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OBD Scanners Discussions on using On Board Diagnostic (OBD) scanners/ Diagnostic Equipment related to TDIs

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Old February 22nd, 2013, 07:10   #61
Zrnojed
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Thank you,FlyTDI Guy,but I found out that the EGR cycle is possible to control,and on my TDI is set to "city" mode(where EGR is returning more gasses into engine,so less polution is going on) It can be reduced to "normal"(where EGR is returning less gasses into engine),with VCDS.
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Old April 12th, 2013, 03:49   #62
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Great thread.
Thanks for your hard work !
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Old August 23rd, 2015, 05:19   #63
ronaldleemhuis
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I have a 2010 Jetta TDI with about 50,000 miles on the clock. I recently had DPF and EGR cooler replaced because of excessive soot in the tailpipe and insufficient EGR flow rate errors. Those errors, and the excessive soot buildup have apparently been corrected, but I now have recurrent P2271 (rear oxygen sensor rich) errors after driving about 60 miles on the highway. Rather than just replace a rear oxygen sensor, I got out my VCDS to figure out what was going on.

Unfortunately, I was not able to find documentation for the measuring groups most pertinent to this issue. When I go into OBD2 mode, oxygen sensor data are reported as 0 or 1 rather than a continuous value one would expect from a wideband oxygen sensor.

After a lot of Googling, it appeared to me that measuring groups 30-39 would probably relate to oxygen sensors and 40-49 to other aspects of emissions. So with some puzzling, logging, analyzing and reflection, I have noticed the followuing:

Measuring group 36 has four fields:

1: blank, apparently
2: labeled Load -- values range from about 10% at idle to values as high as 100% when driving around.
3. labeled Lambda -- values are very similar to those in field 2 of this group, usually off by a fraction of a percentage point. I suspect this is a load value sampled a fraction of a second later, or perhaps calculated by another method. When I graph field 3 vs field 2, I get a straight line of equivalence with very little error.
4. labeled Lambda -- values in this field fluctuate with driving, usually in the range of -0.2 to 0. Occasional values are well above zero, perhaps +0.5 or so. I suspect this is a continuous signal reflecting measurement of the B1S1 (precatalytic) oxygen sensor.

Measuring group 38 has similar content:

1. blank field, apparently
2. labeled Load. Values in this field vary with driving and range from about 43% at idle to 100%. When I graph this value vs the corresponding measurement in group 36, I do not get a good correlation. This leads me to believe that this is not just a rescaled load measurement. I do not know what this is. (Help me, guys). I wonder whether this is really a number reflecting EGR percentage, so that it is 100% with no EGR and lower as the amount of EGR goes up.
3. labeled Lambda. As in group 36, this value is very similar to the reading in field 2. It is off by a fraction of a percentage point. I suspect it may be a second reading of the same value a fraction of a second later. I still do not know what it is, though.
4. labeled Lambda. This varies with driving, and it hovers around -0.2 at idle most of the time and with driving is usually below 0.0, as is the corresponding measurement in group 36. However, quite interestingly it frequently pops up to apparently preprogrammed values in positive range up to +0.6 or so. Some of these values seem to recur every minute or so, more or less. I suspect this is a continuous signal from B1S2 (rear) oxygen sensor.

In group 42, I found this:

1. a negative value that varied with driving and is at times negative -- not sure what it is
2. not sure
3. 3.300 V This looks like a voltage having to do with signal applied to oxygen sensors -- possible
4. Lambda -- a number that varies with driving and looks like the lambda we would expect, from about 2.5 or 3 typically to values as high as 25. I don't know whether this is calculated from oxygen sensor values or from other engine measurements.

Anyway, when I graphed what looked like the two oxygen sensor signals, they varied as I would have expected and looked OK to me.

I'll try logging groups 36, 38 and 42 and see if I can make any more sense of this.

