4BCD

4BCD - P21A9

Information saved in
DDE

Fault code
4BCD - P21A9

Fault text
DDE: Reducing-agent passive-tank fill-level sensor,
plausibility: Plausibility error

Fault description
Plausibility check on passive tank level sensor. When the
physical sensor signal falls below the limit value 35 % the
signal is classified as implausible and the diagnostic trouble
code is logged.

Condition for fault identification
Test condition:
The test routine is executed continuously.
Voltage condition:
The test routine is executed continuously.

Condition for fault memory entry
Debounce (30000 ms)

Action in service
If the reduction agent is frozen: Allow the tank to thaw.
If the reduction agent is not frozen:
Completely fill tank.
1. Check wires and plug connections.
2. If wiring and plug connections are OK:
3. Use diagnostic system to check fluid level.
4. Replace passive tank.

Fault effect and breakdown warning
-

Driver information
Warning light:
MIL

Service instruction
When a plausibility error is present the higher level sensor
recognizes a fluid level (conductivity) while the lower level
sensor does not.
In winter: For example, when a frozen passive tank is
refilled, the lower sensor might be encapsulated in ice, while
the upper sensor would recognize conductivity.
The level sensor's conductivity can also be assessed as
follows:
1. Use the diagnostic system to read out the tank fluid level.
2. Delete the DTCs from the error memory and carry out the
quick-test procedure for the individual control modules.
Watch for new DTC entries.
If the DTC 4BCB is present at the same time it must be
repaired first.