Check engine light or Adblue warning with fault codes P206B31,
P203B31, or P300494 present in AdBlue (SCR) control unit (N118/5)
Topic number

LI49.20-N-078838

Version

11

Function group

49.20 - Exhaust gas aftertreatment

Date

3/23/26

Validity

Model 907

Reason for change

Remedy Update

Complaint
The check engine light or AdBlue warning is activated, accompanied by one or more of the following fault codes related to the AdBlue tank:
P206B31 Malfunction of the concentration sensor for AdBlue; the signal is absent.
P203B31 Malfunction of the AdBlue fill level sensor; the signal is absent.
P300494 The quality of the AdBlue is insufficient.

Cause
A. The software for the AdBlue (SCR) control unit (N118/5) is outdated.
B. The AdBlue concentration is outside ISO 22241 limits (below 31.8 percent or above 33.2 percent).
C. Frozen or partially frozen AdBlue can cause the AdBlue level and concentration functions to report inaccurate values.
D. The AdBlue level and concentration sensing functions in the tank are damaged.
E. Cold soak and freeze thaw exposure can temporarily create concentration gradients in the tank and lead to sampling dependent readings until fully thawed and mixed.
Important
Before evaluating any AdBlue concentration result, follow LI49.20-N-080376 to confirm full thaw, perform a mixing drive, and retest.
Do not use a single urea percent reading taken from frozen, partially frozen, or not fully mixed AdBlue for a repair or
responsibility decision.

Remedy
A. Software
1. Verify no newer software is available for the AdBlue (SCR) control unit (N118/5).
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2. If a software update is available, program the control unit per XENTRY guidance.
3. Only after the system is in proper working condition, complete LI49.20-N-075995 for AdBlue warning reset.
Note:
If there is no updated software available, print the XENTRY screen indicating this and proceed to section B.
B. AdBlue concentration
1. Confirm full thaw and mixing per LI49.20-N-080376, then verify AdBlue concentration.
a. Check concentration using the MB refractometer per the MB tool instructions and applicable WIS guidance.
b. Use freeze frame data when applicable.
2. If AdBlue concentration remains below 31.8 percent or above 33.2 percent after full thaw, mixing drive, and retest,
AdBlue replacement is required.
Warranty and responsibility decisions must follow LI49.20-N-080376 warranty handling guidance and standard warranty policy.
3. If AdBlue concentration is within specification, proceed to section C.
C. Frozen AdBlue or SCR actual values indicate frozen AdBlue
1. Check if AdBlue is frozen. If frozen, allow the vehicle to sit in a warm environment overnight and recheck.
2. With the vehicle running and XENTRY connected, display the actual AdBlue level values.
3. Drain the AdBlue tank and verify the displayed fill level reflects the change.
4. Fill the AdBlue tank slowly. Do not overfill. Verify the fill level reflects the change.
Note:
For model years 23 to 25, Fill Level Sensor 1 functions as the empty sensor and accurately measures only the bottom
portion of the tank. Once the level is above the measurement range, the displayed value may fluctuate. This is normal.
5. Reset the AdBlue tank fill level via XENTRY. Print the XENTRY pages showing that the fill up completed successfully.
6. Only after the system is in proper working condition, complete LI49.20-N-075995 for AdBlue warning reset and print
the XENTRY pages confirming the reset completed successfully.
Note:
If XENTRY still indicates frozen AdBlue or the concentration remains inaccurate after drain and refill and the thaw,
mix, retest requirements were completed, proceed to section D.
D. Damaged AdBlue level and concentration sensing functions
1. Replace the AdBlue tank as required by applicable MB guidance.
2. With the vehicle running and XENTRY connected, display the actual AdBlue level values.
3. Fill the AdBlue tank slowly. Do not overfill. Verify the fill level reflects the change.
4. Reset the AdBlue tank fill level via XENTRY.
5. Only after the system is in proper working condition, complete LI49.20-N-075995 for AdBlue warning reset.

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Note:
During the LI49.20-N-075995 reset, ensure the outside temperature sensor is reading above 10 degrees Fahrenheit.
If not, keep the vehicle inside and repeat the reset steps.

Attachments
File

Description

Adblue Frozen.png

Adblue Frozen

Adblue Concentration.png

Adblue Concentration

Adblue Fill Level.mp4

Adblue Fill Level 1

Urea Quality.png

Urea Quality

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WIS-References
Document number

Title

Note

AP49.20-D-0101TS

AdBlue(R) tank - Check/correct fluid
level

AR49.20-D-2015TS

Empty/fill AdBlue(R) tank

AR49.20-D-2014TSF

Remove/install AdBlue(R) tank

Disclaimer
NOTE: The information contained in this document is intended for use by trained, professional technicians with the
knowledge to properly and safely perform diagnosis and repairs on Mercedes-Benz vehicles, using Mercedes-Benz
approved tools and equipment. It informs service technicians about conditions that could occur in certain vehicles and
provides information that could assist in proper vehicle diagnosis, service, or repair. It does not indicate that a defect
is present in any vehicle referenced in this document nor does it imply warranty coverage. DO NOT assume that a
symptom or condition, or a described cause of a symptom or condition, affects any particular vehicle or groups of vehicles, or that a described repair applies to any particular vehicle or groups of vehicles. There can be multiple causes
resulting in the same or similar symptoms or conditions described in this document, and trained professional service
technicians must use their diagnostic skills to make evaluations on a case-by-case basis. The information contained
in this document does not guarantee warranty coverage nor does it extend the vehicle’s warranty in any way.

Symptoms
Power generation > Fuel system > Fuel tank > Function > Different level
Power generation > Exhaust system > AdBlue > AdBlue consumption > Too high

Parts
Part number

ES1 ES2 Designation

A9104702401

Quantity Note

NOX REDUCING AGENT
TANK

EPC

1

X

Control unit/fault code
Control unit

Fault text

N118/5 | N141 - Selective catalytic reduction (SCR
GEN4) (UDCM2)

P203B31 - The AdBlue® fill level sensor has a malfunction. The signal is not present.
P206B31 - Concentration sensor 'AdBlue®' has a malfunction. The signal is not present.

N3/40 - Motor electronics 'MRD1' for combustion engine
'OM654' (CDI) (MRD1NFZ)

P300494 - The AdBlue® quality is insufficient.

Operation numbers/damage codes
Op. no.

Operation text

470643

Check separate parts of AdBlue(R) sys- 0.7 H
tem: ... as per fault code

499264

Check AdBlue(R) concentration (After
quick test)

H

499290

Empty/fill AdBlue(R) tank

H

499292

Replace AdBlue(R) tank

H

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Time

CCUSHWA

Damage
code

Note
This Operation item is used for completing LI49.20-N-075995.

4707250

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540650

On-board power supply voltage Maintain H
(when checking/testing and troubleshooting)

540990

Program, code ..... control unit (After
quick test)

H

541011

Perform quick test

H

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If newer software is available

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