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Preview Solution CBR-2107-21
Mack DEF Level (UQLS) Sensor
Troubleshooting Guide - US17+OBD16
Emissions and Newer
Published 27aMaya2025
Valid For
All Mack models 2017 and newer

Overview
DEF S
DEF
Sensor
ensor

; Live UI

The Urea Quality Level Sensor (UQLS) is a smart sensor that is located in the top of the
Urea (DEF) Tank, commonly referred to as the DEF Level Sensor or Combined Tank Unit.
This smart sensor has Urea Quality, Level, and Temperature all integrated into it as one unit
and reports on the DL7 (J1939-7) Datalink. This sensor is responsible for sending all the

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urea information to the Aftertreatment Control Module (ACM) and Engine Control Module
(ECM).
a
a
Updated UQLS Functions
a
Basic deţnition
The SSI sensor measures level as well as concentration by means of ultrasonic waves.a
TheaUrea Quality Level Sensor (UQLS)acommunicates onaController Area Network
(CAN)aand is connected to EngineSubnet. The sensor will support automatic baud rate
detection (250/500 kbps).

a
Level measurementa
Level measurement works by bouncing ultrasonic waves towards the liquid level. The
sensors report level in mm where 0 is deţned as bottom of the focus tube (oſset
values can be used for speciţc part number, e.g NA aftermarket sensors). However,
some urea above theapiezoelectric sensor technology (PZT)ais needed in order for
the sensor to detect a level, deţnition is therefore that 20 mm is the lowest level where
the sensor can reliably measure. If the sensor can't detect urea level it will output an
error.
Based on tank geometry and dataset steered by vehicle variant speciţcation, the mm
value from the sensor is converted to urea mass and tank ţll percentage.aa

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Quality measurement
Urea quality (concentration) is measured by bouncing ultrasonic waves between
theaPZTain the toe of the sensor towards a reŤector. By knowing the distance and
temperature the urea concentration is derived from the speed of sound waves in the
liquid.a a
Temperature measurement
Temperature is measured on two locations on theaPrinted Circuit Board (PCB).a
Component list

a
aNOTE:aThe UQLS cannot be ordered in separate parts; it must be purchased as a
complete sensor. Refer to impact for details.
a
a

Diagnosis and Repair
a

Perform a DTC Readout and Review Codes
a
If any of the codes in yellow below appear on the Readout in Active or Confirmed
status: Continue to the diagnostic steps in the following section.
a
DTC
DTC Description
P205B64

; Live UI

P203A13

Reductant Tank Temperature Sensor, Circuit
Range/Performance
Reductant Level Sensor "A", Open Circuit

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P203C00
P206A13

Aftertreatment Reagent Level, Short Circuit Low
Reductant Quality Sensor, Open Circuit

P206C00
P206B64

Reductant Quality Sensor, Short Circuit Low
Reductant Quality Sensor Range/Performance

P205A13
P205C00

Reductant Tank Temperature Sensor, Open Circuit
Aftertreatment Reagent Tank Temperature, Short Circuit Low

P204364

Aftertreatment Reagent Tank Temperature Sensor Stuck, Signal
plausibility failure

a
a
DO NOT REPLACE the DEF Level Sensor for any of the codes in green below: DO
NOT proceed with the diagnostics steps in this solution. None of these codes immediately
indicate a failed DEF Level Sensor. Use the table in conjunction with PTT’s Guided
Diagnostics to find the root cause.
a
DTC
DTC Description
Probable Source
P203F00

Commonly low DEF quantity or
level sensor float in DEF tank stuck

Reductant Level Low

Aftertreatment Reagent Level
System fault. Follow Guided Diagnostics.
Warning
Lost Communication with
U02A200
Reductant
Commonly a wiring harness or connection issue.
Quality Module
Reductant Temperature Too DEF tank temp above 70°C (160°F). Commonly the coolant line
P24FF00
High
to the DEF tank are swapped.
P203B00

P207F00

Reductant Quality

DEF quality low and SCR efficiency low. Commonly a DEF
quality issue.

a
a

Diagnostic Steps For Yellow Codes
a
1. Check the sensor wiring harness connection.
a

• Disconnect the harness and make sure there is no damage to the wires or pins,
corrosion, or water ingress in either of the connectors. If damage is found replace
in accordance with Technical Service Bulletin (only if applicable) 258-040 Aftertreatment Diesel Exhaust Fluid (DEF) Tank, Level Sensor, Replacement.
Located in Impact.
a
1. Navigate to the Service tab.
2. In the Info Type box, make sure Technical Service Bulletins is selected.
3. In the Additional search values box select Keyword

; Live UI

4. Enter 258-040 into the text entry box, press the Search button.

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5. A link for the article will be shown in the results. Click on the title, then choose
one of the options from the window that opens.
a

a
a
2. Verify the DEF quality is at an acceptable value.

• Use refractometer part number 88890105 to verify the DEF quality. See
illustration below.
a

;

Live UI

a
a
3.
a
4.
a
5.
a
6.

Verify that the DEF tank is not frozen.
Ensure that the DEF tank is adequately filled.
Clear the Fault Codes.
Start the engine. Run the engine on high idle for 2 minutes at 1000 rpm or higher.

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7. Turn the Engine Off, Key Off for 15 seconds.
8. Start the engine. Run the engine on high idle for 2 minutes a second time.

Evaluate the Results
If any of the faults shown in the yellow chart above return active: Replace the DEF
Level Sensor in accordance with Impact instructions and the wires routed as shown in
Technical Service Bulletin 258-040.
If none of the faults in the yellow chart above return: No troubleshooting or
replacement of the DEF Level Sensor is necessary.

Rules for Replacement
• In order for a repair to be eligible for warranty coverage, one of the diagnostic trouble
codes (DTCs) listed above must be present. However, in rare cases where there is a
potential failure but no DTCs are detected, please provide supporting evidence of the
failure. This can be in the form of a photo or video, along with an explanation for the need of
replacement. This evidence will be used to determine if the repair qualifies for warranty
coverage. For any additional support with diagnostics, open an e-Service case.
• Standard Diagnostic Time for a DEF Level Sensor is 0.6 hrs.

Related links and attachments
FSB 258-040

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