The DEF level sensor and the DEF pump are two core, functionally complementary components of the SCR exhaust treatment system in diesel vehicles. They work in tandem to ensure the stable operation of the exhaust purification system; their specific interrelationships and roles are outlined below:

I. DEF Level Sensor
This monitoring component is installed on the DEF tank. Models produced after 2019 typically use ultrasonic signals to detect fluid levels, whereas older models utilize radio-frequency (RF) sensors; some units also integrate temperature and concentration detection functions.

*   Core Function: It monitors the remaining DEF volume in real-time and transmits level data to the ECU via the CAN bus. It triggers low-level warnings to prevent issues such as torque limitation and excessive emissions caused by insufficient DEF.
*   **Common Faults:** Freezing in low temperatures or DEF crystallization covering the probe can lead to false level readings. Cleaning the float with hot water or checking resistance values ​​can help determine if the unit is damaged.

II. DEF Pump
This component executes the delivery and injection of DEF. It falls into two categories—delivery-type vane pumps and on-board metering pumps—and is constructed from materials resistant to DEF corrosion, such as 316 stainless steel.

*   **Core Function:** It precisely draws DEF based on ECU commands and delivers it to the exhaust system for atomized injection, ensuring efficient nitrogen oxide (NOx) conversion during the SCR reaction. Some models feature built-in self-heating and dry-run protection capabilities.
*   Common Faults: Crystallization blockages or diaphragm seal failures can result in insufficient injection volume, leading to excessive emissions and a loss of engine power.

III. Synergistic Relationship
The level sensor provides the fundamental data required for the DEF pump's operation: when the sensor detects that the fluid level has dropped below a safe threshold, the ECU restricts the pump's continuous operation to prevent damage caused by dry running (idling without fluid). Additionally, the level data assists the ECU in adjusting the pump's injection metering logic, ensuring injection precision meets emissions standards.