[Mark M]

Got this one over the weekend. Anyone have an All Data Pro account that can look it up? Thanks.

[TDIwyse]

Sounds like the Diesel Oxidative Catalyst isn't providing the temp rise response the DDE checks for. IIRC the car uses post injection and measures the temp rise across the DOC with two of the EGT probes to check its functionality. I'll see if I can dig up the reference to this in the BMW University documents.

[TDIwyse]

Just a guess, but it might be related to this. I've noticed on my car over the years that the post injection is used to raise the EGT's during the warm up phase (usually until the car reaches a coolant temp of ~140F) and I've assumed the following is what is going on.

http://prodcds.bmwuniversity.com/lib...erformance.pdf

Page 114 of the doc.


Oxidation catalytic converter
The oxidation catalytic converter is monitored
with regard to its conversion ability which
diminishes with ageing. The conversion of
hydrocarbons (HC) during cold start is used as
the indicator as heat is produced as part of the
chemical reaction and it follows a defined
temperature progression after the oxidation
catalytic converter.
The exhaust gas temperature sensor after the
oxidation catalytic converter measures the
temperature. The DDE maps the temperature
progression during cold start and compares it
to calculated models. The result determines
how effective the oxidation catalytic converter
is operating. A reversible fault is stored if the
temperature progression drops below a
predetermined value. If this fault is still
determined after two successive diesel
particulate filter regeneration cycles, an
irreversible fault is stored and the MIL is
activated

[Brad335d]

Originally Posted by Mark M

Got this one over the weekend. Anyone have an All Data Pro account that can look it up? Thanks.

Ask and ye shall receive.




4B91 - P0420

Information saved in
DDE

Fault code
4B91 - P0420

Fault text
Oxidation catalytic converter, plausibility: Hydrocarbon
conversion rate during exothermic reaction too low

Fault description
Monitoring of the oxidation catalyst in warmup mode. The
DTC is logged when the calculated conversion rate lies
below the limit value 0.

Condition for fault identification
Test condition:
The monitoring routine executes when the following
conditions are satisfied:
- No other DTCs are logged.
- The oxidation catalyst's surface temperature is above the
minimum limit value 120 �C.
- The oxidation catalyst surface temperature's gradient is
above the minimum limit value OxiCat_dtSurfRHUMin_C.
- Engine running.
- The quantity of fuel consumed since the last particulate
filter regeneration is below the threshold 328 l.
- Temperature model is sufficiently precise: Difference
between the measured and modeled temperatures behind the
oxidation catalyst is less than 500 �C.
- No calculation of HC conversion rate and no monitoring
have taken place in this driving cycle.
- Mass flow rate lies above the threshold 0 kg/h.
Conditions for aborting the process: The test is executed
provided that none of the following conditions lead to
mandatory cancellation:
- Enable conditions are satisfied.
- Warmup mode active for less than maximum threshold
(maximum threshold between 20 sec. at 0 �C and 300 sec. at
200 �C and above). The time is reset when the HC
conversion-rate calculation is not enabled or when a change
in operating mode affecting assessment of the HC
conversion rate is detected.
Trigger condition: The diagnostic routine executes one time
only after the following conditions are satisfied:
- Cumulative exhaust-gas mass flow rate during the
monitoring is less than OxiCat_dmEGAvrgMax_C.
Generation of a cumulative current exhaust-gas mass flow
rate continues for as long as monitoring remains enabled.
Trigger:
- Cumulative HC quantity during the oxidation catalyst's
warmpup phase is above 2 g. Generation of a cumulative
current exhaust-gas mass flow rate continues for as long as
the HC conversion rate calculation remains enabled.
- Oxidation catalyst's surface temperature lies above -
3550 �C.
Once all conditions are satisfied the error is checked one
time with the positive flank.
Voltage condition:
The monitoring routine executes when the following
conditions are satisfied:
- No other DTCs are logged.
- The oxidation catalyst's surface temperature is above the
minimum limit value 120 �C.
- The oxidation catalyst surface temperature's gradient is
above the minimum limit value OxiCat_dtSurfRHUMin_C.
- Engine running.
- The quantity of fuel consumed since the last particulate
filter regeneration is below the threshold 328 l.
- Temperature model is sufficiently precise: Difference
between the measured and modeled temperatures behind the
oxidation catalyst is less than 500 �C.
- No calculation of HC conversion rate and no monitoring
have taken place in this driving cycle.
- Mass flow rate lies above the threshold 0 kg/h.
Conditions for aborting the process: The test is executed
provided that none of the following conditions lead to
mandatory cancellation:
- Enable conditions are satisfied.
- Warmup mode active for less than maximum threshold
(maximum threshold between 20 sec. at 0 �C and 300 sec. at
200 �C and above). The time is reset when the HC
conversion-rate calculation is not enabled or when a change
in operating mode affecting assessment of the HC
conversion rate is detected.
Trigger condition: The diagnostic routine executes one time
only after the following conditions are satisfied:
- Cumulative exhaust-gas mass flow rate during the
monitoring is less than OxiCat_dmEGAvrgMax_C.
Generation of a cumulative current exhaust-gas mass flow
rate continues for as long as monitoring remains enabled.
Trigger:
- Cumulative HC quantity during the oxidation catalyst's
warmpup phase is above 2 g. Generation of a cumulative
current exhaust-gas mass flow rate continues for as long as
the HC conversion rate calculation remains enabled.
- Oxidation catalyst's surface temperature lies above -
3550 �C.
Once all conditions are satisfied the error is checked one
time with the positive flank.

Condition for fault memory entry
Event debounced (1)

Action in service
Replace oxidation catalyst/particulate filter.

Fault effect and breakdown warning
-

Driver information
Warning light:
MIL

Service instruction
none

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