 21-22 mpg overall with a brand new MAF, O2 Sensors, and Coolant Temp Sensor

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Old 01-10-2008, 06:07 AM
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2001, 28, 28l, a4, audi, bank, brown, cat, green, malfunction, miata, o2, oxygen, pre, precat, replace, sensor, voltage


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