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Author Topic: Post Cat Correction - 2001 Audi S4  (Read 24360 times)
phila_dot
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« on: April 11, 2011, 12:23:37 AM »

Despite popular belief that the secondary O2 sensors are for emissions puproses only, I am seeing evidence that they actually have a fairly large impact on performance. 

It all started when I gutted my pre-cats. I installed double-stacked spark plug defoulers to avoid the dreaded CEL from catalyst efficiency codes. When I did this, I did not drill them out at all as some have suggested. I had believed this was done solely to accommodate the sensor and instead used the spacers provided with the defoulers. After a short while, I began experiencing an significant loss of power during part throttle (almost no off-boost power) and my CEL began coming on every 800 miles or so. A quick scan with VAG-Com displayed P1176.

1 17584/P1176/004470 - Bank1: O2 (Lambda) Correction Behind Catalyst: Control Limit Reached

I did not think much of it at the time, so I just cleared it when it came up and continued on. At some point I decided to check the code in the Bentley and was very suprised by what I found.

Quote from: Bentley
Oxygen sensor control behind the catalytic converter is responsible for final control correction. It is the primary sensor and can override the signal from the oxygen sensor control before the catalytic converter.

Oxygen sensor control behind catalytic converter is superior to  oxygen sensor control before catalytic converter and is the primary control. It corrects slight changes in the mixture (i.e. enrichment or thinning) by holding oxygen sensor control before catalytic converter at its highest or lowest point for a specific time (duration). If this time is in the positive range (i.e. 50 ms), mixture is shifted in the -enrich- direction. If this time is in the negative range (i.e. 50 ms), mixture is shifted in the -lean- direction.

After checking my fuel trims and seeing that they were all within a few percent, I once again disregarded it. 

A few months ago I threw a secondary O2 Sensor heater code which activated my CEL. This prompted me to code out my secondary O2's.  Recently, my CEL came back on, primary O2 sensor this time. So I did what any responsible Audi owner would do and replaced both primary sensors immediately. I did this thinking that it would solve all of my problems. It didn't.

Approximately three hundred miles later, remembering my suspicious P1176 research, I unplugged my secondary O2's as a last effort. Almost immediately, my part throttle power improved immensely.

I was searching through the Funktionsrahmen keyword "Hinter Kat" and had some promising hits, however, I recentely moved and don't currently have the internet on my laptop.

Could this have just been a result of adaptation slowly taking effect after replacing my primary O2 sensors?

Has anyone found any maps relating to secondary O2 correction?

Has anyone come across anything related to post cat correction in the Funktionsrahmen?
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coreyj03
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« Reply #1 on: April 12, 2011, 10:09:57 PM »

so can we just code the secondary o2 out so they dont throw a CEL and un plug the rear o2's.

i have a heater circuit code for a secondary o2 and i have also intermittent loss of power in off boost conditions.
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phila_dot
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« Reply #2 on: April 13, 2011, 10:20:35 PM »

Yes, I just coded them out and unplugged them. Problem solved, no CEL, and readiness is set.
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phila_dot
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« Reply #3 on: April 16, 2011, 02:12:05 PM »

I would love to hear some feedback, insight, opinions on this.

Just a quick dash through the G box OLS, it looks like module LRSHK controls post cat O2 correction. I have not looked at it in detail yet though.

« Last Edit: April 20, 2011, 12:07:00 AM by phila_dot » Logged
vwaudiguy
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« Reply #4 on: April 19, 2011, 11:55:41 AM »

I believe the second 02's are there as well for more than just cat monitoring. I had a friend (vw tech) have a problem with his 1.8t golf dumping fuel, and after changing the second 02 problem resolved. It was so long ago he doesn't remember the exact details like fuel trims, etc. I always keep this example in the back of my head when trying to troubleshoot problems..Maybe this isn't a good thing?  Smiley
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phila_dot
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« Reply #5 on: April 20, 2011, 01:17:46 AM »

I found a little more information looking through the funktionsrahmen tonight. I also think that I found the codeword to disable post cat correction.
« Last Edit: April 20, 2011, 01:21:42 AM by phila_dot » Logged
phila_dot
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« Reply #6 on: April 20, 2011, 01:22:01 AM »

Quote from: Funktionsrahmen
The regulation by the probe behind Kat is the lambda control with the probe before the catalytic converter by way of superimposed.
 Control action on the legislation before the catalytic converter, the Delta Lambda Korrekturgr variables dlahi_w and dlahp_w.


Control is post Cat:

Can be disabled by setting Bit 0 in code word CLRSHK to 1 (FALSE).

Effective regulation P-/I-:
The regulation behind Kat is a regulation by integral / proportional action. The P-share dlahp_w immediately,
 because he does not have "memory ", after changing the sensor voltage with the correct sign with respect to the control position
 by the delta-lambda-through enriched or lean.

Through the integral component of the regulation behind the catalytic converter is LRSHK capable of exhaust deterioration caused by a shift
 the static probe characteristic can be caused to compensate to a large extent.

