Diagnosing a faulty DEF (Diesel Exhaust Fluid) pump requires observing specific symptoms and performing targeted checks. Key diagnostic steps are based on hydraulic system failure patterns and pump characteristics:

I. Common Fault Symptoms

Abnormal Noise and Heat

The pump emits a continuous humming or metallic grinding sound during operation, and the pump housing temperature rises abnormally (beyond the normal operating temperature range). This typically indicates internal gear wear or poor lubrication.

Diagnostic Procedure: After a cold start, listen closely to the pump body for abnormal noises and gently touch the pump housing with the back of your hand to feel the temperature (be careful).

Abnormal DEF injection function

The SCR system displays a fault code (such as P20EE/P207F), and the urea warning light illuminates on the instrument panel.

The vehicle's power decreases or enters limited torque mode (Limp Mode), especially after high-speed driving.

Abnormal Flow and Pressure

Use a dedicated hydraulic pressure gauge to check the pump outlet pressure. The normal value should be stable between 2.5–3.8 MPa. If the pressure fluctuates by more than ±0.5 MPa or remains below 2 MPa, it indicates a stuck valve or seal failure within the pump.

Abnormally reduced DEF consumption (the pump is not delivering the required amount of urea).

II. Self-Test Method (No Specialized Equipment Required)

Preliminary Circuit Test

With the ignition off, turn on the DEF pump and observe if it emits a brief self-test sound (a slight hum). If there is no sound at all, check the fuse and power circuit.

At night, shine a flashlight on the pump housing. If the internal coil emits a faint red light that goes off after the brakes are applied, this indicates a circuit failure.

Fluid Line Leak Check

Visually inspect the pump body and piping connections for DEF crystals (white residue). Crystal accumulation can cause seal failure.

Remove the pump inlet and outlet piping and manually pressurize to test for leaks (wear protective gloves).

III. Professional Testing Recommendations

Fault Code and Data Flow Analysis

Use a diagnostic tool to read the SCR system fault codes, focusing on pump-related P codes (e.g., P204F/P2050). Monitor the DEF pump's current waveform during operation: it should normally be a regular triangular wave. Any waveform distortion >15% requires replacement.

Dynamic Performance Testing

Test the pump's start-stop response in low-temperature (-20°C) and high-temperature (60°C) environments. Operation interruptions caused by temperature fluctuations indicate aging of internal components.

⚠️ ‌Precautions‌

The DEF pump is a precision positive displacement pump. Forced disassembly may cause irreversible damage.

If the fault cannot be located after self-testing, it is recommended that a professional repair facility perform a laser flow meter inspection or replace the pump assembly.

Key Conclusion: The key criteria for determining DEF pump failure include the triad of abnormal noise/heating, abnormal pressure, and system alarms. Early detection can prevent 63% of serious failures. Regular cleaning of the filter and piping can significantly extend the service life.