Ron

Last edited by ronaldleemhuis; August 23rd, 2015 at 11:02.
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Old August 23rd, 2015, 08:20   #64
whitedog
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Ron, how about unplugging a sensor to see if a value goes to Zero?

Thanks for spending the time to do this. Collective wisdom often helps.
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Old August 23rd, 2015, 11:50   #65
ronaldleemhuis
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I think I'll log groups 36, 38 and 42 for a trip to work sometime soon and then print out some pertinent graphs.

Then maybe I'll get my daughter to let me log her Golf TDI doing the same thing.

As for taking out the sensors, I haven't found a way to get to either one easily, even though I know where there are.
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Old August 24th, 2015, 19:43   #66
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Some brainstorming ...

The apparent precatalytic O2 sensor signal from group 36 field 4 usually stays around -0.20 when driving around, with some mild fluctuations. The group 38 field 4 signal that may be the postcatalytic O2 sensor signal spends much of the time also at about -0.20 with periodic triangular spikes every minute or two, peaking at a positive value about +0.50 or so. I think these peaks are periodic episodes of deNOx of the NOx storage catalyst. This could be because small amounts of post-compustion fuel is injected for this purpose. Because it is vaporized but not burned before the first catalyst, it doesn't deplete oxygen for the precatalytic reading. However, oxygen gets depleted into the "rich" region as the fuel-laden gases pass through the catalysts.

Now suppose that my NOx storage catalyst was partially inactivated by the deposition of excessive soot over time because of the cracked DPF. The ecu may have had to cycle the deNOx process more frequently, and the monitoring software may have accidentally captured samples with rich readings just because of that.

If so, I would expect that the high temperature DPF regeneration and the high temperature but less frequent desulfation process may remove soot from the NOx storage catalyst as a side effect. Over several hundred miles of driving the deNOx cycles may become slightly less frequent, and I may stop getting the P2271 error codes (rear oxygen sensor rich).

One thing I learned for sure by looking at the signal was that my rear oxygen sensor itself was sending plausible signals which were apparently in the proper lean region except for the normal deNOx peaks.

Does anybody have any link to literature that may confirm or refute these hunches?

Last edited by ronaldleemhuis; August 25th, 2015 at 15:29.
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Old August 25th, 2015, 15:45   #67
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If measuring group 38 field 4 is indeed the rear O2 sensor value, my logs indicate that it is cycling back and forth every few seconds rather than the more conventional one to two minutes used for NOx storage catalyst. Is this what it is supposed to do? Does it reflect something gone wrong? Did the most recent flash to the ecu speed up the cycling? Got me.
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Old August 31st, 2015, 08:09   #68
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For my 2010 Jetta TDI, it appears that measuring group 38, field 4 is the rear oxygen sensor signal. On a typical 7 mile drive, I found that from time it would stay lean at about -0.2 for maybe 10-20 seconds or longer (typically at idle). During regular driving, it jumped back and forth from lean to a +0.5 (rich?) reading. I presume this is from some mild postinjection fueling to cycle the NOx storage catalyst. This is the latest ECU flash, and I suspect it takes advantage of a new fast cycling technology. It cycles every 2-3 seconds.

I found that it spent 30% of the time above zero. The amazing thing is that my daughter's Golf TDI, never than mine, did the same thing and also spent 30% of the time above zero.

I suspect that a lot of owners who think they need a new rear oxygen sensor really have their NOx storage catalyst fouled with soot making its way through a cracked DPF. I think the deSOx measures can clear this soot and restore function of the NOx storage catalyst as long as ongoing buildup is not excessive. Maybe this is why my dealer suggested that I need not replace the rear O2 sensor after my DPF was replaced. It does look like the rear O2 sensor rich codes are less likely to trigger as time goes by. My take home message: If you think you have to replace your rear oxygen sensor in a TDI, it is probably something else that is the problem.
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Old September 12th, 2015, 14:02   #69
ronaldleemhuis
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Up for a VCDS challenge? I am hoping somebody can explain the readings I am getting for the rear oxygen sensor in my 2010 Jetta TDI.

