John Deere 004334.01 – ECU or ACM: DEF System Pressure Low

The Engine Control Unit (ECU) or Aftertreatment Control Module (ACM) has detected that the Diesel Exhaust Fluid (DEF) pressure is below the required minimum level for proper system operation. Adequate pressure is necessary to correctly atomize and inject DEF into the exhaust stream to reduce NOx emissions. Failure to maintain this pressure leads to reduced Selective Catalytic Reduction (SCR) system efficiency, additional fault codes, and eventual engine power reduction.

• 5R Series
• 6M Series
• 6R Series
• 7R Series
• 8R Series
• 8RT Series
• 8RX Series
• 9R Series
• 9RT Series
• 9RX Series
• 210L
• 337E
• 344L
• 344P
• 710L
• 710P
• R4023
• R4038
• R4045

1. 1

   Check for other active fault codes related to the DEF pressure sensor, DEF pump, or DEF tank level and address those first

2. 2

   Inspect DEF level and quality to ensure the tank is not empty (which introduces air) and the fluid is not contaminated

3. 3

   Check for frozen DEF in cold climates (if the heater fails to thaw the fluid within 40 minutes, a fault will log)

4. 4

   Inspect all DEF lines, fittings, and connections between the tank, pump, and dosing valve for leaks (look for white crystalline residue)

5. 5

   Inspect the main DEF pump filter and the suction filter/screen located in the DEF tank header for clogs

6. 6

   Check for air in the system lines caused by low fluid levels or leaks

7. 7

   Examine the electrical circuits and wiring for the DEF pump and pressure sensor for damage, corrosion, or loose connections

1. 1

   Address leaks and contamination

   If any leaks are found, repair or replace the faulty lines or fittings. If the DEF is contaminated, drain the tank, clean it thoroughly, and refill with fresh, high-quality DEF.

2. 2

   Replace clogged filters

   Replace the main DEF dosing unit in-line filter and the tank header suction screen. For tractors with this feature, access the DEF tank header, clean the area around it, disconnect the electrical and fluid lines (coolant, DEF supply, DEF return, vent), and remove the header to access and replace the suction screen. It is recommended to replace both the dosing unit filter and the tank suction screen at the same time.

3. 3

   Test and replace the DEF pump

   If filters are clean and there are no leaks, the pump itself may be faulty. If the pump was recently serviced, the pump filter may have dried out; allow the pump to sit in DEF fluid overnight to re-saturate the filter. If the pump has failed mechanically or electrically, replace the entire unit (the DEF pressure sensor is often integrated into the dosing unit/pump assembly).

4. 4

   Verify the repair

   Clear the fault codes and run the engine. Use a diagnostic tool to perform a system leak test or command the pump to run and verify that it can build and maintain the correct pressure (approximately 72 psi).