Massey Ferguson 3521.31 – Engine ECU: DEF Quality Sensor Has Detected A Problem With The DEF Sample

The DEF quality sensor has measured a urea concentration in the Diesel Exhaust Fluid (DEF) that is outside the acceptable range (normally 32.5% urea and 67.5% deionized water). This deviation can be caused by fluid contamination, evaporation, or the use of incorrect fluid. The fault will likely trigger an engine power reduction (derate) to maintain emissions compliance.

• 4710
• 7724S
• 5712SL
• 7722S
• 7722
• 5713SL
• 7720S
• 7716
• 7718
• T4
• 7715
• 5710SL
• 7726
• 4708
• 7726S
• 4707
• 4709
• 5711SL
• 7718S
• 7719
• 7714
• 7716S
• T4 (2015-2021)
• 7720
• Tier 4 (2015-2021)
• 7715S
• 7724
• 4700 Series
• 5600 Series
• 5608
• 5609
• 5610
• 5611
• 5612
• 5613
• 5700SL Series
• 7600 Series
• 7614
• 7626
• 7700 Series
• 7700S Series
• 8700 S Series

1. 1

   Test the DEF concentration using a DEF refractometer directly from the tank (should read exactly 32.5%).

2. 2

   Visually inspect the DEF tank, filler neck, cap, and breather for dirt, dust, or signs of diesel fuel/water mixed into the DEF.

3. 3

   Inspect the wiring harness and connector going to the DEF quality sensor on the tank unit for corrosion, chafing, or loose pins.

1. 1

   Drain the Tank

   Completely dump/drain the DEF tank to remove the contaminated or degraded fluid.

2. 2

   Clean the Tank

   Flush the tank to ensure no residual dirt, diesel, or crystallized urea remains. Ensure the tank neck and cap are spotless.

3. 3

   Replace the Sensor

   If the fluid tested fine but the code persists, or if the sensor is physically damaged/crystallized beyond cleaning, remove the old DEF quality sensor assembly from the tank and install the new OEM sensor.

4. 4

   Refill

   Refill the tank with fresh, clean, in-date DEF (AdBlue) from a sealed container.

5. 5

   Clear Codes

   Use a compatible diagnostic tool (such as AGCO EDT or Jaltest AGV) to clear the 003521.31 fault code from the Engine ECU.

6. 6

   Test Run

   Run the tractor to allow the system to pressurize, sample the new fluid, and verify that the code does not return.