As above, it appears that measuring block 36, field 4 represents the signal from the front oxygen sensor and block 38, field 4 represents the signal from the rear oxygen sensor.

For a 7 mile drive, I logged measuring block 38. By logging and graphing the values in field 4 for a range of about 30 to 60 seconds, it shows the exhaust running lean (negative signal typically about -0.20) most of the time. Every few seconds there is a positive spike to about +0.50 followed by a drop to 0.126 where it stays precisely steady for a second or two before dropping back to the lean negative range. This keeps on cycling like this most of the time.

When I take 7 miles of data (perhaps 9000 rows on the spreadsheet) and use the spreadsheet percentile function on field 4, I can calculate values for every percentile 0 to 99. When I plot the value on a y axis vs the percentile from 0 to 99, I get a consistent pattern that matches the pattern on my daughter's relatively new Golf TDI.

It shows an upsloping sigmoid curve that starts about -0.5 on the left and crosses the zero line at about the 71st percentile and continues to +0.126 where it levels off precisely for about 10 percentile points and then resumes its upward rise to +0.5 at the 99th percentile.

The +0.126 plateau is precisely the same for both cars.

My hunch is that this indicates that the exhaust at B1S2 sensor is lean about 71+ percent of the time, that deNox is occurring at the +0.126 plateau about 10% of the time and that rich overshoots occur another 10% of the time, approximately.

If this is true, then the TDIs with the latest ecu flash are using the new rapid deNox cycling technology not in widespread use when my car was manufactured.

Is this really what is happening?

Last edited by ronaldleemhuis; September 13th, 2015 at 17:18.
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Old September 14th, 2015, 08:15   #70
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Just a little more insight on the rear oxygen sensor readings for my 2010 Jetta TDI -- as it relates to NOx storage/reduction catalyst system.

The repetitive cycling of O2 sensor reading from the usual lean (negative) range probably reflects some post-combustion fuel injection to cause the first (oxidation) catalyst to burn it and consume enough oxygen to create a rich mixture as the exhaust enters the NOx storage catalyst. If the NOx reduction catalyst were able to respond instantaneously, this spike of rich gases would not be able to make its way through the NOx catalyst to show up on the rear oxygen sensor during an initial spike. Because we do see an initial spike, this means that it takes a little while for the NOx catalyst to warm up a little and start reducing the NOx. The reading holds at +0.126 as long as the NOx is being reduced and the rich gases entering the converter are not overwhelming.

The visible pattern shows that all the sustained values of +0.126 are followed by a DROP in the O2 sensor signal. This means that reduction stops because the post-combustion (rich) environment is stopped by the ecu rather than that all the NOx is reduced. If all the NOx were reduced, the signal would RISE.

I think there is a sort of self-adjusting mechanism whereby the NOx catalyst avoids the extremes of being fully loaded or fully depleted during ordinary cycling. The computer uses a model to guess how much NOx is being formed. Then an appropriate amount of post-combustion fuel is injected. The model used is probably adjusted by some sort of "trim" figure, though I don't know where to find that.

If the catalyst is loaded "too much," the light-off occurs sooner, and more of the rich environment is used, and vice versa.

The P2271 reading I get occasionally still probably reflect slight sluggishness of response of the rear oxygen sensor. I think that the ecu probably looks at the magnitudes (height) of the oxygen sensor spike rather than a more appropriate integration of this spike over a short time period. If this is true, VW may be able to make the system more tolerant of ordinary aging of the rear oxygen sensor by changing the program to look at the AREA of the rich peaks rather than the HEIGHT. While the heights are affected by sluggishness on the O2 sensor, the area would not be so affected.

So in a sense it is possible that yet another software change could make rear oxygen sensors "last longer."
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