DZ128851 | Genuine John Deere® DEF Tank Header Assembly 400mm, 12

1.  Monitor DEF Condition

   • Measures DEF level, temperature, and quality (urea concentration).

   • Sends this data to the engine control unit (ECU) so it can adjust DEF injection accurately.

   • Ensures only properly mixed DEF (32.5% urea, 67.5% deionized water) is used — critical for emission compliance.

2.  Supply DEF to the SCR system

   • The header connects to a DEF pump that draws fluid from the tank and sends it through heated lines to the DEF injector in the exhaust.

   • Without the header’s pickup and delivery system, DEF couldn’t flow properly.

3.  Maintain DEF Temperature

   • Includes a heater element to prevent DEF from freezing in cold weather.

   • Ensures DEF remains in a liquid state for reliable flow and injection.

4.  Communicate with the Vehicle’s ECU

   • The header provides sensor data that helps the ECU determine:

     • When to inject DEF

     • How much DEF to inject

     • Whether there are faults (e.g., low DEF, poor quality, or freezing issues)

1. Warning Lights and Fault Codes

• “Check Engine” light or “DEF system fault” message on the dash.

• SCR or emissions-related fault codes (often starting with P20xx, such as P203F, P205B, P206A, P207F).

• DEF quality sensor errors or temperature sensor circuit faults stored in the ECM.

• “Service DEF system” or “Vehicle will derate in XXX miles” messages.

2. Incorrect DEF Level Reading

• DEF gauge shows empty or full all the time, regardless of actual level.

• Gauge fluctuates or drops suddenly after refilling the DEF tank.
  Cause: DEF level sensor inside the header is malfunctioning or covered in crystallized DEF.

3. DEF Freezing or Not Thawing

• In cold weather, the system takes too long to thaw or never thaws at all.

• The engine may derate because frozen DEF can’t be injected.
  Cause: The heater element in the DEF header has failed.

4. DEF Quality or Concentration Errors

• ECU reports poor DEF quality or incorrect urea concentration even when fresh DEF is used.
  Cause: The quality sensor inside the header is faulty or contaminated with crystallized DEF.

5. Engine Derate or Power Limitation

• The truck goes into limp mode or reduces speed (e.g., limited to 5 mph) after warnings are ignored.
  Cause: The ECM forces derate to prevent emissions violations if it detects the SCR system isn’t working properly.

6. Crystallization or Contamination

• Visible white crystals or buildup around the DEF tank or header connector.

• Poor electrical contact due to corrosion or DEF leakage.
  Cause: DEF exposure to air causes crystallization, which can damage sensors or block passages.