VCDS method of finding oxygen/lambda fault?

#1 2nd February 2010, 03:30 PM

Hi folks. My '02 S8 has just gone through the MOT again (spent a freezing weekend in january replacing all 8 front control arms & track rod ends) and just scraped a pass - emissions were only just within tolerances.

My question is this, I have the engine light on, it's been on since I had some sports cats fitted at a place in Sheffield about 6 months ago. VCDS reads the fault as "CAT bank 1 & 2 inefficient" or something along those lines.

The MOT guy said the cats were fine - the emissions suggest it's more likely a dodgy oxygen sensor. He has a diagnosis machine that he reckons can pinpoint which sensor is at fault, but I'd like to try and find out for myself (can't afford his hourly rates

).

Can anyone give me a run-through on how this is done using VCDS, or can you point me to a guide some place?

I know there are supposed to be pre- and post- cat sensors, but other than that I'm ignorant to these things so any advice much appreciated.

Conan_the_Librarian · #2 14th February 2010, 10:55 AM

The chances are your sports cats are 100CPI and the downstream LAMBA is picking up out of tolerance readings. There is a post on Audiworld about fitting recessed Lamba holders which will cure the engine management light and stop the sensors throwing codes. Have a google for it. If I come across it I will post.

#3 27th March 2010, 01:01 PM

Hi folks. Well I'm sat here totally at a loss as to how to trace which of these o2 sensors are goosed. Here's my VCDS log:-

Address 01: Engine Labels: 4D0-907-560-AVP.lbl
Part No: 4D0 907 559 AA
Component: 4.2L V8/5V G 0002
Coding: 10753
Shop #: WSC 02313
4 Faults Found:
16814 - Catalyst System; Bank 2: Efficiency Below Threshold
P0430 - 35-00 - -
16804 - Catalyst System; Bank 1: Efficiency Below Threshold
P0420 - 35-00 - -
18047 - Accelerator Position Sensor 1/2 (G79/G185): Implausible Signal
P1639 - 35-10 - - - Intermittent
18047 - Accelerator Position Sensor 1/2 (G79/G185): Implausible Signal
P1639 - 35-10 - - - Intermittent
Readiness: 0000 0000

Address 03: ABS Brakes Labels: 8D0-907-389.lbl
Part No: 4D0 907 389 E
Component: ABS/ESP allrad D56
Coding: 08397
Shop #: WSC 02313
1 Fault Found:
01119 - Gear Recognition Signal
35-10 - - - Intermittent

Address 17: Instruments Labels:
Part No: 4D0 919 035 BA
Component: KOMBI+WEGFAHRS.VDO D39
Coding: 00483
Shop #: WSC 02313

No Faults Found or DTCs not supported by controller or a communication error occurred

Address 56: Radio Labels: Redir Fail!
Part No: 4D0 035 192 M
Component: Radio BNO 0001
Coding: 00117
Shop #: WSC 00726

1 Fault Found:
00852 - Loudspeaker(s); Front
36-00 - Open Circuit

I checked with the place that did my cats, they haven't sold or fitted 100cel cats for about 6 years due to MOT failures and too many customer complaints. So I'm confident the cats are 200cel.

I'm about to delve into VCDS measuring blocks to see if I can suss out which sensors are giving out dodgy readings - anybody know what I should be looking for? I did a trial run earlier and had a look through all the different readings and blocks, so I've an idea where to look now, I just don't know HOW to look and WHAT to look for

I've got Elsa on the go now so I'll report back if I find anything.

#4 27th March 2010, 04:21 PM

Hmm, well I guess I'll have to delve deeper. Results:-

Group 034: O2S Aging Bank 1
760 /min Engine RPM 650 to 720rpm
365.0°C Exhaust Temp >200 Celsius
0.64 s Time Period 0.1-1.8s
B1-S1 OK Diag Status Test/ON/OFF|OK

Group 035: O2S Aging Bank 2
760 /min Engine RPM 650 to 720rpm
365.0°C Exhaust Temp >200 Celsius
0.56 s Time Period 0.1-1.8s
B2-S1 OK Diag Status Test/ON/OFF|OK

Group 036: O2S Beyond TWC [B=Bank S=Sensor]
0.845 V Volt Supply B1S2 0-1V
B1-S2 OK Diag Status B1S2 Test/ON/OFF|OK
0.840 V Volt Supply B2S2 0-1V
B2-S2 OK Diag Status B2S2 Test/ON/OFF|OK

Group 037: Oxygen Sensor Control System Bank 1
15.8 % Engine Load 13.5-22.5%
0.080 V O2S Volt B1 S2 0.1-1.0V
0.0 ms Corr val B1 S1-S2 -200-200ms
System OK Diag Status Test/ON/OFF|OK

Group 038: Oxygen Sensor Control System Bank 2
15.8 % Engine Load 13.5-22.5%
0.095 V O2S Volt B1 S2 0.1-1.0V
0.0 ms Corr val B1 S1-S2 -200-200ms
System OK Diag Status Test/ON/OFF|OK

Group 041: Oxygen Sensor Heater Bank 1 @Idle
0.10 kOhm Sensor 1 Resist 0-0.9 KOhm
Htg. S1 ON Heater Bf TWC Status
0.20 kOhm Sensor 2 Resist 0-0.9 KOhm
Htg. S2 ON Heater Af TWC Status

Group 042: Oxygen Sensor Heater Bank 2 @Idle
0.10 kOhm Sensor 1 Resist 0-0.9 KOhm
Htg. S1 ON Heater Bf TWC Status
0.10 kOhm Sensor 2 Resist 0-0.9 KOhm
Htg. S2 ON Heater Af TWC Status

Needless to say, Group 046 test failed again, same inefficient story.

Do these values look ok for the O2 sensors? I presume they must be as the tests concluded as ok with VCDS.

Conan_the_Librarian · #5 27th March 2010, 04:46 PM

I'm out of my depth here. I'll tip Ben off at AVP see if he will comment.

Just_Ben · #6 27th March 2010, 05:26 PM

Got ya PM Mike!

This is a definate issue with the CATS being of a different tolerance to Audi's own. The car will pass an MOT with flying colours even though it won't pass the car's own on-board emissions control checks.
The post CAT lambdas sole job is to monitor the condition of the CATS, the pre-CAt ones doing the job of fuel metering.
The CATS will jsut be not as good as Audi's original, therefore the only fix I have used in the past has been a mod in the engine ECU map. Usually you only have this issue on tuned cars so a map mod is simple and req'd anyway. The modified map just adjusts the sensitivity of the post CAT lambda's (or removes them if no cats are fitted!!)

Hope this helps - don't go spending on new Lambdas!!! You'll be wasting ya cash!