Massey Ferguson 3521.18 – Engine ECU: DEF Quality Sensor Has Detected A Problem With The DEF Sample.

The DEF quality sensor has measured a urea concentration that falls outside the acceptable operating range (which must be a precise 32.5% urea solution in de-ionized water) or has detected contamination such as dirt, diesel, or excess water in the tank.

• 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
• 5608
• 5609
• 5610
• 5611
• 5612
• 5613
• 7614
• 7615
• 7616
• 7618
• 7619
• 7620
• 7622
• 7624
• 7626

1. 1

   Test the DEF fluid using a DEF refractometer to ensure it reads exactly 32.5% urea.

2. 2

   Visually inspect the DEF tank for signs of contamination like diesel fuel, oil, dirt, or cloudiness.

3. 3

   Inspect the DEF tank cap and breather for proper sealing to prevent dust and moisture entry.

4. 4

   Inspect the wiring harness and digital CANBUS connector leading to the DEF sensor/header unit for corrosion, chafing, or loose pins.

1. 1

   Drain contaminated DEF

   If the DEF fluid is contaminated or degraded (Refractometer fails), drain the contaminated DEF completely from the tank.

2. 2

   Flush tank

   Flush the tank thoroughly with distilled water or fresh, known-good DEF.

3. 3

   Replace DEF pump filter/strainer

   Replace the DEF pump filter/strainer to ensure no residual contamination reaches the doser/injector.

4. 4

   Refill tank

   Refill the tank with fresh, high-quality DEF.

5. 5

   Clear codes and verify

   Clear the fault codes using a diagnostic tool and run the tractor to verify the code does not return.

6. 6

   Disconnect battery and sensor

   If the fluid is perfect (32.5%) and wiring is intact, the DEF Quality Sensor itself has failed internally. Disconnect the battery and unplug the electrical connector to the DEF sensor unit on top of the tank.

7. 7

   Disconnect fluid lines

   Disconnect the heated supply and return lines (be careful not to nick the O-rings).

8. 8

   Remove old sensor

   Remove the mounting hardware and extract the old DEF sensor/sender unit from the tank.

9. 9

   Install new sensor

   Install the new OEM DEF sensor, reconnect the fluid lines and electrical harness.

10. 10

    Clear codes and verify

    Clear the ECU fault codes and run the machine to verify normal operation.