4352100C91 | International® Def Quality Level Sensor 10.6/15.9/25 Qls

Part number is an OEM fitment for medium- and heavy-duty International / Navistar commercial trucks equipped with 10.6L, 15.9L, or 25L DEF tanks:

• Class 8 Highway Trucks: International ProStar, LT Series, RH Series, LoneStar.

• Vocational & Regional Trucks: International WorkStar, HV Series, HX Series, MV Series, DuraStar.

• Buses & Commercial Chassis: IC Bus platforms with SCR emissions control systems.

• Compatible Engine Platforms: Cummins ISX15 / X15, Cummins L9 / B6.7, and Navistar N13 / A26 engines.

1. What is it?

Part is a multi-function submerged header unit installed inside the DEF reservoir of medium- and heavy-duty trucks. It serves as the primary communication link between the DEF tank and the Engine Control Module (ECM) to keep Selective Catalytic Reduction (SCR) systems fully compliant with emissions standards.

2. Integrated Functions & Key Features

• Quality, Level & Temperature Sensing (QLS): Utilizes digital ultrasonic sensor technology to measure DEF liquid level, continuously test urea concentration (ensuring 32.5% purity to prevent dilution), and monitor fluid temperature over the CAN-bus network.

• Fluid Suction & Return Pipelines: Heavy-duty pickup tubes transport DEF directly to the dosing pump and return excess fluid safely back to the tank.

• Engine Coolant Heating Loop: Features an internal tube coil connected to the truck's engine cooling system to thaw frozen DEF (which solidifies at 12°F / -11°C) in cold weather.

• In-Tank Protection: Includes a fine mesh filter screen at the suction intake to catch debris and prevent damage to the downstream DEF pump.

1. Active Check Engine Light & Diagnostic Fault Codes

The ECM logs specific SCR Diagnostic Trouble Codes (DTCs) when it loses communication with the integrated Quality, Level, and Temperature Sensor (QLS) or receives out-of-spec signals via the J1939 CAN network:

• Common Fault Codes:

  • SPN 1761 / FMI 9, 11, 19 (DEF Tank Level - Communication Error / Abnormal Rate of Change)

  • SPN 3364 / FMI 2, 9, 11 (DEF Quality - Abnormal Urea Concentration / Non-DEF Detected)

  • SPN 3031 / FMI 9, 11 (DEF Tank Temperature - Out of Range / Signal Invalid)

  • SPN 3868 / SPN 4572 (Navistar QLS Header Data Performance Faults)

2. Incorrect or Fluctuating DEF Level Readings

• Symptom: The DEF gauge on the dashboard reads empty immediately after refilling a 10.6L, 15.9L, or 25L tank, stays permanently stuck at full, or fluctuates randomly while driving.

• Cause: Failure of the internal float, degradation of the ultrasonic transducer, or fluid ingress shorting out the internal circuit board.

3. False "Incorrect DEF" Warnings & Engine De-rate

• Symptom: The driver display triggers a "Poor DEF Quality" warning along with a countdown timer, eventually reducing vehicle speed (typically down to 55 mph and later to 5 mph).

• Cause: Internal calibration drift causing the sensor to falsely report incorrect urea concentration (below the standard 32.5% ISO requirement) even when fresh, high-quality DEF is used.

4. Cold Weather Heating & Thawing Failures

• Symptom: In freezing conditions, the vehicle logs SCR pressure codes (SPN 3556) or tank thaw timeout errors because fluid remains frozen in the reservoir.

• Cause: A faulty integrated temperature sensor sending incorrect liquid temperature readings to the ECM, or internal blockage within the coolant heating loop.