The calculation of LRSHK is continuous on lambda level. For this purpose, first the probe voltage ushk_w by way of the
 Characteristic LALIUSH (lamsonh_w) linearization. A similar linearization with the voltage set point USRHK
 made (lamsolh_w). The pseudo-feedback is lamsonh_w project specific operates by way of code word CLRSHK resold.

 a) directly, (-> default in Modulating Before regulation cat, as intervention all is possible 10ms)
 b) filtered by way of a PT1-element (-> project-specific)
 c) fr-synchronously averaged (-> default for two-point control, since depends only before tv fr-jump prospective is)

as a rule lamhm_w subtraction dlashkm_w are fed.

By assessing the characteristics of and KDLASHKP KDLASHKI, the control difference dlashkm_w
 will be corrected separately according to the catalyst properties before the calculation of the P and I_ proportion.
 The distorted or control differences dlashkp_w dlashki_w result now by weighting by KPLRHML = f (ml) of the P-proportion
 dlahp_w, or by weighting by KILRHML = f (ml) of the I-proportion dlahi_w.


Code word CLRSHK
 ----------------
 To w application during the treatment of the adaptation value dlahi_w bein k it is possible to influence, has the code word
 CLRSHK inserted leads. The meaning of the control bits in CLRSHK are described in the block comments.
 Useful combinations - in decimal - are listed following leads:
 
 CLRSHK = disabled is odd LRSHK
 CLRSHK = 16 dlahi_w resetting erasure is at fault memory l with the value DLAHIINI
 otherwise default state for LRSHK
 CLRSHK = 24 dlahi_w is resetting during engine start with the value DLAHIINI
« Last Edit: April 20, 2011, 01:23:54 AM by phila_dot » Logged
phila_dot
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« Reply #7 on: April 20, 2011, 01:24:21 AM »

0x11A87 CLRSHK  Code word for Lambda - Control post Kat on/off (original value 72) set to odd?
0x19FCF CLRSHKA Code word for Lambda - Control post Kat          (original value 0)   set to 1?
« Last Edit: April 20, 2011, 01:27:02 AM by phila_dot » Logged
nyet
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« Reply #8 on: April 20, 2011, 09:49:19 AM »

0x11A87 CLRSHK  Code word for Lambda - Control post Kat on/off (original value 72) set to odd?
0x19FCF CLRSHKA Code word for Lambda - Control post Kat          (original value 0)   set to 1?

Sounds reasonable. Added to tuning wiki. However, in some DAMOS they may be CLRHK and CLRKHA?
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« Reply #9 on: April 23, 2011, 12:55:42 AM »

I have attached the full (except schematics) LRSHK module translated to English.
« Last Edit: April 24, 2011, 11:00:47 PM by phila_dot » Logged
nyet
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« Reply #10 on: May 12, 2011, 10:15:14 AM »

0x11A87 CLRSHK  Code word for Lambda - Control post Kat on/off (original value 72) set to odd?
0x19FCF CLRSHKA Code word for Lambda - Control post Kat          (original value 0)   set to 1?

Added to latest map pack

http://www.nefariousmotorsports.com/forum/index.php/topic,200.msg4966.html#msg4966
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« Reply #11 on: May 12, 2011, 11:04:38 AM »

In my testing I did not find that the post cat O2 sensors made a difference if they were connected or not. I only tested with working O2 sensors though, so it is possible they help when one or more of the O2 sensors is failing.

In my car I completely disconnected the rear O2 sensors. Then I disabled the cat efficiency code, and the codes for the rear O2 heaters.
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« Reply #12 on: May 17, 2011, 10:29:56 AM »

In my testing I did not find that the post cat O2 sensors made a difference if they were connected or not. I only tested with working O2 sensors though, so it is possible they help when one or more of the O2 sensors is failing.

In my car I completely disconnected the rear O2 sensors. Then I disabled the cat efficiency code, and the codes for the rear O2 heaters.
This is exactly what I did in my car.  No problems after several hundred miles of driving.
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phila_dot
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« Reply #13 on: May 17, 2011, 12:00:46 PM »

In my testing I did not find that the post cat O2 sensors made a difference if they were connected or not. I only tested with working O2 sensors though, so it is possible they help when one or more of the O2 sensors is failing.

In my car I completely disconnected the rear O2 sensors. Then I disabled the cat efficiency code, and the codes for the rear O2 heaters.
This is exactly what I did in my car.  No problems after several hundred miles of driving.

Yes, that is what corrected my problem. Code out and uplugged sensors. I plan to play around with disabling correction when I get the chance.
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phila_dot
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« Reply #14 on: May 22, 2011, 01:39:34 AM »

Messing around with VCDS tonight I decided to check my secondary O2 sensor readings. With the sensors unplugged, the readings oscillated from 0.435v to 0.43v. I believe this to be ~Lambda 1.

I then plugged them in and of course the readings are all over the place. The average voltage bank 1 was 0.274, while bank 2 averaged 0.9 volts.
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