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Old 03-18-2017, 11:20 PM
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Mein Auto: 2011 BMW X5 35D
46A4 - DDE: DeNox Catalyst, Understanding root cause

Haven't found a thread with this specific code, thought I would open new one.
Also, cross-posted in that other forum.

2011 35d, 108K miles.

So, this non-CEL fault is lately coming back (triggered a CEL a couple weeks ago, I recorded it and removed from memory to keep an eye on reocurrences):
Quote:
0046A4 DDE: NoX sensor after DeNox Catalyst, plausibility NoX
I have searched potential answers on forums, with potential causes being:
-Bad urea active tank
-Clogged SCR metering valve
-Bad cat
-Bad or fouled NoX sensor

Being fully aware that there is a warranty extension on Active Tank and Metering Valve, I need to make sure those are the culprit before heading out to dealer for claim (and risking wasted diagnostics fee).

So, pulled this fault code documentation in Rheingold:
Quote:
Fault Description
Monitoring the NOx signal offset value (nitrogen oxide sensor downstream of SCR catalytic converter). The fault is detected if the average offset value exceeds a limit value.

Limit value: between 30 ppm and 60 ppm (version-dependent).
This is pure legalese!

Can one of the veterans help me decipher this text (calling you ard, Doug H., ZetaTre)?

This snippet:
Quote:
NOx signal offset value (nitrogen oxide sensor downstream of SCR catalytic converter)
..is it safe to assume that "offset value = difference in reading between pre and post NoX sensor"?

Can it be said this way?:
"The fault is detected if the average detected NoX reduction (over certain period of time, on that below) between pre and post Cat sensor is less than 30-60 parts per million".

Continuing to read..
Quote:
Condition for fault identification
Monitoring takes place continuously when the following preconditions are met:

- The difference between the maximum and minimum offset value must be less than the limit value during this minimum duration.

Limit value: between 4 ppm and 7 ppm (version-dependent).

...
[talks about engine rpm and other preconditions
...
I think it can be said like this in plain English:

"Non-stop monitoring will start as soon as largest and smallest difference between pre and post NOx sensors is less than about 7 parts per million.
In other words, system will start continuous watching as soon as it detects no improvement in NoX.
"

The rest of documentation, more-less understandable.
Quote:
Condition for fault memory entry
The fault is entered if it has been detected more than the following number of times (fault debouncing counter): 1

Action in service
Check the NOx sensor downstream of SCR catalytic converter with an SCR catalytic converter system check.

1. Check line and plug connections
2. If wires and plug connections are OK:
Replace nitrogen oxide sensor after SCR catalytic converter.
Next, hooked up laptop and went for a drive to :
1. Confirm post-cat sensors working
2. Gather live NOx sensor data under various driving conditions

Observations:
40 mph on cruise control, 1,500 rpm, UPHILL
pre-cat: ~600 ppm
post-cat: ~200 ppm

40 mph on cruise control, 1,500 rpm, LEVEL
pre-cat: ~110 ppm
post-cat: ~50 ppm

0 mph, 800 rpm, IDLING
pre-cat: ~130 ppm
post-cat: ~12 ppm

If it means what I think it means, I may have hit this "Limit value: between 30 ppm and 60 ppm" on this test drive:
Quote:
40 mph on cruise control, 1,500 rpm, LEVEL
pre-cat: ~110 ppm
post-cat: ~50 ppm
If that's true, I need to get to the culprit of worse-than-expected NoX readings after SCR.
They could be:
-DEF injection issues
-Bad Catalyst
-Bad NoX sensor. Which one?
For example, pre-SRC sensor could be fouled by soot, giving lower initial readings and thus misleading post-SRC sensor...

It doesn't help that there's no information available for NoX reduction baseline in well-working SCR system in 35d.

Any info is appreciated.
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